McHenry and Bogin estimated the age of maturation for A. afarensis and A. africanus to have been similar to that found in the modern chimpanzee (Pan troglodytes), 10 to 12 years.Read more at location 5660
Infancy is the time of suckling, and juvenility is the time between weaning and reproductive maturation; reproductive maturation is the final development period,Read more at location 5666
Unlike most other primates, chimpanzees have a 12- to 18-month delay between the age of first molar eruption and weaning. During this time they learn, through observation and imitation, how to “fish” for termites, crack open nuts, and perform other survival-related skills (Goodall, 1986).Read more at location 5669
Bogin (1999) proposed that human childhood emerges between infancy and juvenility and extends from 2 to 3 years of age (weaning in traditional societies) to the age of eruption of the first molar at 6 to 7 years.Read more at location 5671
The juvenile period is the same as that found in other primates and lasts from age 7 to the onset of the hormonal changes that begin reproductive maturation.Read more at location 5687
The 15- to 20-year period between weaning and reproductionRead more at location 5693
compared with 5 to 7 years in chimpanzees and our early ancestors,Read more at location 5694
the development of secondary sexual characteristics can be costly,Read more at location 5700
The costs include potential suppression of immune functions, increased risk of predation (e.g., for brightly colored males), and increased aggression from conspecifics, among others.Read more at location 5701
delayed maturation allows males to grow larger and gain the social and behavioral competencies needed to compete for and attract mates.Read more at location 5718
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)Read more at location 5721
The slower maturation of boys (see Garai & Scheinfeld, 1968; Hutt, 1972) appears to heighten their risk of early mortality but contributes to their adult height; later puberty results in longer legs in men than in women, relative to overall height.Read more at location 5724
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)Read more at location 5727
physical changes are the result of the increase in sex hormonesRead more at location 5740
also influence the expression of a wide range of social, behavioral, and sexual differences (Hayward, 2003). As examples, the hormonal changes influence the emergence of sexual fantasyRead more at location 5740
For boys, there is a marked increase in sexual behavior, especially masturbation,Read more at location 5744
There are a few sex differences in physical competencies prior to puberty,Read more at location 5747
During childhood there are small to moderate differences favoring boys in tasks such as grip strength, jumping distance, and running speeds, with large differences emerging during adolescence (Thomas & French, 1985); by 17 years of age, more than 9 out of 10 boys outperform the average girl in these areas.Read more at location 5748
As a result of the sex difference in leg length, muscle mass, and cardiovascular capacity, men can run faster, on average, than women (Deaner, 2006). By far, the largest differences in physical competencies are for throwing distance and throwing velocity (Thomas & French, 1985).Read more at location 5755
The sex differences in throwing skills are related to differences in the structure of the supporting skeletal system. Relative to overall body height, boys have a longer ulna and radius (i.e., forearm), on average, than do girls (Gindhart, 1973).Read more at location 5762
some theorists have argued that the physical differences evolved from the division of labor, including men’s hunting, rather than male–male competition (e.g., Kolakowski & Malina, 1974).Read more at location 5774
Across polygynous primates with intense male–male competition, there is a characteristic pattern of female and male growth.Read more at location 5779
Of course, some physical sex differences, such as the wider pelvic region in women, have evolved through natural rather than sexual selection. Once the large pelvis evolved, the waist-to-hip ratio that men find attractive emerged and began to be shaped by male choiceRead more at location 5782
Girls are also more physically flexible than boys and have an advantage in fine eye–motor coordination.Read more at location 5784
Kimura (1987) argued that the advantage of girls and women might be related to manipulating objects “within personal space, or within arm’s reach, such as food and clothing preparation and child care” (p. 145).Read more at location 5787
Related to the need to grow larger and stronger than girls is slower growth, beginning prenatally: Boys are born “premature” relative to girls. They have higher activity levels and higher basal metabolic rates than girls, resulting in higher caloric requirements for normal development.Read more at location 5803
Among other social and behavioral sex differences is higher risk taking and accidental injury in boys.Read more at location 5806
boys will be more sensitive—suffer more physical, social, and cognitive deficits—than girls when growing up in poor conditions, including poor nutrition, inadequate health care, or poor social stimulationRead more at location 5807
underlying vulnerability becomes most apparent in less than optimal conditions.Read more at location 5810
An analysis of 16,000 infant deaths in the United States between 1983 and 1987, inclusive, revealed that boys had a 38% higher mortality rate due largely to infectious disease during the 1st year of life (J. S. Read, Troendle, & Klebanoff, 1997).Read more at location 5821
Poor early physical development, exposure to parasites, frequent illness, poor nutrition, and inadequate social and cognitive stimulation have been shown to be related to poor long-term cognitive, academic, and social outcomes, even after controlling for SES and other confounds (S. P. Walker, Chang, Powell, & Grantham-McGregor, 2005). Sex differences are not typically reported in these studies, but boys’ risk of poor early growth and development is likely to result in an overrepresentation of boys and men with poor long-term outcomes.Read more at location 5830
