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male, female: Chapter 7

Developmental Sex Differences

Is sexual selection related to differences in the physical, social, and psychological development of boys and girls? The goal of this chapter is to address this question by examining the pattern of sex differences across a variety of domains and by relating these sex differences to adult sex differences in the nature of intrasexual competition, parental investment, and so on. Developmental sex differences in the pattern of physical development, infancy, play patterns, social development, and parenting influences are described in the respective sections below. The pattern that emerges across these sections is consistent with the view that many developmental sex differences are indeed related to sexual selection and involve a largely self-directed preparation for engaging in the reproductive activities described in Chapter 4 and Chapter 5. In keeping with the position presented in Chapter 6, this self-directed preparation is manifested in terms of differences in the types of activities in which girls and boys prefer to engage and results in an adaptation of functional systems to local conditions.

Physical development

With any consideration of the relation between sexual selection and developmental sex differences, the first question that must be addressed is evolutionary change in the developmental period. This is so because selection will always favor early maturation and therefore faster reproduction unless delayed reproduction results in considerable reproductive advantages. In Chapter 6, it was argued that one important function of development is to provide the experiences needed to elaborate the basic skeletal knowledge associated with evolved competencies, such that these competencies are tailored to local conditions (Mayr, 1974). In other words, a period of immaturity allows the individual to flesh out the skeletal competencies that appear to comprise evolved cognitive modules and develop the associated behavioral and social skills needed to function in adulthood, including those skills associated with intrasexual competition and parenting.

For any given species, a relatively long developmental period would suggest the need for relatively sophisticated social, cognitive, and behavioral skills for adequate functioning in adulthood, relatively open genetic programs supporting these skills, and on the basis Sawaguchi’s (1997) analysis intense intrasexual competition (see below and the Social modules section of Chapter 6). The first section below addresses the issue of evolutionary change in the length of the developmental period and the second provides discussion of sex differences in the pattern of physical development and physical competencies.

Evolutionary change

On the basis of the strong relation between adult size and age of physical maturity, McHenry (1994b) estimated that the age of maturation for A. afarensis, A. africanus, and H. habilis was very similar to that found in the modern chimpanzee (Pan troglodytes), that is, nine to ten years (see also Conroy & Kuykendall, 1995). The estimated age of maturation for H. erectus, in contrast, was between 12 and 13 years. The roughly three year delay in the maturation of H. erectus, combined with a relatively large brain size (see Figure 3.7), suggest a significant increase, relative to their ancestors, in the complexity of the social, cognitive, and behavioral competencies needed to function in adulthood (see McKinney, 1998, for a general discussion).

The evolutionary trend is extended further in modern humans, where physical maturation is achieved as late as the early 20s (Garai & Scheinfeld, 1968; Tanner, 1990) and brain volume increased 50% relative to H. erectus. Moreover, in comparison to other primates, Leigh (1996) found that human physical development is characterized by a distinct period of relatively slow growth--especially between about 6- to 10-years-of-age--but is unexceptional in other respects (e.g., the sex difference in the growth spurt; Leigh, 1995, 1996). Leigh’s (1996) findings, combined with the overall evolutionary pattern, suggest that the relatively long maturational period of modern humans is largely due to an increase in the length of childhood--the period between birth and puberty--and that childhood is the period in the life span where basic social, behavioral and cognitive competencies are elaborated and adapted to local ecologies.

Comparative studies also reveal a strong relation between the length of the juvenile period, the size of the neocortex, and the complexity of the species social system. These patterns suggest that the acquisition and refinement of social competencies is especially important during childhood (Joffe, 1997) and that the more refined social competencies that can be achieved during a longer as opposed to a shorter developmental period provided a strong selection advantage, especially in H. erectus and H. sapiens. Moreover, the finding that neocortical size in primates is related, in part, to the intensity of intrasexual competition suggests that sexual selection contributed to the evolution of a long developmental period in humans (Sawaguchi, 1997).

Sex differences

Two general issues associated with physical sex differences are reviewed in the two sections below. In the first, the pattern of sex differences in physical development and physical competencies is discussed, while in the second, discussion of the sex difference in vulnerability to illness and developmental difficulties is provided.

Physical development. For many species, including humans, physical dimorphisms are smaller prior to puberty than after puberty. As noted in Chapter 2, the development of secondary sexual characteristics is often costly, especially for males (see below). Such costs include suppression of immune functions and increased risk of predation (e.g., for brightly colored males), among others. The increased cost associated with the development of secondary sexual characteristics will result in natural selection favoring the delay of the emergence of physical dimorphisms until the individual has gained the social, behavioral, cognitive or physical (e.g., weight) competencies needed for successful intrasexual competition. Among other things, this view suggests that the maturational period reflects the portion of the life span during which sex-specific skills--those related to intrasexual competition and reproductive strategies in general (e.g., degree of parental investment)--are refined, as suggested above. Puberty, in turn, results in the physical changes associated with adult reproduction, as well as those physical changes that directly facilitate intrasexual competition or influence mate choice (e.g., an increase in body size for male-male competition or the acquisition of bright plumage for female choice), as described by Darwin more than 125 years ago:

There is ... a striking parallelism between mammals and birds in all their secondary sexual characteristics, namely in their weapons for fighting with rival males, in their ornamental appendages, and in their colours. In both classes, when the male differs from the female, the young of both sexes almost always resemble each other, and in a large majority of cases resemble the adult female. In both classes the male assumes the characters proper to his sex shortly before the age for reproduction (Darwin, 1871, Vol. II p. 297)

Studies of the northern elephant seal (Mirounga angustirostris) and the satin bower bird (Ptilonorhynchus violaceus), described in Chapter 2, provide examples of the relation between sex differences in the maturational period and sexual selection. Male northern elephant seals mature at around 8 years of age, as compared to 3 years of age for females (Clinton & Le Boeuf, 1993). Among other things, the males’ relatively long maturational period allows them to gain the body mass needed to compete for a harem. Male satin bower birds do not achieve the full-adult blue plumage until they are 7 years old, many years after conspecific females have sexually matured (Collis & Borgia, 1992). Prior to this point, young males maintain a green plumage and do not differ significantly in appearance from young and adult females. During this period of immaturity, "young males spend a great deal of time observing older males at their bower, and practice bower building and display behaviors when the owner is absent from the bower site" (Collis & Borgia, 1992, p. 422; see Figure 2.6). Young males also engage in agonistic encounters with their same-age peers, which appears to provide the experience needed for dominance-related encounters in adulthood. Thus, in addition to the development of larger body sizes, delayed maturation also allows for the refinement of those social, behavioral, and presumably cognitive skills associated with intrasexual competition.

Sex differences in human physical development are characterized by the same general features described by Darwin (1871), that is, the most prominent physical dimorphisms emerge during puberty, although there are some earlier differences. Basically, "girls grow up faster than boys: that is, they reach 50% of their adult height at an earlier age..., enter puberty earlier and cease earlier to grow. ... At birth the difference corresponds to 4 to 6 weeks of maturation and at the beginning of puberty to 2 years" (Tanner, 1990, p. 56; see also Garai & Scheinfeld, 1968; Hutt, 1972). The slower maturation of boys appears to heighten their risk for early mortality (see below) but contributes to their adult height. For instance, the slower maturation of boys results in longer legs, relative to overall body height, than would otherwise be the case, which, in turn, contributes to the larger overall size of men relative to women (Tanner, 1990).

The most prominent sex differences to emerge during puberty are a widening of the hips and pelvis in girls and a widening of the width of the shoulders in boys. During puberty--which can last from 1.5 to 5 years--boys also "develop larger hearts as well as larger skeletal muscles, larger lungs, higher systolic blood pressure, lower resting heart-rate, a greater capacity for carrying oxygen in the blood, and a greater power of neutralizing the chemical products of muscular exercise. ... In short, the male becomes more adapted at puberty for the tasks of hunting, fighting and manipulating all sorts of heavy objects" (Tanner, 1990, p. 74). Other sex differences include greater changes in the facial features of boys than girls and greater increases in body fat in girls than boys. The former reflects the emergence of those facial features that members of the opposite sex find attractive, that is, a masculine jaw in men and a youthful appearance (i.e., less change from childhood to adulthood) in women (see Figure 5.1 and Figure 5.3).

These changes in physical structure are accompanied by marked changes in physical competencies. The longer legs of men, relative to overall body height, allows for faster running and running for longer distances than women, on average. There are, in addition, some sex differences in physical competencies prior to puberty, although these differences are not typically as marked as those associated with pubertal changes. During childhood there are small to moderate sex differences, favoring boys, in tasks such as grip strength, jumping distance, and running speeds, with large differences emerging during puberty (Thomas & French, 1985); by 17 years of age, more than 9 out of 10 men outperform the average women in these areas. Infant boys are also more physically active than infant girls--about 3 out of 5 boys are more active than the average girl--and, again, this difference becomes more pronounced with maturation; by adolescence, just over 7 out of 10 boys are more active than the average girl (Eaton & Enns, 1986; Eaton & Yu, 1989). Girls, on the other hand, show greater physical flexibility and fine eye-motor coordination (e.g., as in threading a needle) than boys. The advantage of girls in these areas does not appear to vary with age and is modest in size (about 3 out of 5 girls outperform the average boy) (Thomas & French, 1985); Kimura argued that the advantage of girls and women in this area might be related to manipulating objects "within personal space, or within arm’s reach, such as food and clothing preparation and child care" (Kimura, 1987, p. 145).

By far, the largest documented sex differences in physical competencies are for throwing distance and throwing velocity (Thomas & French, 1985). As early as 4- to 7-years-of-age, more than 9 out of 10 boys show a higher throwing velocity than the average same-age girl, despite the fact that girls are physically more mature at this age. By 12 years of age, there is little overlap in the distribution of the throwing velocities of boys and girls; the very best girls show throwing velocities that are comparable to the throwing velocities of the least skilled boys. The sex difference is somewhat larger for throwing distance. By 2- to 4-years-of-age, more than 9 out of 10 boys can throw farther than the average girl, and by 17 years of age only the very best girls can throw as far as the least skilled boys. At this age, men also have moderate to large advantages in visual acuity, throwing accuracy and in the ability to track and intercept (i.e., block) objects thrown at them; about 3 out of 4 males outperform the average female in these areas (Jardine & Martin, 1983; Kolakowski & Malina, 1974; Law, Pellegrino, & Hunt, 1993; Velle, 1987; Watson & Kimura, 1991; see the Physical modules section of Chapter 8).

The finding of large sex differences in throwing skills as early as 2 years of age indicates that it is very unlikely that these differences result from the differential socialization of boys and girls (see the Parenting section below). In fact, these sex differences are almost certainly related, at least in part, to differences in the structure of the skeletal system that supports throwing. For instance, relative to overall body height, boys have a longer ulna and radius (i.e., forearm), on average, than do girls--a difference that emerges in utero (Gindhart, 1973; Tanner, 1990). For neonates, the radii of 3 out of 4 boys is longer than the radii of the average girl, while for 18 year olds, the radii of more than 19 out of 20 men is longer than the radii of the average woman. There are also sex differences in the timing and pattern of skeletal ossification in the elbow and in the length and robustness of the humerus (i.e., upper arm; Benfer & McKern, 1966; Frisancho & Flegel, 1983; Tanner, 1990).

These differences in skeletal structure and the associated throwing competencies, combined with the large male advantage in arm and upper body strength, indicate strong selection pressures for these physical competencies in men. In fact, these sex differences are consistent with the view that the evolution of male-male competition in humans was influenced by the use of projectile (e.g., spears) and blunt force (e.g., clubs) weapons (Keeley, 1996; see also the Physical modules section of Chapter 8); during agonistic encounters, male chimpanzees often use projectile weapons (e.g., stones) and sticks as clubs and do so much more frequently than female chimpanzees (Goodall, 1986). The finding that men have a higher threshold and greater tolerance for physical pain than do women, on average, is also in keeping with the view that male-male competition is related to human physical dimorphisms, given that success at such competition is almost certainly facilitated by the ability to endure physical pain (Berkley, 1997; Velle, 1987); of course, women can endure considerable pain under some circumstances, such as childbirth.

Nonetheless, it might be argued that these physical sex differences have emerged from a sex difference in the division of labor, such as hunting, rather than direct male-male competition (e.g., Frost, in press; Kolakowski & Malina, 1974). Although the sexual division of labor contributes to the differential mortality of men and women in preindustrial societies and might influence the reproductive variance of men, comparative studies of the relation between physical dimorphisms and male-male competition suggest that the sexual division of labor is not likely to be the primary cause of these physical dimorphisms (see also The evolution of sex differences and the sexual division of labor section of Chapter 3). Recall, for primates and many other species, there is a consistent relation between physical sex differences and the nature of intrasexual competition (see Chapter 3). For monogamous primates--those with little direct male-male competition over access to mating partners--there are little or no differences in the physical size or the pattern of physical development of females and males (Leigh, 1995). For nonmonogamous primates--those characterized by direct male-male competition over access to mating partners--males are consistently larger than females, and this difference in physical size is consistently related, across species, to the intensity of physical male-male competition and not to the foraging strategy of the species (Clutton-Brock et al., 1977; Mitani et al., 1996; Plavcan & van Schaik, 1997a).

Across these nonmonogamous species, there is also a characteristic pattern of female and male growth. In those species where it occurs, the female pubertal growth spurt begins at an earlier age, reaches it’s peak more quickly, and lasts for a shorter period of time than is found in male conspecifics (Leigh, 1996). The pattern of human sex differences in physical development fits this general pattern (Leigh, 1996; Tanner, 1990; Weisfeld & Berger, 1983), a pattern that is consistent with the position that physical sex differences in humans have evolved largely by means of sexual selection (Tanner, 1992). Of course, some physical dimorphisms, such as the wider pelvic region in women, have likely emerged through natural selection. Nonetheless, once this difference emerged--creating the WHR (recall, waist-to-hip ratio) that men find attractive--it appears to have influenced mate choice and is thus also influenced by sexual selection (see Male choice section of Chapter 5).

In other cases, it is likely that some physical sex differences are largely due to natural selection, as noted in Chapter 3 (see The evolution of sex differences and the sexual division of labor section). Female and male chimpanzees, for instance, have different foraging strategies, which, in turn, can result in the evolution of sex differences independent of sexual selection. Goodall notes that "females not only crack nuts more frequently than males, (they also) show more dexterity in their manipulation of hammer stones (Goodall, 1986, p. 564); whether the advantage of girls and women in fine motor dexterity is related to a sex difference in the foraging strategies of our ancestors is not known.

Vulnerability. The delayed maturation of boys relative to girls and the general tendency for male hormones to suppress immune functions appears to put boys and men at risk for a wider array of illnesses and for premature death than same-age girls and women (Davis & Emory, 1995; McEwen et al., 1997; Tanner, 1990). In addition, boys and men have higher basal metabolic rates and higher activity levels than girls and women, on average, which, in turn, results in higher caloric requirements for boys than for girls for normal development to occur (Aiello, 1992). The net result of these differences appears to be a greater sensitivity of developing boys than developing girls to poor environmental conditions, such as poor nutrition or inadequate health care (see Stinson, 1985, for related discussion). In fact, a greater sensitivity of males than females to environmental conditions is often found in species where physical sex differences appear to have been shaped through sexual selection (Clutton-Brock et al., 1985; McDonald, 1993; Müller, 1994a; Potti & Merino, 1996; Rowe & Houle, 1996). As described in Chapter 2, the greater environmental sensitivity of males appears to reflect, at least in some cases, the evolution of condition-dependent secondary sexual characteristics, such that these characteristics are only fully developed in the most healthy males (see the Sex hormones and parasites section).

Whether a similar process is operating in humans is not currently known, but is a distinct possibility (Thornhill & Müller, 1997). For instance, there is some evidence that boys and men respond to social and other stressors differently than girls and women. Davis and Emory (1995), as an example, found that newborn boys showed an increase in cortisol levels following exposure to mild but prolonged stressors, but newborn girls showed no such increase, although they did show a larger increase in heart rate than did boys. It is likely that any such sex differences in the nature of stressor-related physiological responses varies across social and other contexts (Flinn et al., 1996; Sapolsky, 1993). Nonetheless, an overall sex difference in cortisol responses, among other factors, would make boys and men more susceptible to growth disorders and other diseases--through suppression of immune functions and growth hormones--than girls and women, and might contribute to the higher mortality rates in boys and men relative to same-age girls and women (McEwen et al., 1997; Tanner, 1990).

As an example, Martorell and his colleagues followed the physical, intellectual, and educational development of 249 rural Guatemalans from early childhood up to 26 years of age (Martorell, Rivera, Kaplowitz, & Pollitt, 1992). Growth failure at age 3 years--due to an inadequate diet and high rates of infection--was related to stunted physical and educational development for both men and women in adulthood. Men, however, were more severely affected, as a group, than women on a number of dimensions, including the proportion of lean muscle mass, years of education, and literacy scores. For instance, of those individuals showing early growth failure, nearly 1 out of 3 men but only 1 out of 10 women exhibited reading difficulties in adulthood. In contrast, for individuals showing normal growth at age 3 years, more men than women passed the literacy test.

No doubt there are other physiological and social mechanisms involved in the apparent sex difference in sensitivity and reactivity to environmental conditions (see, e.g., Gualtieri & Hicks, 1985). Stinson (1985), for instance, suggested that in many preindustrial cultures, parents might compensate for the greater sensitivity of boys through a preferential treatment of boys, such as providing them with more calories relative to girls. Whatever the mechanisms, these sex differences also appear to result in greater variability across many characteristics within groups of boys and men relative to groups of girls and women (Hedges & Nowell, 1995), as would be expected for condition-dependent traits (implications are discussed in Chapter 9). In other words, poor environmental conditions appear to more adversely affect larger numbers of boys than girls, while more boys than girls might benefit from more optimal rearing conditions. However, it is not known whether the greater variability within groups of boys and men extends to most domains (e.g., physical and cognitive development) or whether girls and women are more susceptible than boys and men in some areas. The finding that female-female competition and male choice have operated during the course of human evolution suggests that sex-specific sensitivities to poor environments are possible, and in fact expected (Thornhill & Müller, 1997; see Social modules section of Chapter 8).

Infancy

The study of sex differences in infancy is inherently more difficult than the study of sex differences in older children, because the behavior of infants is more variable than the behavior of older children (which would obscure many sex differences) and because there are fewer methods that can be used to study infants than older children (Maccoby & Jacklin, 1974). As a result, the pattern of sex differences in infants has not been as systematically assessed, at least in recent years, as the pattern in older children. Given these limitations, many of the sex differences described in this section need to be interpreted with some caution.

With the study of developmental sex differences, it is often assumed that biologically-influenced sex differences will manifest themselves early in development, whereas sex differences that are largely influenced by cultural factors, such as gender roles, will manifest themselves later in development and as a result of the cumulative effects of socializing agents (e.g., parents’ stereotyped interactions with girls and boys; e.g., Adamson & McArthur, 1995; Serbin, Powlishta, & Gulko, 1993; Whiting & Edwards, 1973). As noted earlier, this assumption is incorrect. In species where sexual selection has resulted in the evolution of sex differences, such differences are often not manifested until puberty (Darwin, 1871). In this view, infant boys and infant girls are expected to be more similar than different, and this appears to be the case (Hsu, Soong, Stigler, Hong, Liang, 1981; Maccoby & Jacklin, 1974; Rothbart, 1989).

In theory, those sex differences that are found in infancy should reflect the seeds of the later described sex differences in play and social development, which, in turn, should provide the experience and practice needed to acquire the behavioral, social, and cognitive competencies associated with the sex difference in reproductive activities, such as differences in the nature of intrasexual competition (see Chapter 5). There are indeed several patterns in the infancy literature that suggest that the skeletal structure of the later described sex differences in social and play activities are evident in the in first year or two of life and in some cases in the first few days of life (Block, 1976; Cohen & Gelber, 1975; Davis & Emory, 1995; Fagan, 1972; Gunnar & Donahue, 1980; Gunnar & Stone, 1984; Haviland & Malatesta, 1981; Kujawski & Bower, 1993; Rosen, Adamson, & Bakeman, 1992; Simmer, 1971; Zahn-Waxler, Radke-Yarrow, Wagner, & Chapman, 1992; Zahn-Waxler, Robinson, & Emde, 1992).

One of the more consistent of these differences is the general orientation of boys and girls toward other people (Freedman, 1974; Garai & Scheinfeld, 1968; Haviland & Malatesta, 1981; McGuinness & Pribram, 1979). For infants, the degree of orientation toward other people has been measured in terms of the duration of eye contact, empathic responses to the distress of other people, recognition of faces, and time spent looking at faces, among other behaviors. In a review of sex differences in nonverbal behavior, Haviland and Malatesta noted that "there is no doubt that girls and women establish and maintain eye contact more than boys and men. The earliest age for which this is reported is one day" (Haviland & Malatesta, 1981, p. 189). In addition, boys and men gaze-avert much more frequently than girls and women, a sex difference that has been found as early as 6 months of age. In the first few days of life, it appears that girls orient to faces and voices more frequently, on average, than do boys (Haviland & Malatesta, 1981) and, by least at 6 months of age, girls might have a better memory for faces and might be more skilled than boys in discriminating two similar faces (e.g., Fagan, 1972); these latter differences, however, are not found as consistently in infants as they are in older individuals (Hall, 1984; Haviland & Malatesta, 1981; Maccoby & Jacklin, 1974; McGuinness & Symonds, 1977).

A number of other studies suggest that infant girls react with greater empathy to the distress of other people than do infant boys (Hoffman, 1977). Simner (1971), for instance, found that infant girls cried longer than infant boys when exposed to the cry of another infant, but no sex difference in reflexive crying was found when the infants were exposed to artificial noise of the same intensity as infant crying. More recently, Zahn-Waxler and her colleagues found a sex difference in the responses of 12- to 20-month-olds to the distress of other people (Zahn-Waxler, Radke-Yarrow et al., 1992; Zahn-Waxler, Robinson, & Emde, 1992). In both studies, girls responded to the distress of other people with greater empathic concern than boys; defined as "emotional arousal that appears to reflect sympathetic concern for the victim ... manifested in facial or vocal expressions (e.g., sad looks, sympathetic statements ...) or gestures" (Zahn-Waxler, Radke-Yarrow et al., 1992, p. 129). In one of the studies, girls also responded to the distressed individual with more prosocial behavior (e.g., comforting) and engaged in more information seeking behaviors (e.g., "what’s wrong") than did boys. Boys, in contrast, were unresponsive or affectively indifferent to the victim’s distress more frequently than were girls.

However, these differences were only found for distress that was witnessed and not caused by the child. In other words, girls did not show more empathy than boys when they caused the distress in another individual (e.g., in the course of some conflict), although boys behaved more aggressively (e.g., hit) than did girls in these situations. Moreover, the magnitude of the sex differences in empathic concern and indifference were modest. At 20 months of age, about 3 out of 5 girls responded with greater empathic concern to the distress of another person than did the average boy, whereas 2 out of 3 boys showed more affective indifference than did the average girl. Finally, both empathic concern and affective indifference were found to have moderate genetic influences for both 14- and 20- month-olds, suggesting the these social behaviors are influenced by a mix of biological, social, and contextual factors; between 29 and 35% of the individual differences in these social behaviors appear to be heritable at these ages (Zahn-Waxler, Robinson, & Emde, 1992).

The results of several studies of the nature and quality of social interactions between parents and infants are also consistent with the view that infant girls are more responsive, and perhaps more sensitive, to social cues than are infant boys (Freedman, 1974; Gunnar & Donahue, 1980; Gunnar & Stone, 1984; Rosen et al., 1992). Gunner and Stone found that in ambiguous situations--such as in the presence of an unfamiliar and potentially threatening toy (i.e., a monkey)--12 month olds of both sexes would approach the unfamiliar object if their mother signaled positive emotions (e.g., smiling) in reference to this object. Rosen et al. found the same pattern, as well as a sex difference when mothers signaled fear in response to the unfamiliar object. In this situation, girls tended to withdraw from the object, whereas boys tended to approach the object. Independent coders rated the intensity of the mothers’ fear signal and judged that these signals were more intense when directed toward boys than when directed toward girls, suggesting that the difference in the reaction of boys and girls was not likely to be due to the behavior of their mothers. Rather, the tendency of boys to approach unfamiliar objects more frequently than girls, on average, might be one early manifestation of the sex difference in risk taking --mentioned later (Social development section)--and mothers’ more intense signals to boys might be a reflection of their prior experiences with unresponsive sons (see Adamson & McArthur, 1995, for an alternative explanation).

In a related study with 6- to 12-month-olds, Gunnar and Donahue (1980) found that mothers were just as likely to attempt to initiate social interactions with their sons as with their daughters, but daughters were much more responsive than sons to their mother’s verbal requests; Whiting and Edwards (1988) found the same pattern with older children, across cultures. For instance, for 12 month olds, girls responded to 52% of their mother’s verbal requests to engage in some form of social interaction, as compared to a 25% response rate in same-age boys. In contrast, boys and girls were equally responsive to their mother, when she used a toy to attempt to initiate a social interaction. In addition, girls initiated about 30% more social interactions with their mother than did boys, on average. Gunnar and Donahue’s results suggest that the occasional finding that mothers sometimes interact more with their daughters than with their sons (e.g., Klein & Durfee, 1978) might stem from a sex difference, favoring girls, in social responsiveness and the degree to which social interactions are initiated, rather than a maternal preference for girls per se (Freedman, 1974).

While girls appear to orient more to other people and show greater sensitivity to some social cues than do boys, boys appear to orient more to physical information and show greater sensitivity to certain physical cues, such as geometric shape, than do girls (Cohen & Gelber, 1975; Freedman, 1974; McGuinness & Pribram, 1979). When "differences are found, males from 4-6 months onwards respond preferentially to blinking lights, geometric patterns, colored photographs of objects and three-dimensional objects" (McGuinness & Pribram, 1979, p. 19). A similar conclusion was drawn by Cohen and Gelber, based on a review of research on infants’ visual memory. On the basis of this review, they argued that "males and females are processing and storing different kinds of information about repeatedly presented (visual) stimuli. Males appear to be more likely to store information about the various components of a repeatedly presented stimulus, for example, its form and color. ... (while) females, unlike males, are more likely to store information about the consequences of orienting" (Cohen & Gelber, 1975, p. 382). In short, it appears that by about 4 months of age, boys selectively attend to the physical properties of objects, such as shape, while girls selectively attend to the consequences of orienting to objects in their environment, rather than to the objects themselves (except when these objects are people); consequences refer, for instance, to how the objects might be related to the behavior of other people (e.g., whether their behavior changes when the objects are present).

In all, the pattern of sex differences in infancy suggests that girls and boys orient, process, and react to certain social and physical cues differently. These sex differences, in turn, appear to reflect early differences in the skeletal competencies underlying the sex differences in certain social and physical cognitive modules, as described in Chapter 8, as well as the precursors to some of the differences in the play styles and social development of boys and girls, as described below. In other words, infant boys and girls appear to be differentially biased in the ways in which they process information associated with the individual-level social modules and certain physical modules, described in Chapter 6, and their attentional and processing biases, along with the activity differences described below, appear to be related to many of the cognitive sex differences described in Chapter 8 (McGuinness & Pribram, 1979).