Breast-fed infants of both sexes had normal IQ scores at age 8 years,Read more at location 5840
For the two groups that were not breast fed, the risk of cerebral palsy or other significant cognitive impairments was 5 to 6 times higher for boys who received standard formula compared with boys who received the supplement. The risk was not elevated for girls. In fact, the mean IQ of girls was in the average range and did not vary across conditions.Read more at location 5842
The flip side of boys’ greater vulnerability may be an enhanced potential to benefit more than girls from an enriched environment. This hypothesis has not been tested, although there were hints of such an effect in the Lucas et al. (1998) study.Read more at location 5861
M. Alexander (2003) hypothesized that some of the early sex differences in orientation toward people (more in girls) or things (more in boys) reflect the evolved skeletal structure of the visual system, specifically, biases in the what and where visual pathways. Prenatal and early postnatal exposure to androgens may enhance development of the latter and result in attentional and perceptual biases for processing spatial location and object motion.Read more at location 5874
Orientation toward other people is measured in terms of the duration of eye contact, empathy for others’ distress, and time spent looking at faces, among other behaviors.Read more at location 5886
“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” (p.Read more at location 5888
The sex difference in time spent looking at the face was small but consistent with studies of older infants (McClure, 2000).Read more at location 5891
By this age, girls might also have a better memory for faces and might be more skilled in discriminating two similar faces (e.g., J. F. Fagan, 1972);Read more at location 5893
sex difference in social organization will manifest in the attentional interests of boy (i.e., a focus on groups) and girl (i.e., a focus on individuals) infants.Read more at location 5903
Infant girls may react with greater empathy than infant boys to the distress of other people (M. L. Hoffman, 1977). Simner (1971) found that infant girls cried longer than infant boys when exposed to the cry of another infant, but there was no sex difference in reflexive crying when the infants were exposed to artificial noise of the same intensity.Read more at location 5914
Studies of the quality of social interactions between parents and infants also reveal that girls are more responsive, and perhaps more sensitive, to social cues than boys (Freedman, 1974; Gunnar & Donahue, 1980; W. D. Rosen, Adamson, & Bakeman, 1992).Read more at location 5924
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; Whiting and Edwards (1988) reported the same pattern with older children across cultures.Read more at location 5932
K. A. Buss, Brooker, and Leuty (2008) found that 2-year-old girls sought contact with and stayed closer to their mother than did boys in a fear-eliciting situation,Read more at location 5943
In an intensive naturalistic study of mother–infant interactions from birth to 3 months, Lavelli and Fogel (2002) found that girls spent more time in face-to-face communication with their mothers than did boys.Read more at location 5949
In Connellan et al.’s (2001) study of newborns, the sex difference in looking time was larger for the mobile than for the face.Read more at location 5959
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. (p. 382)Read more at location 5968
Baron-Cohen and his colleagues have been studying how prenatal exposure to testosterone influences sex differences in infancy and in older children (Baron-Cohen, Lutchmaya, & Knickmeyer, 2004; Lutchmaya, Baron-Cohen, & Raggatt, 2002a, 2002b).Read more at location 5982
Testosterone gets into the amniotic fluid by diffusion through the fetus’s skin and through urination.Read more at location 5985
Lutchmaya et al. (2002a) examined the relation between prenatal testosterone levels and the frequency with which 12-month-olds made eye contact with their mother.Read more at location 5986
Prenatal testosterone levels were, however, related to how often the boys made eye contact but not in a straightforward way.Read more at location 5989
Lutchmaya et al. (2002b) assessed the vocabulary of boys and girls at 18 and 24 months. At 18 months, the vocabulary of girls was more than double that of boys. At 24 months, the typical girl knew 40% more words than did the typical boy. At this age, higher prenatal testosterone levels were associated with lower vocabulary scoresRead more at location 5991
sex differences in play activities are a universal aspect of children’s behavior.Read more at location 5998
Infants begin to make sex-based discriminations (e.g., between male and female voices) as early as 6 months of age (C. L. Martin, Ruble, & Szkrybalo, 2002). By 18 months, they are beginning to categorize some activities as male typical and others as female typical (Eichstedt, Serbin, Poulin-DuBois, & Sen, 2002), and they talk about these as 2-year-olds (S. A. Gelman, Taylor, & Nguyen, 2004).Read more at location 6008
children are often more stereotyped than their parents and other adults (e.g., S. A. Gelman et al., 2004).Read more at location 6012
Girls and boys segregate into same-sex groups whether or not they are engaging in sex-typed activities (Maccoby, 1988), and children raised by egalitarian parents—those who actively discourage sex typing—have less stereotyped beliefs about sex differences than do children raised in other types of families, but their toy and play preferences are the same as these other children (Weisner & Wilson-Mitchell, 1990).Read more at location 6016
Rough-and-tumble play is more common among boys and play parenting is more common among girls.Read more at location 6029
The sex difference is most evident with groups of three or more children and in the absence of adult supervisionRead more at location 6042
The sex difference in rough-and-tumble play emerges by age 3 yearsRead more at location 6046