Play

In order to illustrate the apparent functions of play, the first section below focuses on nonhuman species. The second section focuses on human sex differences in play-related activities.

The functions of play

Play in one form or another is found in most mammalian species and some species of bird, but not generally in fishes, reptiles or insects (Fagen, 1981). Across mammalian species, play is typically categorized as social, locomotor, or object oriented (Aldis, 1975; Fagen, 1981) and it is often assumed that play provides delayed benefits to the individual (see Archer, 1992, and Fagen, 1995, for discussion). "The consensus that emerges from the scores of definitions is that play incorporates many physical components of adult behavior patterns, such as those used in aggression, but without their immediate functional consequences" (Walters, 1987, p. 360). The delayed benefits of play are in terms of practicing those behaviors that are important for survival and reproduction in adulthood. In addition, Barber (1991) hypothesized that play resulted in a number of more immediate benefits, although this interpretation is debated (Archer, 1992). For instance, play increases body temperature, which, in turn, can enhance resistance to infections.

In mammals, play often occurs in social contexts and very frequently involves rough-and-tumble play and chasing (Archer, 1992; Panksepp, Siviy & Normansell, 1984; Walters, 1987). Rough-and-tumble play or play fighting appears to be especially common in species where social conflict in adulthood is resolved through physical contests (e.g., elephant seals) and, given this, it has been proposed that these play activities provide the practice needed to develop social-competitive skills (Pellis, Field, Smith, & Pellis, 1997; Smith, 1982). Moreover, across those mammalian species where it has been studied, rough-and-tumble play is generally more common and more vigorous in males than in females, and appears to occur relatively more frequently and more vigorously with males of polygynous species--those characterized by physical male-male competition--than with males of monogamous species (Aldis, 1975; Smith, 1982). One apparent exception is for carnivores, where both males and females hunt, defend territory, and engage in physical competition with conspecifics. In these species, it appears that both juvenile males and females regularly engage in rough-and-tumble play (Aldis, 1975; Fagen, 1981).

Extensive experimental studies of rough-and-tumble play have been conducted with the laboratory rat and the rhesus macaque (Macaca mulatta; e.g., Meaney & Stewart, 1981; Panksepp, 1981; Panksepp et al., 1984; Pellis et al., 1997; Wallen, 1996). These studies indicate that the expression of rough-and-tumble play is influenced by a mix of hormonal, social-rearing, and contextual factors. For the polygynous rhesus macaque, males consistently show more rough-and-tumble play than females, although the magnitude of this sex difference varies somewhat with rearing environment (Wallen, 1996). In comparison to males reared in typical male-female social groups, males reared only with other males show relatively more rough-and-tumble play, whereas males reared in isolation show relatively less rough-and-tumble play when introduced to their peers. Neonatal castration and other manipulations that suppress androgens after the male is born have little effect on the frequency of rough-and-tumble play. For females, prolonged prenatal exposure to androgens significantly increases the frequency of rough-and-tumble play, regardless of rearing environment. On the basis of these results, it appears that, at least for the rhesus macaque, the frequency of rough-and-tumble is primarily influenced by prenatal exposure to androgens (Wallen, 1996), although prenatal exposure to estrogens, as well as social and contextual factors also appear to be important for the expression of this form of play in some other species (Panksepp et al., 1984; Pellis et al., 1997).

It appears that chasing and associated behaviors such as stalking, rushing, and pawing are related to the development of prey capture or predator avoidance skills (Smith, 1982). Sex differences in these behaviors have not been as systematically studied as the sex difference in rough-and-tumble play, although there is some indication that, at least for some species, this form of play might be relatively more common in females than in males (Fagen, 1981).

Another form of social play that is more common in females than in males is alloparenting, or play parenting, although this form of play can occur in both sexes (Nicolson, 1987; Pryce, 1992, 1993, 1995). In primates, play parenting is most frequently observed in young females who have not yet had their first offspring. In a number of these species early play parenting (e.g., caring for siblings) is associated with higher survival rates of the first-born, and sometimes later-born, offspring (Nicolson, 1987). For instance, across five primate species it was found that first-born survival rates were 2 to more than 4 times higher for mothers with early experience with infant care--obtained through play parenting--than for mothers with no such experience (Pryce, 1993). As noted in Chapter 4 (Hormonal influences section), maternal care is also influenced by the hormonal changes that occur during pregnancy and the birthing process, such that a combination of early play parenting and these hormonal changes contribute to the adequacy of female care-giving in many primate species (Lee & Bowman, 1995; Pryce, 1995).

In contrast to social play, which involves dyads and sometimes larger groups, locomotor play is typically a solitary activity and often involves an exaggeration of those movements involved in common functional activities, such as predator avoidance (Fagen, 1981; Smith, 1982). Byers and Walker (1995) have recently argued that early locomotor play results in long term changes in the synaptic organization of the cerebellum--which is involved in the coordination of complex motor movements--and in the distribution of fast and slow muscle fibers. In this view, locomotor play results in neural and muscular changes that support complex functional activities in adulthood. As an example, with the Siberian ibex (Capra sibirica), a species of mountain goat, "social play is equally likely on flat and sloped terrains, but solitary locomotor play was much more frequent on sloped terrain, despite the greater dangers of falling" (Smith, 1982, p. 142). From the perspective introduced by Byers and Walker, the ibex kids play on sloped terrain would result in neural and muscular adaptations that would facilitate later locomotion in mountainous terrain.

Object play occurs relatively infrequently in primates, except for chimpanzees and humans, and involves the nonfunctional manipulation of objects, such as throwing them, banging them, and so on (Byrne, 1995; Fagen, 1981). The function of this type of play appears to be to learn about the different ways in which various objects in the environment can be used, which, in turn, appears to facilitate later tool use and perhaps later problem-solving skills as related to tool use (Byrne, 1995; Smith, 1982). Goodall provides an example with the chimpanzee:

Youngsters utilize many objects during solitary play, demonstrating the extent to which infants make use of the objects in their environment--sometimes in a very inventive way. Fruit-laden twigs, strips of skin and hair from an old kill, or highly prized pieces of cloth may be draped over the shoulders or carried along in the neck or groin pocket (that is, tucked between the neck and shoulder or thigh and belly); stones or small fruits may be hit about on the ground, from one hand to the other, or thrown short distances into the air and retrieved. (Goodall, 1986, p. 559).

At times, these play episodes result in important discoveries. For Gombe chimpanzees, sticks are often used to fish for termites. On one occasion, "a juvenile male, Wilkie, poked his stick into an ant nest, causing a stream of fierce black ants to emerge. Wilkie avoided them. His mother, who had been watching, immediately approached and ate the ants (Goodall, 1986, p. 563). Chimpanzees who lack this type of object-oriented play in their childhood are "poor at problem-solving later in life with tasks that involve objects" (Byrne, 1995, p. 86). Stated otherwise, these individuals do not use common chimpanzee tools, such as the stick used by Wilkie, to solve everyday problems, such as getting food.

In sum, play provides a pattern of early experience that results in the fine-tuning of a number of social and physical competencies associated with survival and reproduction in adulthood. For instance, these experiences appear to result in long-term neural and muscular adaptations associated with the facile execution of essential behavioral competencies, such as predator avoidance. In some species, it also appears that early play--object-oriented play in particular--results in learning about the potential usefulness of objects in the local ecology, which, in turn, appears to be related to later tool use. It also seems likely that social play influences the development of social-cognitive competencies, such as those associated with theory of mind, at least to the extent that the particular species, such as the chimpanzee, develops a theory of mind.

Sex differences

Sex differences in play activities are a universal feature of children’s behavior and the study of these differences has generally focused on three relatively independent components; gender schemes--knowledge about the sex-typed activities of girls and women and boys and men; child-initiated activities, such as rough-and-tumble play; and, the formation of same-sex play groups (Aldis, 1975; Boulton & Smith, 1992; Brown, 1991; Lever, 1978; Maccoby, 1988; Maccoby & Jacklin, 1974; Pitcher & Schultz, 1983; Sandberg & Meyer-Bahlburg, 1994; Weisfeld & Berger, 1983; Whiting & Edwards, 1973, 1988). Children’s gender schemes are only very weakly related to the actual play and social activities of girls and boys (Serbin et al., 1993; Turner & Gervai, 1995) and girls and boys play in same-sex groups, regardless of the degree to which their activities are sex-typed (Maccoby, 1988). Boys who engage in high levels of rough-and-tumble play, for instance, spend most of their play time with other boys, as do boys who do not engage in high levels of rough-and-tumble play.

The focus of this section is primarily on child-initiated play activities; a discussion of the social styles of girls and boys--including the formation of same-sex play groups--is presented in the section below and discussion of parental influences, or a lack thereof, on children’s social development is presented in the last section of the chapter. As is the case with many other mammals, the pattern of human sex differences in child-initiated play activities appears to be influenced by a mix of hormonal, social, and contextual factors and involves at least six different forms of play--rough-and-tumble, locomotor, exploratory, parenting, object-oriented, and fantasy (Berenbaum & Hines, 1992; Berenbaum & Snyder, 1995; Boulton, 1996; Collaer & Hines, 1995; DiPietro, 1981; Hines & Kaufman, 1994; Loy & Hesketh, 1995; Maccoby, 1988; Serbin, Connor, & Citron, 1981). Sex differences in the pattern and the frequency of engagement in each of these forms of play are described in the respective sections below.

Rough-and-tumble play. One of the more consistently found child-initiated sex differences, favoring boys, is in the frequency and the nature of rough-and-tumble play, although sex differences are not always found for all components of this form of play and the nature of rough-and-tumble play can vary from one culture to the next (Boulton, 1996; Whiting & Edwards, 1988). As noted earlier, females of many primates species engage in rough-and-tumble play but do so less frequently and less vigorously than do conspecific males (Aldis, 1975; Smith, 1982). Similarly, female-on-female aggression frequently occurs in these primates--typically over access to food--but results in severe injury and death much less frequently than does male-on-male aggression (see Female-female competition and male choice section of Chapter 3). The same pattern is found in humans--rough-and-tumble play is not an exclusively male activity but it occurs much more frequently and more vigorously with boys than with girls and parallels the sex difference in the intensity of physical intrasexual competition (see Chapter 5).

Across contexts, sex differences in rough-and-tumble play are most evident with groups of three or more boys and in the absence of adult supervision (Maccoby, 1988; Pellegrini, 1995); adults often discourage this type of play. In situations where play activities are not monitored by adults and in contexts where their activities are not otherwise restricted (e.g., a play area that is too small), groups of boys engage in various forms of rough-and-tumble play--including playful physical assaults and wrestling--three to six times more frequently than do groups of same-age girls (DiPietro, 1981; Maccoby, 1988). As an example, in an analysis of the activities of triads of same-sex four year olds who did not know that they were being observed, DiPietro found that boys engaged in playful physical assaults--including hitting, pushing, and tripping--4 1/2 times more frequently than did girls; biting is not typically a feature of this form of play (see Sexual dimorphisms section of Chapter 3 for related discussion). Research conducted in the United States indicates that the sex difference in playful physical assaults and other forms of rough-and-tumble play begin to emerge by about three-years-of-age (Maccoby, 1988) and the same general pattern is found in other industrial, as well as many preindustrial societies, although the magnitude of the sex difference in this form of play varies across these cultures (Eibl-Eibesfeldt, 1989; Whiting & Edwards, 1973, 1988).

The nature of boys’ rough-and-tumble play also varies somewhat across cultures. In societies characterized by relatively high levels of adult male-on-male physical aggression, the play fighting of boys tends to be rougher than the play fighting found in societies with relatively less male-on-male physical aggression. For instance, intergroup aggression is a pervasive feature of Yanomam??ont> society (Chagnon, 1988; Male-male competition section of Chapter 5) and young Yanomam??ont> boys often play fight with clubs or bows and arrows, practices that are typically discouraged in settings where male-on-male physical aggression occurs infrequently (Chagnon, personal communication, July, 1997). Loy and Hesketh (1995) provide a number of other examples, in their analysis of the war-related games of the Native American warrior societies of the central plains. "Evidence suggests that all Plains Indian tribes were, to greater or lesser degrees, involved in a wide range of warring activities ... confined primarily to small war parties, raids, forays; that is, conflicts which were brief and usually indecisive" (Loy & Hesketh, 1995, p. 80). For the Sioux, and many other Indian tribes, the activities of young boys were designed to encourage aggression--both one-on-one and coalition-based--and physical endurance (Culin, 1902/1903; Hassrick, 1964).

Games for the Sioux frequently were contrived life-situations in miniature. They ran the gamut from the more complex diversion of the Moccasin Game enjoyed by adults to the raucously rough Swing-kicking game played by young boys. ... The Swing-Kicking Game took first place as a rugged conditioner, and there was no pretense at horseplay. Here two rows of boys faced each other, each holding his robe over his left arm. The game was begun only after the formality of the stock question, "Shall we grab them by the hair and knee them in the face until they bleed?" Then using their robes as a shield, they all kicked at their opponents, endeavoring to upset them. There seems to have been no rules, for the boys attacked whoever was closest, often two boys jumping one. Kicking from behind the knees was a good way of throwing an opponent, and once down he was grabbed at the temples with both hands and kneed in the face. Once released, the bloody victims would fight on, kicking and kneeing and bleeding until they could fight no longer. ... As Iron Shell explained, "Some boys got badly hurt, but afterwards we would talk and laugh about it. Very seldom did any fellows get angry. ... Throw at Each Other with Mud was a slightly more gentle spring pastime where teams of boys attacked (each other) with mud balls which they threw from the tips of short springy sticks. Each boy carried several sticks and an arsenal of mud as he advanced. "It certainly hurt when you got hit, so you must duck and throw as you attack." Sometimes live coals were embedded in the mud balls to add zest to the game. (Hassrick, 1964, pp. 127-130).

Similar types of play fighting are evident even in cultures where most men do not engage in intergroup aggression or physical one-on-one competition. For instance, in several large-scale studies of the play activities of boys and girls in the United States, consistent sex differences, favoring boys, in one-on-one and coalition-based play fighting and intrasexual competition have been found (Lever, 1978; Sandberg & Meyer-Bahlburg, 1994; Willingham & Cole, 1997); see Sherif et al. (1961) for an engaging illustration of social-competitive play in boys. These sex differences have changed little from one decade to the next and are evident whether observations, questionnaires, interviews, or diaries of leisure activities are used to assess play behavior. As an example, Lever asked 181 10- and 11-year-old children to record their after-school activities during the course of one week, resulting in 895 cases of social play. An analysis of sex differences in social play indicated that boys participated in group-level competitive activities, such as football and basketball, three times as frequently as did girls. Observation of the spontaneous (i.e., not organized by adults) play activities of these same children confirmed the pattern noted in the diaries and indicated that boys’ social play involves larger groups, on average, than does girls’ social play and greater role differentiation within these groups.

More often, boys compete as members of teams and must simultaneously coordinate their actions with those of their teammates while taking into account the action and strategies of their opponents. Boys interviewed expressed finding gratification in acting as a representative of a collectivity; the approval or disapproval of one’s teammates accentuates the importance of contributing to a group victory. (Lever, 1978, p. 478).

A more recent questionnaire-based assessment of the play activities of 355 6- to 10-year-old girls and 333 same-age boys revealed the same pattern (Sandberg & Meyer-Bahlburg, 1994). For 6 year olds, 44% of the boys regularly played football, compared to 2% of the girls. For 10 year olds, 70% of the boys regularly played football, compared to 15% of the girls. The magnitude of the sex difference was smaller, though still substantial, for basketball; 85 and 86% of the 6- and 10-year-old boys, respectively, played regularly, as compared to 25 and 36% of the same-age girls. These differences, along with many other sex differences, were essentially the same as those found three decades earlier by Sutton-Smith and his colleagues (Sutton-Smith, Rosenberg, & Morgan, 1963).

The sex difference in one-on-one and group-level competitive play is related, at least in part, to prenatal exposure to androgens. Indeed, the "clearest evidence for hormonal influences on human behavioral development comes from studies of childhood play. Elevated androgen in genetic females ... is associated with masculinized and defeminized play" (Collaer & Hines, 1995, p. 92). For example, Berenbaum and Snyder (1995) administered the same questionnaire used by Sandberg and Meyer-Bahlburg (1994) to boys and girls who were prenatally exposed to excess levels of androgens (i.e., CAH), unaffected children, and their parents. Based on self and parental report, girls affected by CAH engaged in more athletic competition than their unaffected peers--between 7 and 8 of the girls affected by CAH engaged in athletic competition more frequently than the average unaffected girl. This differences, however, was not as large as the difference between unaffected boys and unaffected girls; more than 9 out of 10 unaffected boys reported engaging in athletic competition more frequently than the average unaffected girl.

In an observational study, Hines and Kaufman (1994) found that girls affected by CAH engaged in more playful physical assaults, physical assaults on objects, wrestling, and rough-and-tumble play in general than did unaffected girls, but none of these differences were statistically significant. The lack of significance was possibly due to the testing arrangements used in this study. Here, most of the girls affected by CAH were observed as they played with one unaffected girl, a situation (two girls), as noted above, that does not typically facilitate rough-and-tumble play (Maccoby, 1988).

In all, the cross-cultural pattern of sex differences in rough-and-tumble play and group-level competition, combined with the finding that at least some components of this type of play are influenced by prenatal exposure to androgens, is consistent with the view that these activities represent fundamental differences in the way in which the typical boy and typical girl plays. Moreover, this form of play appears to serve the function of developing the component competencies associated with intrasexual competition in adulthood. Boulton (1996), for instance, found that the component behaviors associated with rough-and-tumble play, such as hitting, were the same as those involved in actual one-on-one physical fights. Play fighting also provides the practice of those component skills associated with coalition-based intergroup warfare.

As an example, consider the game of baseball played in modern America and the game of Throw at Each Other with Mud played by Sioux Indians 200 hundred years ago; similar games were common throughout Native American tribes (Culin, 1902/1903), as shown in Figure 7.1, and are found in many other parts of the world as well (Eibl-Eibesfeldt, 1989). Both of these games, as well as most if not all other forms of athletic competition, require many of the same physical, social, and cognitive competencies involved in coalition-based warfare (Geary, 1995b). Both baseball and Throw at Each Other with Mud require the formation of in-groups and out-groups, the strategic coordination of the activities of in-group members as related to competition with the out-group, the throwing of projectiles (baseballs and mudballs) at specific targets, and the tracking and reacting to the movement of these projectiles (to catch the baseball or to avoid being hit by the mudball).

The latter activities are in keeping with the earlier described sex differences, favoring boys and men, in throwing distance, velocity, and accuracy, as well as skill at intercepting thrown objects (Thomas & French, 1985; Watson & Kimura, 1991). These component skills, along with the male advantage in upper body strength and length of the forearm (Tanner, 1990), are the same competencies involved in the use of--and in the avoidance of being hit by--projectile weapons. Or stated differently, the rough-and-tumble play and fighting games of boys appear to provide the activities needed to fine tune the competencies associated with physical one-on-one and coalition-based male-male competition, and, given this, they have likely evolved by means of sexual selection (Darwin, 1871).

Moreover, the finding that the nature of play fighting is influenced by cultural factors (Whiting & Edwards, 1988), in particular the intensity and frequency of intergroup warfare, supports the position that the function of childhood is to adapt evolved traits to local conditions (Loy & Hesketh, 1995). In many cases, the form of play fighting--such as club fights or spear throwing--mirrors the actual form of male-on-male aggression in adulthood and thus provides direct practice of the associated component skills (Eibl-Eibesfeldt, 1989). In addition to providing the practice of those social, cognitive, and behavioral competencies associated with primitive warfare, it is also likely that the intensity of the play fighting results in changes in the sensitivity of the associated emotional systems (MacDonald, 1988). For instance, the physical pain that was associated with playing the Swing-Kicking Game almost certainly resulted in more aggressive boys than would otherwise be the case, as well as boys who were less sensitive to the distress of other people and better able to suppress their fear and their reactions to physical pain (see Parenting section below).

Locomotor and exploratory play. In addition to rough-and-tumble play, boys also engage in gross locomotor play more frequently than girls, a sex difference that has been found in industrial as well as many preindustrial societies (Eibl-Eibesfeldt, 1989; Munroe & Munroe, 1971; Whiting & Edwards, 1988). The sex difference in locomotor play is related in part to the different types of activities in which boys and girls engage. As noted above, boys engage in group-level competitive play, such as football or soccer, about three times as frequently as girls and engagement in this type of play results in a sex difference in gross locomotor activities, in particular, running (Eaton & Enns, 1986; Lever, 1978). Boys also engage in competitive running games, such as relay races, more frequently than girls, but the sex difference in these activities is not nearly as large as the sex difference in the frequency of engagement in group-level competitive play (Sandberg & Meyer-Bahlburg, 1994). Nonetheless, on the basis of Byers and Walker’s (1995) model, it appears that the sex difference in gross locomotor activities will create neuromuscular changes that will result in men being better adapted for running and traveling long distances on foot than women.

In addition to any such neuromuscular adaptations, the sex difference in gross locomotor activities results in larger play ranges for boys than for girls. Within these ranges, boys not only engage in these locomotor activities more frequently than girls they also explore and manipulate (e.g., build things, such as forts) the environment much more frequently than girls (Matthews, 1992; Munroe & Munroe, 1971). The sex difference in the size of the play range and the associated exploratory play is potentially important because these factors appear to be related to the development of certain spatial competencies, and, given this, might be one factor contributing to the sex differences in the elaboration of certain physical modules, described in Chapter 8 (Matthews, 1992). There are two associated issues related to developmental sex differences: The extent to which the play ranges of boys and girls differ and any associated sex difference in the ability to mentally represent these ranges.

The sex difference in the area of the play range appears to be related, at least in part, to greater parental restrictions on the ranges of girls than boys. However, a sex difference in the size of the play range is found in the absence of any such restrictions and has been found in both industrial and some preindustrial societies (Matthews, 1992; Munroe & Munroe, 1971). In studies of the exploratory play of children in suburban England, for instance, Matthews (1992) found that younger children--both boys and girls--tended to play within close proximity of one or both of their parents (see also Whiting & Edwards, 1988). Older children, in contrast, were more likely to play away from home, and, at this point, a sex difference in the area comprising the play range emerges. For 8- to 11-year-olds, the unrestricted play range of boys was found to cover from 1 1/2 to nearly 3 times the unrestricted play-range area of same-age girls. Whiting and Edwards (1988) report a similar sex difference for older children in three separate groups in Kenya, as well as for children in Peru, and Guatemala. Nonetheless, the age at which this sex difference emerges appears to vary with the ecology of the group. For the Ache, who live in dense tropical rain forest, the size of the range of boys and girls does not typically diverge until adolescence (Hill & Hurtado, 1996).

Research on the exploratory behavior of animals suggests that, among other things, this activity allows for the development of cognitive representations of the local ecology, such as a mental map of the relative position of major landmarks. These cognitive representations, in turn, appear to support navigation within this ecology (Poucet, 1993; see Physical modules section of Chapter 6). Similar conclusions have been drawn about the relation between the size of children’s play ranges and their spatial representations of local environments (Matthews, 1992). In a review of the relation between childhood experiences and cognitive abilities, Matthews concluded that exploration of the physical environment improved children’s ability to mentally represent this environment, as assessed, for instance, by the ability to later draw of a map of the environment.

Nonetheless, overall research on the relation between children’s play activities and spatial and other abilities has yielded mixed results. Sometimes relations between play activities and cognitive competencies are found and sometimes they are not (e.g., Matthews, 1987; Munroe, Munroe, & Brasher, 1985; Rubin et al., 1983; Serbin & Connor, 1979; Webley, 1981). In fact, it is very likely that the relation between play experiences and the development of physical, social, and cognitive competencies is very specific and thus difficult to assess. For instance, it is likely that the relation between locomotor play in ibex kids, described earlier, and any associated neuromuscular changes that result from this form of play are specific to movement in mountainous terrain and thus are not evident for other types of motor behaviors (Byers & Walker, 1995). Similarly, it appears that environmental exploration improves the ability to generate mental maps of physical environments but is not consistently related to other forms of spatial cognition (e.g., the ability to copy geometric figures; e.g., Munroe et al., 1985).

Matthews’ (1987) study of the relation between exposure to a novel environment and the pattern of sex differences in the spatial representations of this environment illustrates the basic point. Here, 8- to 11-year-old boys and girls were taken on a one-hour tour of an unfamiliar area in suburban England. In one condition, the children were given a map of the entire area and were then taken on the tour, with the guide pointing out various environmental features. In the second--more difficult--condition, another group of children was given a map of 1/2 of the area and their tour was interrupted for 30 minutes at the halfway point, although the same environmental features were pointed out to the children; these conditions placed greater memory demands on the children when later asked to draw a map of this environment. At the end of the tour, the children were asked to draw a map of the entire area. Various features of these maps--such as the inclusion of landmarks and the clustering and relative orientation of these landmarks--were then analyzed, in order to make inferences about the ways in which boys and girls mentally represented this unfamiliar environment.

The maps of boys and girls did not differ in the overall amount of information provided, but sex differences did emerge for other map features. For the first group--those taken on the uninterrupted tour--the only difference was that girls included more landmarks in their maps and boys included more routes (e.g., roads). Under the more difficult conditions--with the interrupted tour--boys outperformed girls on a number of map features. At all ages, but especially for 10- and 11-year-olds, "boys showed a keener appreciation of the juxtaposition of places" (Matthews, 1987, p. 84). Boys were also better able than girls to integrate clusters of environmental features in ways that reflected their actual topographical positions and showed significantly fewer topographical distortions than girls. Moreover, "some of the older boys ... managed to show a euclidean grasp of space" (Matthews, 1987, p. 86). In other words, under conditions with fewer supports--such as 1/2 versus an entire map of the area--boys were better able than girls, on average, to mentally reconstruct the topography of an unfamiliar environment, retaining general orientation, clustering, and Euclidean (e.g., relative direction) relations among important environmental features. Under conditions with many supports--an entire map of the area and an uninterrupted tour--girls and boys remembered different features of the environment (landmarks vs. routes) but did not differ in the complexity of the maps they later drew of this environment.

The pattern found by Matthews (1987) suggests that the relation between childhood play experiences and later competencies is very complex. Studies that assess only global play activities and global physical, social, or cognitive competencies are not likely to find a strong relation between early experiences and later competencies, even when these relations do in fact exist (e.g., Greenough et al., 1987; Resnick, Berenbaum, Gottesman, & Bouchard, 1986). In this case, the sex of the child and the types of experiences they received interacted in complex ways in the expression of certain types of spatial competencies, such that sex differences for mentally representing an unfamiliar environment were only found with experiences that did not provide many external supports (e.g., a map). The sex difference in the ability to generate accurate mental maps of unfamiliar environments was evident under some, but not all, experiential conditions and appears to affect only specific types of spatial cognitions (see, e.g., Grimshaw, Sitarenios, & Finegan, 1995). More important, Matthews (1987) findings suggest that under natural play conditions (i.e., no map or tour), boys develop much more accurate mental representations of unfamiliar territory than do girls.