The sex difference in infants’ interest in groups (Benenson et al., 2004) continues into childhood and beyond. These studies confirm that boys organize themselves into much larger social groups than do girls, engage in intergroup competition once such groups are formed, form within-group dominance hierarchies, and show within-group role differentiation and specialization when engaged in group-level competition (Eder & Hallinan, 1978; Lever, 1978).Read more at location 6085
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.Read more at location 6096
Collaer and Hines (1995) concluded that 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” (p. 92).Read more at location 6108
excess levels of androgens (i.e., affected by congenital adrenal hyperplasia [CAH]),Read more at location 6115
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.Read more at location 6121
Throughout the world, girls are assigned child-care roles, especially for infants, more frequently than are boys (Whiting & Edwards, 1988). Girls also 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 more frequently than do same-age boys (Pitcher & Schultz, 1983).Read more at location 6181
In sum, a sex difference in play parenting favoring girls is found in both modern and traditional societies and in fact most other species of primate (Eibl-Eibesfeldt, 1989; Nicolson, 1987).Read more at location 6203
In this section, exploratory play refers to exploration of the ecology,Read more at location 6216
Across traditional societies, men travel farther from the home village than do women for many reasons, including finding mates, developing alliances with the men of neighboring villages, hunting, and intergroup warfare (Chagnon, 1997; K. Hill & Hurtado, 1996; K. Hill & Kaplan, 1988).Read more at location 6217
Women’s foraging, in contrast, typically occurs within the group’s territory,Read more at location 6220
Unlike women’s foraging for fruit or tubers, hunting requires an ability to track and predict the movements of evasive prey, human and nonhuman.Read more at location 6224
The sex difference is related in part to the sex difference in group-level competitive play and to a greater engagement of boys than girls in solitary running (Eaton & Enns, 1986; Lever, 1978).Read more at location 6231
During juvenility and beyond, boys have a larger play range than girls;Read more at location 6237
Whenever it emerges, boys not only engage in more locomotor activities over a larger range than girls but also explore and manipulate (e.g., build things, such as forts) the ecology much more frequently (Matthews, 1992).Read more at location 6241
ecological exploration is correlated with the ability to generate mental maps of the physical layout of the ecology but is not consistently related to other forms of spatial cognition, such as the ability to copy geometric figures (R. H. Munroe et al., 1985).Read more at location 6247
Scientists do know that 4-year-old boys, and perhaps even infant boys (D. S. Moore & Johnson, 2008; Quinn & Liben, 2008), have an advantage over same-age girls on some spatial tasks (Levine, Huttenlocher, Tayler, & Langrock, 1999) and that this advantage grows during juvenility, puberty, and adolescence (Matthews, 1992; D. Voyer, Voyer, & Bryden, 1995).Read more at location 6250
Levine et al. assessed 547 children from high-, middle-, and low-income backgrounds across second and third grades on two spatial tasks—map reading and two-dimensional mental rotation—and a syntax comprehension test. No sex differences were found on the syntax test, but boys from high- and middle-income families had an advantage on both spatial tasks. There was no sex difference on either spatial task for children from low-income families. In other words, low family income was associated with lower scores for both boys and girls on all three tests.Read more at location 6285
Boys engage in much more object-oriented play than do girls (Eibl-Eibesfeldt, 1989; Freedman, 1974; Sandberg & Meyer-Bahlburg, 1994; Sutton-Smith et al., 1963).Read more at location 6317
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 on the basis of, for example, color and shape.Read more at location 6336
As I review elsewhere (Geary, 1998b), there are sex differences in interest in and pursuit of careers that involve designing and working with mechanical objects, as in engineering and computer technology.Read more at location 6342
Chen and Siegler (2000) found that as early as 18 months of age, boys have small to moderate advantages over girls in several aspects of early tool use, as in using a hooked stick to retrieve a desired toy. Boys were better at applying tool-related knowledge learned in one setting to another setting, were more consistent in the use of tools across settings, and were more successful in the use of tools in problem solving. Without any hints from an adult, 79% of the boys and 31% of the girls were able to use such tools to retrieve the toy.Read more at location 6345
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. (p. 79)Read more at location 6386
The length of the developmental period has increased considerably during human evolution (Bogin, 1999), corresponding with an increase in the complexity of human social systems (Dunbar, 1993; Flinn, Geary, & Ward, 2005).Read more at location 6402
The seeds of children’s self-initiated activities appear to be found in the attentional biases of boy and girl infants.Read more at location 6411
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 found in other polygynous primates (Leigh, 1996).Read more at location 6415
longer forearm and greater upper body strength in men)Read more at location 6419
The sex differences in rough-and-tumble play, exploratory behavior, size of the play range, and the tendency of boys to form competitive coalitions and within-coalition dominance hierarchies are also consistent with an evolutionary historyRead more at location 6423
Girls’ more frequent engagement in play parenting follows readily from women’s greater investment in children and is consistent with sex differences found in other speciesRead more at location 6435