Indeed, the sex difference in skill at generating cognitive representations of unfamiliar environments (see also the Physical modules section of Chapter 8), the earlier described sex difference in physical activity levels, and the finding that in prehistoric and preindustrial contexts men travel farther from the home village than women indicates stronger selection pressures on men than women for the competencies associated with traveling in unfamiliar territories; for prehistoric fossils, patterns of bone wear indicate that men walked and ran more frequently than women (Ruff, 1987). Across preindustrial societies, men travel farther from the home village than women, on average, for a number of reasons, including finding mates, developing alliances with the men of neighboring villages, hunting, and intergroup warfare (Chagnon, 1977; Hill & Hurtado, 1996; Hill & Kaplan, 1988; Symons, 1979).

As described in Chapter 5, many of these activities--especially finding mates and intergroup warfare--are an important feature of male-male competition. Given this, the just mentioned sex differences, along with the associated play patterns, have likely been shaped by means of sexual selection (Parker, 1984). In this view, the sex differences in the size of the play area, in gross locomotor activity levels and in exploratory behavior within these ranges function to make the neuromuscular adaptations associated with running and traveling long distances on foot, as well as providing the experiences that will later facilitate the generation of cognitive maps of unfamiliar ecologies. All of these competencies are associated with intergroup warfare, as well as other sex-typed activities (e.g., hunting).

Play parenting. In contrast to rough-and-tumble play and the size of the play range, both of which favor boys, play parenting occurs much more frequently with girls (Lever, 1978; Pitcher & Schultz, 1983; Sandberg & Meyer-Bahlburg, 1994; Sutton-Smith et al., 1963). The sex difference in play parenting is related, in part, to the fact that girls are assigned child-care roles, especially for infants, much more frequently than are boys throughout the world (Whiting & Edwards, 1988). In addition, girls seek out and engage in child-care, play parenting and other domestic activities (e.g., playing house)--with younger children or child substitutes, such as dolls--much more frequently than do same-age boys (Pitcher & Schultz, 1983), as is the case with many other species of primate (Nicolson, 1987).

Nevertheless, as with most other social sex differences, the magnitude of these differences varies across age and context (Berman, 1980; Whiting & Edwards, 1988). Prior to about 6 years of age, both girls and boys are generally responsive to infants, but after this age and continuing into adulthood girls are more responsive, on average, to infants and younger children than are boys (Berman, 1986; Berman, Monda, & Myerscough, 1977; Edwards & Whiting, 1993; Fogel, Melson, & Mistry, 1986). The emergence of this sex difference is related to a significant drop in the frequency with which older boys attend to and interact with infants and younger children (e.g., Berman, 1986) and to an increase in girls’ interest in children following menarche (Goldberg, Blumberg, & Kriger, 1982). The latter finding suggests that girls’ interest in play parenting is heightened as a result of the hormonal changes associated with puberty, as is the case with many other species of primate (Nicolson, 1987). Nonetheless, the sex difference in the nature and frequency of interactions with infants and younger children varies with the relationship between the child-caregiver and the infant and whether the caregiving is an assigned responsibility or not. Generally, the sex difference, favoring girls, is largest when she is caring for a sibling and when she has been assigned this role by an adult (Berman, 1986).

Studies of children’s self-initiated play activities and the relation between these activities and prenatal exposure to androgens suggest that the sex difference in caregiving and play parenting is not simply due to a sex difference in socially-assigned roles (but see Fogel et al., 1986), in keeping with the just mentioned changes associated with menarche (Goldberg et al., 1982). Studies conducted in the United States, for instance, consistently find that girls engage in play parenting much more frequently than do same-age boys. Sandberg and Meyer-Bahlburg (1994) found that nearly 99% of 6 year old girls frequently played with dolls, as compared with 17% of same-age boys (it was not clear if this included play with "action figures" such as G.I. Joe). By 10 years of age, 92% of girls frequently played with dolls, as compared with 12% of same-age boys. Similar differences were found 30 years earlier, despite significant changes in the social roles of men and women in the United States (Sutton-Smith et al., 1963; see also Lever, 1978). Moreover, sex differences in play parenting have been documented across many preindustrial societies, such as the Yanomam??ont> , the !Ko Bushman of the central Kalahari and the Himba of Southwest Africa (Eibl-Eibesfeldt, 1989). Figure 7.2 shows a Himba girl using a sandal as a child substitute. While holding and cuddling the sandal, she sings "This is also a human being, this is my child, this is my child." Toward the end of the sequence, she punishes the "child" by beating it with a leather string and after this she blows on the sandal in order to reduce the pain and then comforts her "child" (Eibl-Eibesfeldt, personal communication, September, 1997); play parenting, like actual parenting, involves nurturance and discipline.

Research on children affected by CAH suggests that engagement in play parenting is influenced, at least in part, by prenatal exposure to androgens (Berenbaum & Hines, 1992; Berenbaum & Snyder, 1995; Collaer & Hines, 1995). As noted earlier, prenatal exposure to androgens is associated with a defeminization of play activities, including significantly less play with dolls and less interest in infants (Collaer & Hines, 1995). In a study involving the direct observation of the play activities of 5- to 8-year-old boys and girls affected with CAH and unaffected same-sex relatives, Berenbaum and Hines (1992) found that unaffected girls played with dolls and kitchen supplies 2 1/2 times longer than girls affected by CAH. These girls, in turn, played with boys’ toys (e.g., toy cars) nearly 2 1/2 times longer than unaffected girls. The same pattern was found in a follow-up study 3- to 4-years later (Berenbaum & Snyder, 1995). With this latter study, the children were also allowed to choose a toy to take home, after the assessment was complete. Unaffected girls most frequently chose a set of markers or a doll to take home, whereas girls affected by CAH most frequently chose a transportation toy (e.g., toy car) or a ball to take home; 7% of the girls affected by CAH chose a doll, as compared to 28% of the unaffected girls.

In sum, a sex difference in play parenting, favoring girls, is found in both industrial and preindustrial societies and in fact many other species of primate (Eibl-Eibesfeldt, 1989; Nicolson, 1987). For children, this sex difference is related, in part, to the fact that child-care responsibilities are assigned more frequently to girls than to boys throughout the world (Whiting & Edwards, 1988); of course, girls might be assigned these roles because they are more attentive to their siblings than are boys, on average (Edwards & Whiting, 1993). Even in the absence of assigned roles, girls engage in play parenting much more frequently than do boys, a pattern that is a precursor of the later sex difference in the level of parental investment (see Chapter 4). Moreover, the finding that early play parenting substantially improves the survival rate of first-born offspring, in many species of primate, suggests that this form of play has evolved by means of natural--as contrasted with sexual--selection, and serves the function of providing experiences that result in the improvement of later care-taking competencies (Nicolson, 1987; Pryce, 1993).

Object-oriented play. In keeping with the earlier described sex difference in the general orientation of infant girls to people and infant boys to objects, object-oriented play occurs much more frequently with boys than with girls (Eibl-Eibesfeldt, 1989; Freedman, 1974; Goodenough, 1957; Sandberg & Meyer-Bahlburg, 1994; Sutton-Smith et al., 1963). However, the sex difference in this form of play cannot simply be described as play with things versus play with people. Girls, in fact, engage more frequently than boys in the broader category of construction play, including play with puzzles, markers, clay, and so on (Christie & Johnson, 1987; Jennings, 1977; Rubin et al., 1983). It appears that boys more frequently engage in a more restricted category of play with things, in particular inanimate mechanical objects (such as toy cars) and construction play that involves building (e.g., with blocks; Garai & Scheinfeld, 1968; Hutt, 1972). Moreover, boys, more than girls, frequently engage in the experimental manipulation of these objects, such as taking them apart and trying to put them back together (Hutt, 1972).

The degree to which boys are interested in play with inanimate mechanical objects is illustrated by the earlier described Sandberg and Meyer-Bahlburg (1994) study. Here, it was found that 97% of 6-year-old boys frequently played with toy vehicles (e.g., cars), as compared with 51% of same-age girls. At ten years of age, 94% of boys frequently played with toy vehicles but only 29% of girls did so. Sutton-Smith and his colleagues (Sutton-Smith et al., 1963) found the same sex difference thirty years earlier and Eibl-Eibesfeldt (1989) described a similar pattern with !Ko children. Here, an analysis of 1,166 drawings revealed that boys drew technical objects, such as wagons and airplanes, 10 times more frequently than did girls (20% versus 2%). The same pattern has in fact been found in the drawings of children in the United States, Japan, Hong Kong, Bali, Ceylon, India, and Kenya (Freedman, 1974).

Moreover, studies of children affected by CAH suggest that an interest in mechanical objects and the associated play style is influenced, in part, by prenatal exposure to androgens (Berenbaum & Hines, 1992; Berenbaum & Snyder, 1995: Collaer & Hines, 1995). As noted earlier, girls affected by CAH played with boys’ toys--a helicopter, two cars, and a fire engine--nearly 2 1/2 times longer than unaffected girls and played with these boys’ toys more than 3 times longer than they played with girls’ toys, such as a doll (Berenbaum & Hines, 1992). When given an opportunity to take a toy home, 43% of the girls affected by CAH chose a toy car or airplane, while none of the unaffected girls chose these items; boys affected by CAH and unaffected boys chose these items 57 and 61% of the time, respectively.

As with the chimpanzee, it appears that object-oriented play helps children to learn about the physical properties of objects and the different ways in which these objects can be used and classified (Jennings, 1975; Rubin et al., 1983). Jennings, for instance, found that the free play activities of preschool children could be classified as largely people-oriented or object-oriented. An analysis of the relation between the focus of play activities and the pattern of cognitive abilities indicated that children whose play was object-oriented "performed better on tests of ability to organize and classify physical materials" (Jennings, 1975, p. 515), as assessed by tests of spatial cognition (e.g., the ability to mentally represent and manipulate geometric designs) and the ability to sort objects based on, for example, color and shape. In other words, manipulative and exploratory play with objects appears to be involved in the elaboration of the engineering modules described in Chapter 6 (Jennings, 1975; Rubin et al., 1983), which, in turn, appears to be related to the evolution of tool use in humans (Byrne, 1995; Pinker, 1997).

If this is the case, then--given the sex difference in object-oriented play--the construction and use of tools would have occurred more frequently with our male ancestors than with our female ancestors, or at least the range of tool use would have varied more for men than for women. In support of this argument is the finding that men work with a wider range of objects than do women across preindustrial societies (Daly & Wilson, 1983; Murdock, 1981). The activities that are performed exclusively or primarily by men include metal working, weapon making, the manufacture of musical instruments, work with wood, stone, bone and shells, boat building, the manufacture of ceremonial objects, and netmaking (Daly & Wilson, 1983). Across cultures, nearly 92% of those activities that appear to be most similar to the likely tool-making activities of H. habilis and H. erectus--weapon making and work with wood, stone, bone, and shells--are performed exclusively by men; just over 1% of these activities are performed exclusively by women and about 7% are performed by both men and women (Daly & Wilson, 1983; Gowlett, 1992a). At the same time, there are no object working activities--at least of those recorded--that show the same degree of exclusivity for women, although across cultures women engage in pottery making, basketmaking, and weaving much more frequently than do men (Murdock, 1981).

From this perspective, the sex difference in manipulative and exploratory object-oriented play reflects an evolved bias in children’s activities, such that boys, more than girls, play in ways that elaborate the skeletal competencies associated with the engineering modules and later tool use. As noted in Chapter 6, natural selection would favor the evolution of tool use to the extent that their use afforded greater control over the biological resources in the group’s ecology, such as expanding the array of foods available to individual (e.g., stone hammers can be used to extract bone marrow) and greater control over physical resources (e.g., tools for starting a fire; Gowlett, 1992b). To the extent that some tools--such as stones used as projectile weapons or spears--provided an advantage in male-on-male aggression their use, along with any associated play patterns, was likely influenced by sexual selection. Given this, the sex difference in manipulative and exploratory object-oriented play is likely to be the result of a combination of natural and sexual selection, with the former potentially related to an early difference in the foraging strategies of male and female hominids and the latter to male-male competition.

Fantasy play. Finally, there is also a sex difference in the fantasy elements of children’s sociodramatic play (Pitcher & Schultz, 1983; Rubin et al., 1983; Ruble & Martin, 1997). Recall, sociodramatic play involves groups of children enacting some social episode--often with great flair and emotion--centered on an everyday or imaginary theme, such as dinner or dragon slaying (Rubin et al., 1983; see the Children’s play and exploration section of Chapter 6). This form of play appears to be involved in the rehearsal and the development of social and social-cognitive (e.g., theory of mind) competencies (Leslie, 1987). Both boys and girls regularly engage in sociodramatic play, but differ in the associated themes and the roles they tend to adopt, as noted by Pitcher and Schultz (1983):

Boys play more varied and global roles that are more characterized by fantasy and power. Boys’ sex roles tend to be functional, defined by action plans. Characters are usually stereotyped and flat with habitual attitudes and personality features (cowboy, foreman, Batman, Superman). Girls prefer family roles, especially the more traditional roles of daughter and mother. Even at the youngest age, girls are quite knowledgeable about the details and subtleties in these roles. ... From a very early age girls conceive of the family as a system of relationships and a complex of reciprocal actions and attitudes. (Pitcher & Schultz, 1983, p. 79).

In other words, the sociodramatic play of boys focuses, more often than not, on themes associated with power, dominance, and aggression, as in enacting conflicts between cowboys and Indians, while the sociodramatic play of girls focuses, more often than not, on family-related themes, such as taking care of children. These activities, of course, reflect the same sex differences found in rough-and-tumble play and play parenting, respectively. In addition to practicing the behaviors associated with physical male-male competition and child care (i.e., parental investment), the fantasy element of sociodramatic play might also be involved in the development of the psychological control mechanisms described in Chapter 6. More precisely, the fantasy component of this form of play might provide practice at using fantasy to rehearse later social strategies, as well as to provide a vehicle for the expression of the motivational and emotional mechanisms associated with adult activities. For instance, in this view, Freud’s Oedipus and Electra complexes (i.e., fantasy about replacing the same-sex parent) are an early manifestation of intrasexual competition and provide early experiences in coping with the emotional features of this competition, such as sexual jealousy (Daly & Wilson, 1988a; Freud, 1923/1957).

Social development

The first section below provides a description of the tendency of boys and girls to segregate into same-sex social groups and the second provides discussion of the different social styles and motives that are manifest within these groups. The final section presents an evolutionary consideration of the patterns described in the first two sections.

Segregation

One of the most consistently found features of the social behavior of children is the formation of same-sex play and social groups (Maccoby, 1988; Moller & Serbin, 1996; Strayer & Santos, 1996; Whiting & Edwards, 1988). The formation of these groups is evident by the time children are about 3 years of age and becomes increasingly frequent through childhood. For instance, in a longitudinal study of children in the United States, Maccoby and Jacklin (1987) found that 4- to 5-year-old children spent three hours playing with same-sex peers for every single hour they spent playing in mixed-sex groups. By the time these children were 6- to 7-years-old, the ratio of time spent in same-sex versus mixed-sex groups was 11:1. Strayer and Santos found a similar pattern for French-Canadian children, as did Turner and Gervai (1995) for children in England and Hungry and Whiting and Edwards for children in Kenya, Mexico, the Philippines, Japan, India, and the United States, although the degree of segregation varied across these societies. It appears that children’s social segregation is most common in situations that are not monitored by adults, that is, situations in which the children are free to form their own social groups (Maccoby, 1988; Strayer & Santos, 1996).

The tendency of children to segregate themselves into same-sex groups appears to be related, in part, to the different play and social styles of girls and boys (Maccoby, 1988; Serbin et al., 1993). Girls and boys not only play differently (e.g., in terms of rough-and-tumble play), as described above, they also tend to use different social strategies to attempt to gain control over desired resources (e.g., toys) or to influence group activities. For instance, in situations where access to a desired object--such as a movie-viewer that can be watched by only one child at a time--is limited, boys and girls use different strategies, on average, for gaining access to this object (Charlesworth & Dzur, 1987). More often than not, boys gain access to this object by playfully shoving and pushing other boys out of the way, whereas girls gain access by means of verbal persuasion (e.g., polite suggestions to share) and sometimes verbal commands (e.g., "It’s my turn now!").

On the basis of these and other studies, Maccoby (1988) argued that segregated social groups emerge primarily because children are generally unresponsive to the play and social-influence styles of the opposite sex. Boys, for instance, sometimes try to initiate rough-and-tumble play with girls but most girls withdraw from these initiations, whereas most other boys readily join the fray. Similarly, girls often attempt to influence the behavior of boys through verbal requests and suggestions but boys, unlike other girls, are generally unresponsive to these requests (Charlesworth & LaFrenier, 1983). In this view, children form groups based on mutual interests and the ability to influence group activities and sex-segregation results, at least in part, from the sex differences in play interests and styles of social influence. There are also peer pressures that promote this segregation, such as teasing about "cooties" if one interacts with a member of the opposite sex (Maccoby, 1988).

In addition, studies of children affected by CAH suggest that the tendency to segregate into same-sex groups might be moderated by the categorization of other children as boys or girls, combined with a tendency to congregate with children in the same social category (Berenbaum & Snyder, 1995). In keeping with this view, most girls affected by CAH prefer other girls as playmates, even though their play activities tend to be masculinized (Berenbaum & Hines, 1992). However, the same-sex segregation occurs before many children consistently label themselves and other children as a boy or a girl, indicating that the categorization of children as boys and girls is not likely to be a sufficient explanation for this phenomenon (Maccoby, 1988; Moller & Serbin, 1996). In addition, many girls affected by CAH do in fact prefer boys as playmates, consistent with Maccoby’s (1988) position. Hines and Kaufman (1994), for instance, found that 44% of the girls affected by CAH in their study indicated that their most frequent playmates were boys, as compared to 11% of their unaffected peers and compared to more than 80% of the boys affected by CAH and unaffected boys. Girls affected by CAH thus show a pattern intermediate to that found in unaffected boys and unaffected girls. A substantial minority of these girls do tend to play in boys’ groups, whereas the majority of similarly affected girls tend to play in girls’ groups.

At this point, it appears that a combination of sex differences play and social styles, along with social categorization, are the proximate mechanisms underlying the segregation of boys and girls into same-sex groups. The net result is that boys and girls spend much of their childhood in distinct peer cultures (Harris, 1995; Maccoby, 1988). And, it is in the context of these cultures that differences in the social styles and preferences of boys and girls congeal, as described in the next section (see Harris, 1995, for a more general discussion).

Social behavior and motives

In those communities in which there are enough same-age children to form peer groups, children and adolescents throughout the world form these groups and spend much of their free time engaged in social discourse within these groups. Within these groups, the nature of the social relationships that develop among boys’ and among girls’ differs in a number of important ways. Most generally, the social relationships that develop among girls are more consistently communal--manifesting greater empathy, more concern for the well-being of other girls, more nurturing, intimacy, social/emotional support, and so on--than are the relationships that develop among boys, whereas relationships among boys are more consistently instrumental or agentic--more concern for the establishment of dominance, control of group activities, task orientation, and greater risk taking (Ahlgren & Johnson, 1979; Alfieri, Ruble, & Higgins, 1996; Archer, 1992, 1996; Block, 1976; Ginsburg & Miller, 1982; Jones & Costin, 1995; Charlesworth & Dzur, 1987; Feingold, 1994; Jarvinen & Nicholls, 1996; Knight & Chao, 1989; Maccoby, 1988; Miller & Byrnes, 1997; Savin-Williams, 1987; Strough, Berg, & Sansone, 1996; Ryff, 1995).

These sex differences are evident with the use of direct observation of social behavior, self-report measures, personality tests, tasks that involve the allocation or control of desired resources, and, depending on how it is assessed, for moral judgments about the "rights and wrongs" of social behavior (Carlo, Koller, Eisenberg, Silva, & Frohlich, 1996; Charlesworth & Dzur, 1987; Feingold, 1994; Knight & Chao, 1989; Petrinovich, O’Neill, & Jorgensen, 1993; Whiting & Edwards, 1988; Woods, 1996) and appear to be a reflection of the sex difference in social motives described in Chapter 6 (The Motivation to Control section). Recall, men are more concerned with the establishment and maintenance of social dominance than are women, on average, while women are more motivated then men to develop and maintain a reciprocal and socially stable system of interpersonal relationships, relationships that are characterized by a relatively equal distribution of resources and less obvious social hierarchies.

As described earlier, sex differences in some of the associated component competencies--such as girls’ greater sensitivity to social cues (e.g., facial expression) and their greater empathy to the distress of others--are found during infancy. Differences in the social relationships that form within groups of boys and groups of girls are evident as earlier as three years of age and are found at every age thereafter, although the magnitude of these differences is relatively larger during puberty and relatively smaller during old age (Ahlgren & Johnson, 1979; Alfieri et al., 1996; Knight & Chao, 1989; Maccoby, 1988, 1990; Turner, 1982). The sex differences in social behaviors and motives are also found across industrial as well as preindustrial societies (e.g., Feingold, 1994; Whiting & Edwards, 1988). As an example, in their study of the social development of children in Liberia, Kenya, India, Mexico, the Philippines, Japan, and the United States (with less extensive observations in Peru and Guatemala), Whiting and Edwards concluded:

Of the five major categories of interpersonal behavior explored in (these studies)--nurturance, dependency, prosocial dominance, egoistic dominance, and sociability -- two emerge as associated with sex differences. Across the three older age groups (knee, yard, and school-age children) girls on average are more nurturant than boys in all dyad types ... while boys are more egoistically dominant than girls" (Whiting & Edwards, 1988, p. 270).

The same pattern is evident for adolescents and adults. In one associated analysis, Feingold (1994) focused on the pattern of sex differences on personality tests normed in the United States. The analysis of test norms is especially informative, given that these involve large and often times nationally-representative samples (Feingold’s analysis included the scores of 105,742 people). Across tests, he found moderate to large sex differences for "tender-mindedness" (i.e., nurturance and empathy), favoring women, and assertiveness (e.g., dominance-related activities), favoring men; about 6 out of 7 women scored higher than the average man on measures of tender mindedness, while about 7 out of 10 men scored higher than the average woman on measures of assertiveness. The magnitude of these differences varied little for samples assessed from the 1940s to the 1990s and varied little across groups of drawn from high schools, colleges, or the general population. Multiple (i.e., > 1) studies of sex differences in personality were also available for adults from Canada, Finland, Germany, and Poland, and these studies revealed the same pattern as was found in the United States, although the magnitude of the sex differences varied across these nations.

Ahlgren and Johnson (1979) found a similar pattern in the social motives of 2nd to 12th graders. The social motives of these children and adolescents were captured by two salient themes, cooperation (e.g., "I like to learn by working with other students.") and competition (e.g., "I like to do better work than my friends."). At all grade levels, girls endorsed cooperative social behaviors more frequently did than boys, while boys endorsed competitive social behaviors more frequently than did girls. A more recent study of 250 14 year olds revealed the same pattern (Jarvinen & Nicholls, 1996). Here, boys’ social goals were relatively more focused on the achievement of dominance and leadership, while girls’ social goals were relatively more focused on the establishment of intimate and nurturing relationships, a pattern that did not differ across the academic track (i.e., low, average, or high achieving) of these students. The largest sex differences were for the establishment of intimacy--more than 4 out of 5 girls rated this goal as being more important than the average boy--and dominance--3 out of 4 boys rated this goal as being more important than the average girl.

Knight and Chao (1989) found the same pattern in the rules that 3- to 12-year-olds use to distribute a valuable resource amongst themselves and their social group (money in this case). The associated studies were designed to determine whether the children had preferences for equality (minimizing differences between oneself and others), group enhancement (enhancing the overall resources of the group, regardless of how this effects one’s own resources), superiority (trying a maximize one’s resource relative to other group members), or individualism (enhancing one’s resources independent of peer resources). Self-interest was evident in the resource distributions of 3- to 5-year-old boys and girls, as about 1/2 of these children showed an individualism preference. At the same time, 25% of the girls but none of the boys showed an equality preference, whereas 19% of the boys but only 5% of the girls showed a superiority preference. By 6 years of age, the majority of boys showed a superiority preference, while the girls were largely split between the individualism and equality preferences. For instance, for 9- to 12-year-olds, 75% of the boys showed a superiority preference, as compared to 20% of the girls. The remaining girls were split evenly (40% each) between the individualism and equality preferences; only 7% of the boys showed an equality preference. Parallel sex differences are often, but not always, found in the moral judgments of boys and girls and men and women (Gilligan, 1982; Petrinovich et al., 1993; Woods, 1996). For instance, girls and women more consistently than boys and men endorse a moral ethos that espouses equality in social relationships and an avoidance of the harm of others (e.g., Gilligan, 1982).

Qualitative differences in the nature of the social relationships that form within groups of boys and groups of girls are nicely illustrated by Savin-Williams’ (1987) ethological study of adolescent social groups. Here, the social relationships that developed during the course of a five-week summer camp were systematically observed and documented for groups of 12- to 16-year-old boys and girls assigned to the same cabin. These observations revealed a number of similarities in the social behaviors that emerged within these same-sex groups. Both boys and girls formed dominance hierarchies and frequently used ridicule to establish social dominance, such as name calling ("homo," "perverted groin") or gossiping. For both sexes, the establishment of social dominance resulted in greater access to desired resources (e.g., larger desserts) and greater control over the activities of the group, relative to lower-status peers. At the same time, there were sex differences in the stability of the social hierarchies, the degree to which dominance displays were direct or indirect, the use of physical strength and skills to establish dominance, and the benefits of achieving dominance, among others.

In some groups, boys began their bid for social dominance within hours of arriving in the cabin, whereas most of the girls were superficially polite for the first week and then began to exhibit dominance-related behaviors. For boys, dominance-related behaviors included ridicule, as noted above, as well as directives ("Get my dessert for me."), counter dominance statements ("Eat me."), and physical assertion (e.g., play wrestling, pillow fights, etc., and sometimes actual physical fights). More than 90% of the time these behaviors were direct and overt, that is, they were visible to all group members, clearly directed at one other boy, and were essentially attempts to establish some type of control over this individual.

Girls used ridicule, recognition, and verbal directives to establish social dominance, but used physical assertion only 1/3 as frequently as boys. Unlike boys, more than 1/2 of the girls’ dominance-related behaviors were indirect. For instance, a girl might suggest to another girl that she "take her napkin and clean a piece of food off of her face," while under the same conditions a boy would simply call his less-kept peer a "pig" and then try to enlist other boys in a group-wide ridicule session of this boy; once "down" lower-status boys would typically use this opportunity to attempt to establish individual dominance over this peer. Moreover, girls, in contrast with boys, often overtly recognized the leadership of another girl. In fact, recognition was the second most frequently used form of dominance-related behavior with girls, but occurred infrequently with boys (23% vs. 6% of the dominance-related behaviors for girls and boys, respectively). In these cases, less-dominant girls would approach their more dominant peers for advice, social support, grooming (e.g., having her hair combed), and so on.

Boys’ dominance hierarchies were also much more stable across situations and across time than were girls’ hierarchies. By the end of summer camp, boys’ groups showed greater stability and cohesiveness relative to the first week of camp, whereas most of the girls’ groups were on the verge of splintering or had already spit into "status cliques based on popularity, beauty, athletics, and sociability" (Savin-Williams, 1987, p. 124). In some cases, dominant girls disengaged from the cabin-group and spent most of their free time with one or two friends, consistent with the finding that girls’ groups are often comprised of dyads or triads (Lever, 1978). During this free time, the girls would typically walk and talk. Dominant boys, in contrast, never disengaged from the group and spent most of their free time directing the group in competitive athletic activities against other groups. In other words, dominant boys more actively and more successfully controlled group activities than did dominant girls, as illustrated by the following flag making exercise:

Andy (the alpha male) immediately grabbed the flag cloth and penciled a design; he turned to Gar for advice, but none was given. Otto (low ranking) shouted several moments later, "I didn’t say you could do it!" Ignoring this interference, Andy wrote the tribal name at the top of the flag. Meanwhile, Delvin and Otto were throwing sticks at each other with Gar watching and giggling. SW (the counselor) suggested that all should participate by drawing a design proposal on paper and the winning one, as determined by group vote, would be drawn on the flag. ... Andy, who had not participated in the "contest," now drew a bicentennial sunset; it was readily accepted by the others. Without consultation, Andy drew his design as Gar and Delvin watched. Gar suggested an alteration but Andy told him "Stupid idea," and continued drawing. Otto, who had been playing in the fireplace, came over and screamed, "I didn’t tell ya to draw that you Bastard Andy!" Andy’s reply was almost predictable, "Tough shit, boy!" (Savin-Williams, 1987, p. 79).

Andy’s mode of domination was more physically assertive and verbally aggressive than was the social style of the dominant boys in the some of the other cabins. The result was the same, however. The dominant boys got first choice of what to eat (e.g., they almost always got the largest desserts), where to sleep, and what to do during free time. Across cabins, dominant girls also differed in their social styles. Although some girls were physically assertive and direct in their attempts to dominate other girls, the most influential girls (over the course of the five weeks) were much more subtle, as exemplified by Anne.

(Her) style of authority (was) subtle and manipulative, she became the cabin’s "mother." She instructed the others on cleanup jobs, corrected Opal’s table manners ("Dottie, pass Opal a napkin so she can wipe the jelly off her face."), and woke up the group in the morning. ... Anne became powerful in the cabin by first blocking Becky’s (the beta female) dominance initiations through refusing and shunning and then through ignoring her during the next three weeks. By the fifth week of camp Ann effectively controlled Becky by physical assertion, ridicule, and directive behaviors. (Savin-Williams, 1987, p. 92).

For both boys and girls, the achievement of social dominance was related to athletic ability, pubertal maturity, and leadership. Dominant girls were more socially popular than were many of the dominant boys--Andy was not well liked by his cabin-mates but they followed his directives nonetheless--and physical attractiveness was more important for achieving social dominance within boys’ groups than within girls’ groups. Moreover, other studies suggest that risk taking is more frequently used by boys than by girls in their attempts to achieve social status, with high risk takers being afforded a higher social status, on average, than their risk-avoiding peers (MacDonald, 1988; Miller & Byrnes, 1997). In all, the boys described ideal leaders as instrumental, "determined and tries hard at what he does, considerate in tolerating underlings, organizes activities, and knows what to do and makes the right decisions. The (girls’) groups emphasized expressive attributes: relates to my problems, friendly, outgoing, patient, considerate in respecting the needs and feelings of others." (Savin-Williams, 1987, p. 127).

Ethological and other studies suggest that these social patterns congeal as adolescents approach physical maturation (Ahlgren & Johnson, 1979; Block, 1993; Savin-Williams, 1987). Ahlgren and Johnson, for instance, found that at about the time of puberty, girls’ social motives become more cooperative and less competitive than their younger peers. Savin-William’s found that by the end of adolescence, there was a significant reduction in ridicule, "backbiting, bickering, and cattiness" (Savin-Williams, 1987, p. 150) in girls’ interpersonal relationships, compared to early adolescence. By late adolescence, girls’ relationships also showed greater stability (i.e., less changing of "best friends"), more recognition, greater sensitivity to the needs and emotions of their friends, more helping behavior, and fewer attempts at establishing dominance (i.e., the relationships were more often among equals) than was found during early adolescence.

Boys’ relationships changed, as well. By late adolescence, boys’ group-level games were characterized by greater focus and organization, with fewer within-group negative criticisms and more encouragement, than was found with younger boys (Savin-Williams, 1987). At the same time, during their dominance-related encounters, older boys used physical assertion less frequently and used recognition more frequently than their younger peers. Under some conditions, however, male dominance encounters can become very physically aggressive and even deadly by late adolescence (sometimes earlier; Wilson & Daly, 1985).

Social development and evolution

When viewed from the perspective of human evolution in general, children’s relationships within the above described social groups provide one mechanism through which the individual- and group-level cognitive modules, along with the associated motivational and emotional components, become elaborated and adapted to the group’s social structure and customs (see Chapter 6). For instance, the nature of the play fighting that emerges within boys’ groups appears to be related, in part, to the nature of male-male competition in adulthood, and the intensity of this play fighting likely contributes to the preparation of these boys--as described earlier--for later participation in the adult community; they learn, for instance, how to achieve cultural success (Irons, 1979; see Male-male competition section of Chapter 5). Moreover, social relationships within these groups likely allow children and adolescents to refine those social skills that will later be used to influence other people and to garner important resources. They learn which social strategies work well for them and which do not (e.g., social persuasion vs. attempts to dominate; MacDonald, 1996).

When viewed from the perspective of sexual selection in particular, the tendency of children to segregate themselves into same-sex groups and to manifest a different pattern of social behavior and motives within these groups follows, at least in part, from the sex differences in reproductive strategies, described in Chapter 4 and Chapter 5. In other words, the types of social competencies needed to compete with members of the same sex or to successfully raise children differ for men and women, and, given this, the developmental experiences that support the refinement of these competencies should differ as well. In this view, one important function of children’s relationships within peer groups is to develop the social competencies that will later be employed in the context of intrasexual competition and to refine those social competencies that will later be used to attempt to organize and control other people; examples of attempts to organize the social world were provided in Chapter 6, with the description of the sex difference social motives (The motivation to control section).

Any evolutionary interpretation of the specific sex differences described above must be based on a consideration of the social groups within which our female and male ancestors likely lived (Foley & Lee, 1989; Rodseth et al., 1991). Of particular importance is the fact that men--and our male ancestors--are the philopatric sex for humans, that is, men tend to stay in the birth group and women tend to emigrate to a different group, although women often maintain contact with their kin (Pasternak et al., 1997; Rodseth et al. 1991). These social patterns indicate that as adults our female and male ancestors lived in very different social worlds. Our male ancestors, more often than not, lived in the same community as their male kin, whereas our female ancestors, more often than not, lived in a community with nonkin. As described in Chapter 6 (Social modules section), social relationships with kin and nonkin often differ. Relationships with friends are characterized by higher levels of reciprocity than are relationships with kin (Hartup & Stevens, 1997; Trivers, 1971). Moreover, as described in Chapter 3 (Male-male competition section), whether males or females are the philopatric sex influences coalition formation and the nature of intergroup relationships, with the philopatric sex being more likely to form same-sex coalitions and more likely to engage in intergroup coalition-based competition (e.g., Rodseth et al., 1991; Wrangham, 1980).

From an evolutionary perspective, coalition-based intrasexual competition is therefore expected to be and, in fact, does occur more frequently in men than in women, as described in Chapter 5. From a developmental perspective, the social culture that emerges within boys’ groups should provide a context for refining individual-level dominance-related competencies and an opportunity to develop the competencies necessary to form and maintain cohesive and effective large-scale coalitions. The latter activities would be designed to improve one’s position within the coalition and the former for competition against other coalitions. The finding that boys, in relation to girls, are more concerned with achieving social dominance and more frequently organize themselves into relatively large and cohesive groups that compete against other groups of boys is entirely consistent with this view, that is, these activities involve a preparation for later coalition-based male-male competition. The findings that the striving for social dominance, the tendency to engage in group-level athletic competition, and the physical competencies associated with male-on-male aggression (i.e., upper body strength) are all related to exposure to sex hormones (Collaer & Hines, 1995; Mazur & Booth, in press; Tanner, 1990) further support the position that the social behavior within boys’ groups has been shaped by sexual selection and involves a preparation for later male-male competition.

As with other primates species in which males are the philopatric sex, girls and women do not form coalitions to compete against groups of other girls or women--bonobo females form coalitions but not for the purpose of intergroup aggression (de Waal & Lanting, 1996)--nor are they as concerned as boys and men about establishing social dominance. Nonetheless, some form of one-on-one dominance-related behaviors are expected for girls and women, given that social dominance results in greater access to important resources and because--across primate species--access to these resources (especially food) often has reproductive consequences (see Female-female competition and male choice section of Chapter 3; Pusey et al., 1997). Moreover, for those species in which males provide some level of postnatal parental investment--which includes humans (Chapter 4)--some level of female-female competition is expected (e.g., Parker & Simmons, 1996), although the intensity (e.g., risk of injury or death) of intrasexual competition should be lower for females than for males, as described in previous chapters (e.g., Chapter 2 and Chapter 5); males generally benefit more than females by intense and risky competition (e.g., by gaining additional mates) and physical injury can be particularly costly to females because any such injury will reduce their ability to care for offspring.

Adolescent girls and women do indeed develop relatively subtle dominance relationships (Bjorkqvist, Osterman, & Lagerspetz, 1994). Achieving relative dominance within these relationships is probably important for securing important resources and for controlling the social behavior of other people (e.g., Grotpeter & Crick, 1996), and is likely to be related, in part, to female-female competition. Although in many societies and presumably during the course of human evolution (Hrdy, 1997), men often attempt to control the marriage and sexuality of their daughters and other women, female-female competition could readily manifest itself in the context of polygynous marriages (i.e., conflict between co-wives) and in the context of less formal and furtive social and sexual relationships (Betzig, 1986; Daly & Wilson, 1983; Hill & Hurtado, 1996). Within polygynous marriages, for instance, the first and usually dominant wife typically has more children than her younger peers (Daly & Wilson, 1983), although it is not known, at this point, if dominant women somehow--for example, through creating social stress--suppress the reproduction of their co-wives.

Across contexts, female-female competition often involves relational aggression, where girls and women attempt to exclude their competitors from the social group, by means of gossiping, shunning, and so on. These, of course, are exactly the form of dominance-related behaviors that are manifest in the social groups of adolescent and sometimes younger girls (Savin-Williams, 1987; see Female-female competition section of Chapter 5). Thus, as with boys, social behavior within girls’ groups appears to provide experience in using the social strategies associated with later intrasexual competition. The particular form of intrasexual competition in girls and women follows from the sex difference in the valuation of intimate and reciprocal relationships. As described below, the formation of these relationships provides an important source of social support for women and, given this, relational aggression would be particularly effective in competing against other women. This is so because relational aggression disrupts the ability of potential competitors to develop these relationships. As noted in Chapter 5 (Female-female competition section), the disruption of social relationships often reduces fertility in females, at least in some primate species (e.g., Abbott, 1993). Given this, this form of competition could very well have influenced reproductive outcomes in women and could represent a form of female-on-female aggression that has been shaped by sexual selection. The finding that these social behaviors are influenced by hormonal and genetic factors further supports the position that these are evolved aspects of girls’ and women’s social relationships (Skuse et al., 1997; Van Goozen et al., 1995).

As noted above, it is likely that hominid females more frequently than hominid males transferred into groups where they were not genetically related to other individuals in the group. Under these circumstances, hominid females were more likely to be socially isolated and thus at greater risk for exploitation than were hominid males (Pasternak et al., 1997). The greater attentiveness of girls and women to social cues (e.g., facial expressions), their greater positive social signaling (e.g., smiling), their skill at strategically using emotion cues in social contexts, and their general motivation to develop intimate social relationships as an end in itself--rather than as a means to compete as is the case with boys and men--might reflect an adaptation to these social conditions (Bjorklund & Kipp, 1996; Freedman, 1974); for instance, in adulthood, about out 7 out of 10 women smile more frequently in noncompetitive social situations than does the average man and directs these smiles more frequently to other women than to men (Hall, 1984; see Social modules section of Chapter 8).

Any such adaptation--shaped by natural selection--might reflect the evolutionary elaboration of the social and emotional systems that support friendships, that is, relationships with nonkin. Or stated differently, kin bias would more or less automatically provide males with a system of social support, but no such kin-based support system would be available for immigrant females. Under the latter conditions, intimate relationships would provide an important social resource in a potentially hostile social environment. In keeping with this view, girls and women are indeed more concerned than boys and men about the nuances of interpersonal relationships (e.g., the other person’s feeling) and use these relationships as a source of social and emotional support more frequently than do boys and men (Brodzinsky, Elias, Steiger, Simon, Gill, & Hitt, 1992; Causey & Dubow, 1992; Simon & Ward, 1982; Strough, Berg, & Sansone, 1996).

Alternatively, it might be argued that the sex difference in the motivation to develop intimate relationships is related to the sex difference in parental investment, such that girls’ intimate relationships provide practice for the later development of intimate family relationships. Of course, this is possible. However, for children and adolescents these relationships develop with same-age (not younger) peers and the relationships that develop with these relationships differs from play parenting. The distinction between peer relationships and play parenting suggests that the development of these intimate relationships is not simply practice for later parent-child relationships, although in adulthood these peer relationships might provide a source of social support and assistance in child rearing (e.g., through alloparenting, such as baby-sitting). In addition, such relationships would also provide an important support system for dealing with the dynamics of adult-on-adult social relationships, such as providing social support in the context of male-on-female aggression (Smuts, 1992, 1995).

Parenting

The study of parental influences on developing children is complicated because in most families parents provide their children with genes and a rearing environment (Scarr & McCartney, 1983; Scarr, 1992). As a result, the often found correlation between parental characteristics and the characteristics of young children--which are often assumed to be due to parental socialization--might well result from the overlapping genes between parents and children, socialization, or differences in the ways in which children with different genotypes react to similar rearing environments (Maccoby & Martin, 1983; Reiss, 1995; Scarr, 1992). Indeed, behavioral genetic and other studies suggest that parental influences on individual differences in children’s personality, social behavior, intelligence and so on are much weaker than many people believe, or would like to believe (Harris, 1995; Plomin et al., 1997; Rowe, 1994; Scarr, 1992). This is not to say that parents are not important. They are. As described in Chapter 4, parental investment can have a profound--sometimes life or death--effect on the physical and physiological development of children (Flinn et al., 1996; Hill & Hurtado, 1996; Sapolsky, 1997).

Moreover, behaviors outside of the range that naturally occurs in parent-child relationships, such as severe neglect, can adversely affect the social and psychological development of children (Scarr, 1992), just as neglectful parenting can adversely affect offspring development in other primates (Goodall, 1986). Most parents, however, provide a level of investment that allows for normal development and variations within this normal range do not appear to be systematically related to variations in child outcomes. Stated differently, parents provide an evolutionarily expected rearing environment for the infant and child, including exposure to language, synchronized parent-child interactions, and so on, that provides the experiences that begin to flesh out the skeletal competencies of evolved modules, in particular the individual-social modules (Kuhl et al., 1997; MacDonald, 1993; Papouš ek & Papouš ek, 1995). Experiences that go above and beyond these evolutionarily expected experiences, in contrast, do not appear to have strong influences on the developing child (Scarr, 1992).

In a similar vein, studies conducted in Western nations indicate that parents, on average, treat boys and girls in very similar ways--with a few exceptions noted below--and given this the earlier described sex differences in infancy, play, and social development are not likely to be the result of parental treatment (Lytton & Romney, 1991; Maccoby & Jacklin, 1974). The most comprehensive assessment--involving 172 studies and 27,836 participants--of the parental treatment of boys and girls was conducted by Lytton and Romney. In this meta-analysis, parental treatment was assessed across eight broad socialization areas, including amount of interaction, achievement encouragement, warmth and nurturance, encouragement of dependency, restrictiveness, disciplinary strictness, encouragement of sex-typed activities, and clarity of communication directed toward the child. For studies conducted in North America, there were very few differences in the ways in which parents treated girls and boys, as assessed by observation, parental report, and child report.

One exception was for encouragement of sex-typed activities, although the difference was relatively small; for about 2 out of 3 boys, parents encouraged sex-typed activities more frequently than they did with the average girl. This result appears to largely reflect an active discouragement of boys, especially by fathers, from playing with girls’ toys, such as dolls. Doll play in boys appears to prompt concerns of homosexuality (Maccoby & Jacklin, 1974). Studies conducted in Western nations outside of North America indicated that boys received more physical discipline than girls, but again the difference was small. A more recent meta-analysis revealed some differences in the ways in which parents talk to boys and girls (Leaper, Anderson, & Sanders, 1998). Mothers, for instance, talked more and provided more encouraging speech to their daughters than to their sons.

Lytton and Romney (1991) suggested that even these differences might result from parental reactions to differences in the behavior of girls and boys. The greater use of physical punishment with boys, relative to girls, might be a reflection of the tendency of boys to be less responsive to verbal requests than girls. Similarly, paternal concerns over doll play might be a reaction to the fact that such play occurs relatively infrequently in boys (Sandberg & Meyer-Bahlburg, 1994) and is, in fact, correlated with later homosexuality (Green, 1987). Similarly, the difference in the ways in which parents talk to boys and girls likely reflects the earlier described (Infancy section) differences in the social responsiveness and behavior of boys and girls.

It is not likely that the sex differences described in this chapter are due to children’s selective imitation of the same-sex parent or other same-sex people. On the basis of a review of 23 studies of children’s imitative behavior, Maccoby and Jacklin tentatively concluded "that early sex typing is not a function of a child’s having selectively observed, and selectively learned, the behavior of same-sex, rather than opposite-sex, models" (Maccoby & Jacklin, 1974, p. 299). Barkley and his colleagues reached the same conclusion, after reviewing 81 relevant studies (Barkley, Ullman, Otto, & Brecht, 1977). Moreover, in an empirical study of their own, they found that girls tended to imitate traditionally feminine behavior--such as playing house--whether these behaviors were enacted by a male or female model and boys tended to imitate traditionally masculine behavior--such as play fighting--regardless of the sex of the model (Barkley et al., 1977).

It is not likely that these findings result from children only imitating behavior that is considered to be sex appropriate. As noted earlier, children’s knowledge of gender roles is only weakly related to the actual behavioral differences that are observed between boys and girls (Serbin et al., 1993; Turner & Gervai, 1995) and many of the sex differences described in this chapter emerge before children know that they are a boy or a girl or have any knowledge of what is "expected" of boys and men or girls and women (Maccoby, 1988). Rather, in keeping with the sex differences described in the Infancy section, it is more likely that boys and girls selectively attend to and find more attractive or engaging behaviors that are traditionally defined as sex-typed, although young boys who label themselves as boys and understand that they will someday be men do attend to men more frequently than do boys who do not yet understand that one’s biological sex is constant through time (Slaby & Frey, 1975).

In other words, boys and girls selectively attend to different types of behaviors in adults, although for very young children this selective attention does appear to be related to knowledge of their biological sex. Endicott provides an illustration of the selective attention of boys, with her description of the play of Batek children in Malaysia; the Batek are a relatively egalitarian hunter-gatherer society in which "no gender distinctions are made in the terms for children, siblings, cousins, and grandchildren" (Endicott, 1992, p. 282):

Playgroup activities range from pretending to move camp to imitating monkeys to play-practicing economic skills such as blowpipe-hunting, digging tubers, collecting rattan, and fishing. Fathers sometimes intervene in the activities of children to offer advice about how to perform these skills. For example, when several children were pretending that they were harvesting honey by smoking bees out of a hive high in a tree in the middle of camp, a father who often participated in honey collecting showed the children how to properly construct rattan ladders to use for climbing up to the hive. It was the older boys, in the 10- to 12-year-old range, who paid closest attention tothis informal lesson. (emphasis added; Endicott, 1992, p. 288).

Although studies conducted in Western nations suggest that parenting does not strongly influence the personality and social behavior of children, as noted above, cross-culture comparisons do find a relation between parenting and child development (Low, 1989; MacDonald, 1992; Rohner, 1976). These seemingly contradictory findings are due, in part, to differences in how the relation between parenting and child outcomes are measured in these studies. For within-culture studies, individual differences in children’s behavior are related to individual differences in parenting style, whereas cross-cultural studies involve comparisons of average differences across groups of people from different societies. Moreover, within- and cross-culture studies often yield different results because the range of parenting behaviors is larger when assessed across than within cultures (McDonald, 1992).

To illustrate, in societies characterized by high-levels of intergroup aggression, parenting practices for both boys and girls tend to be harsher--including more physical discipline, less responsiveness to the child’s emotional state and so on--than parenting practices found in more peaceful societies (Bonta, 1997; West & Konner, 1976; MacDonald, 1993). One apparent result of these differences in the modal style of parenting is a cross-cultural difference in the average level of aggression found in societies with relatively harsh as opposed to relatively warm parenting styles (MacDonald, 1992). At the same time, the pattern of sex differences remains within cultures, although girls or women from one culture might be more aggressive, on average, than boys and men from another culture. The cross-cultural pattern suggest that parenting can in fact accentuate or attenuate the expression of certain social behaviors (e.g., frequency of physical aggression) but these effects largely result in cross-cultural differences in the behavior of same-sex children and not the creation of sex-differences in one culture but not another.

The largest cross-cultural analysis of the relation between child-rearing practices and children’s social behavior was conducted by Low (1989). In this analysis of 93 cultures, child-rearing practices were examined as they were related to social structure (i.e., stratified vs. nonstratified societies and group size) and marriage system (i.e., polygynous vs. monogamous). In nonstratified polygynous societies--societies where men can improve their social status and thus increase the number of women they can marry--the socialization of boys focuses on fortitude, aggression, and industriousness; traits that will likely influence economic and thus reproductive success in adulthood. Moreover, for these nonstratified societies, there was a very strong linear relation between the socialization of competitiveness in boys and the maximum harem size allowed within the society. The larger the maximum harem size, the more the competitiveness of boys was emphasized in parental socialization.

In contrast, for stratified societies--where men cannot improve their social status--boys are not strongly socialized to exhibit aggression and fortitude, although industriousness is still important. For girls, there was a relation between the amount of economic and political power held by women in the society and socialization practices. In societies where women could inherit property and hold political office, girls were socialized to be less obedient, more aggressive and more achievement oriented relative to girls who lived in societies in which men had more or less complete control over economic and political resources. Based on these patterns, Low concluded that "there is thus some evidence that patterns of child training across cultures vary in ways predictable from evolutionary theory, differing in specifiable ways between the sexes, and varying with group size, marriage system, and stratification" (Low, 1989, p. 318).

These differences in socialization patterns appear to reflect differences in the ways in which social, economic, and reproductive success can be achieved in different cultures and the associated child rearing practices appear to prepare the child, in terms of social scripts, emotional reactions to social relationships, and so on, for the social environment they will experience as adults (MacDonald, 1992). These parenting practices, however, appear to largely amplify or suppress, rather than create, the pattern of sex differences described in this chapter, with parents tending to emphasize those characteristics that will enhance the reproductive opportunities of their children--boys and girls--and thus increase the likelihood that they will have grandchildren. In this view, the earlier described cross-cultural differences in the intensity of boys’ play fighting results from differences in parental disciplinary techniques--for instance, punishment that results in physical pain appears to result in relatively angry and aggressive children--and the extent to which parents encourage or discourage boys’ natural play fighting.

Summary and Conclusion

The period between birth and puberty appears to represent the portion of the life span during which the skeletal competencies associated with those cognitive, social, and behavioral skills that support survival and reproduction are elaborated and tailored to the local ecology. The length of this development period has increased considerably during the course of hominid evolution (McHenry, 1994b) and it appears that much of this change is related to an increase in the complexity of human social systems (Dunbar, 1993; Joffe, 1997). The complexity of these systems, in turn, is related, in part, to the reproductive concerns and strivings of human beings (Chagnon, 1988; Symons, 1979). From this perspective, one important function of childhood is to provide the experiences needed to refine those competencies that are associated with intrasexual competition and other reproductive activities (e.g., parental investment) in adulthood.

On the basis of the sex differences in parental investment (Chapter 4), the nature of intrasexual competition and in mate choice criteria (Chapter 5) in adulthood, sex differences in the self-initiated developmental experiences of boys are girls are expected and are found. Although there are, of course, many similarities in the childhood experiences of boys and girls, there are also considerable differences. Girls and boys show different patterns of physical development (Tanner, 1990), different play interests and styles, as well as different social behaviors and motives, and many of these differences can be readily understood in terms of sexual selection in general and intrasexual competition in particular (Darwin, 1871).

As an example, the delayed physical maturation of boys, relative to girls, and the sex difference in the timing, duration, and intensity of the pubertal growth spurt follow the same pattern as is found in other nonmonogamous primates (Leigh, 1995, 1996). Across these primate species, the sex differences in these features of physical maturation are consistently related to the intensity of physical male-male competition, as contrasted with any sex differences in foraging strategy (Mitani et al., 1996). The sex difference in the pattern of human physical competencies, such as a longer forearm and greater upper body strength in men than in women, is also readily explained in terms of selection for male-on-male aggression, selection that involved the use of projectile and blunt force weapons (Keeley, 1996). Stated more directly, the sex differences in physical development and physical competencies have almost certainly been shaped by sexual selection, and the majority of these differences have resulted from male-male competition over access to mates (Tanner, 1992); of course, some physical sex differences, such as the wider pelvis in women, have been shaped by natural selection.

It is very likely that many of the sex differences in play interests and social behaviors have also been shaped by sexual selection. The sex differences in rough-and-tumble play, exploratory behavior and size of the play range, the tendency of boys to form coalitions in their competitive activities with other boys, and the formation of within-coalition dominance hierarchies are also patterns that are associated with male-male competition in other primates, particularly primates in which males are the philopatric sex (Goodall, 1986; Smith, 1982). In this view, all of these features of boys’ play and social behavior involve a preparation for later within-group dominance striving and coalition formation for intergroup aggression. Through parenting practices, such as degree of physical discipline, the selective imitation of competitive activities, and actual experiences within same-sex groups, boys learn how to best achieve within-group social dominance and practice the specific competencies associated with male-male competition in their particular culture. They learn how to achieve cultural success (e.g., by leading raids on other villages or becoming a star football player).

Not all developmental sex differences are related to male-male competition, however. For instance, the relational aggression that is common in girls’ groups might be a feature of female-female competition and a number of other physical and behavioral sex differences that become evident during development have likely been shaped by natural selection or mate choice. The sex difference, favoring boys, in manipulative and exploratory object-oriented play appears to be related to the evolution of tool use and a sex difference, favoring men, in the range of tool-related activities in adulthood. Although it is not certain, these sex differences have likely been shaped, in part, by natural selection (e.g., through a sex difference in the foraging strategies of our ancestors). Similarly, the sex difference in play parenting, favoring girls, reflects the later sex difference in parental investment, favoring women, and has almost certainly been shaped by natural selection (Pryce, 1995). Finally, many of the physical changes associated with puberty, such as the development of a masculine jaw in men and relatively large breasts in women, have likely been shaped by the mate choice preferences of the opposite sex and might be condition-dependent indictors of physical and genetic health (Thornhill & Müller, 1997).

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