Short Paper: Gender Dysphoria

The Early Development of Gender Differences Author(s): Matthew H. McIntyre and Carolyn Pope Edwards Source: Annual Review of Anthropology, Vol. 38 (2009), pp. 83-97 Published by: Annual Reviews Stable URL: https://www.jstor.org/stable/20622642 Accessed: 15-01-2019 12:57 UTC

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The Early Development of Gender Differences

Matthew H. Mclntyre1 and Carolyn Pope Edwards2 1 Department of Anthropology, University of Central Florida, Orlando, Florida 32816; email: [email protected]

2 Departments of Psychology and Child, Youth, and Family Studies, University of Nebraska, Lincoln, Nebraska 68588; email: [email protected]

Annu. Rev. Anthropol. 2009.38:83-97

First published online as a Review in Advance on June 17,2009

The Annual Review of Anthropology is online at anthro.annualreviews.org

This article's doi: 10.11467annurev-anthro-091908-1643 3 8

Copyright ? 2009 by Annual Reviews. All rights reserved

0084-6570/09/1021-0083$20.00

Key Words reproductive ecology, evolutionary psychology, patriarchy, dominance, temperament

Abstract

This article reviews findings from anthropology, psychology, and other

disciplines about the role of biological factors in the development of sex differences in human behavior, including biological theories, the developmental course of sex differences, and the interaction of biologi cal and cultural gendering processes at different ages. Current evidence suggests that major biological influences on individual differences in human gender, to the extent that they exist, operate primarily in early

development, during and especially prior to puberty. Biological effects are likely to be mediated by relatively simple processes, like temper ament, which are then elaborated through social interactions (as with mother and peers) into more complex gendered features of adult person

ality. Biological anthropologists and psychologists interested in gender should direct more attention to understanding how social processes in fluence the development and function of the reproductive endocrine system.

83

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INTRODUCTION

The purpose of this review is to summarize the

current evidence about the role of biological factors in the development of human gender over the life course. Rather than accept the distinction between biological sex and cultural gender, we employ the term gender very broadly to include both sex differences them selves and the cultural and biological processes that shape them. At the risk of over-reaching, we address between-sex differences, related within-sex variation, and broader features of

human social systems such as patriarchy. Our review begins with biological theory about gender and its application to the evolution of human sex differentiation, followed by a discussion of the developmental course of human sex differences and the various

biological and social gendering processes. As such, we also consider research from many

disciplines, including tentative consideration of sociocultural studies conducted from a

humanistic perspective. One important topic that we unfortunately leave out is sexuality.

BIOLOGICAL THEORIES ABOUT HUMAN GENDER Biological theory about gender (even if that term is not always used) refers to the exis tence, in sexually reproducing species, of two distinct reproductive strategies called parental investment and mating effort, which have been elaborated from Darwin's description of sexual selection. Parental investment encom

passes activities that are cosdy to parents but direcdy contribute to the growth or survival of offspring (Trivers 1972). For some species, this investment consists almost entirely of the

initial cytoplasm contained in the gametes, with

no further support provided by parents, but mammals have a number of additional parental functions including lactation. Parental invest ment is, in principle, common to both sexual and asexual reproduction. However, finding a mate is only relevant to sexual reproduction. In some species, finding a mate may involve travel

over long distances, displays of health or beauty,

84 Mclntyre Edwards

physical conflict with others who are seeking mates, or coercion of the potential mates them selves (Bateman 1948, Clutton-Brock & Parker

1992, Dewsbury 1982). For reasons that are not

fully understood (Kokko et al. 2006, Wade & Shuster 2002), parental investment activities of

many kinds are often, but not always, enacted by

one physical form, which is also often the form

with larger gametes, called female, and mating activities by another physical form, often with smaller, more motile gametes, called male.

In most mammals, virtually all parental in vestment is done by females and all mating ef fort by males, resulting in more notable sex differences than in other taxa (Clutton-Brock

1989, Orians 1969). The few exceptions are in species in which roles may be partially mixed and the sexes have less notable differences, and

which more often have mating systems charac terized as monogamous (Jarman 1983, Plavcan

2001). The primate order includes a relatively large number of monogamous species, often characterized by some level of male parental investment (Fuentes 1998). The characteriza tion of patterns of human parental investment and mating effort has been the subject of debate

among evolutionary anthropologists (Hawkes et al. 1991, 2001; Hill & Kaplan 1993; Kaplan et al. 2000), partly because of the substantial variation among even hunter-gatherer societies in foraging and marriage systems (Wobst 1978).

Geary (2002, 2006) has suggested that evolved human psychological sex differences in

clude {a) adaptations for child care in women and interpersonal dominance striving in men, both of which should be largely primitive evolutionarily in that similar sex differences are present even in nonprimate mammals, (b) adaptations for coalitional aggression in men, which might be homologous with chim panzees (Wrangham 1999), and (c) adaptations supporting the sexual division of labor, with par

ticular focus on hunting. The latter two do mains can be considered relatively more derived as they would have evolved later.

Feminine psychological adaptations for parental care have been linked to the psy chometric construct of empathy, and reduced

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empathy in men has been linked, in turn, to lower thresholds for aggression (Baron-Cohen 2002, Campbell 2006). Although dominance striving has been studied using a variety of tech

niques, it has not yet been closely linked with, or developed as, a particular psychometric con struct (Burgoon et al. 1998). Weak associations with narcissism, sensation seeking, instrumen tal motivations, and externalizing behavior are likely, and there may be a developmental link between low empathy and dominance striving, making femininity-masculinity at least partly unidimensional (Campbell 2006, Mclntyre & Hooven 2009). Theorists have proposed that the primitive sex differences in parental care and interpersonal dominance striving should be reduced in humans owing to relatively low levels of polygyny and high levels of male parental investment (Geary 2002). Despite such a reduction, it would be surprising not to find associations of basic psychological di

mensions of parental investment or male-male

competition with biological factors, such as sex hormones, given the established role of these factors in nonhuman sex differences.

Wliere interesting and surprising results might be found is in the interaction of these primitive

biological factors with social forces. How do the evolved processes related to biological gen der operate in different cultural and economic conditions?

Human sex differences in coalitional aggres sion and the division of labor are of particular interest to anthropologists because of their rel atively recent evolution and probable role in the origins of patriarchy (Smuts 1995). It is difficult

to predict how such biological systems might operate given the relative uniqueness, among all animals, of coalitional aggression and hunt

ing as sex dimorphic features. The psycholog ical construct that has been most commonly proposed as reflecting adaptations for coali tional aggression is called social dominance ori entation, defined as "the extent to which one

desires that one's in-group dominate and be superior to out-groups" (Pratto et al. 1994, p. 742), which shows substantial sex differences.

Although many physical sex differences may be

related to hunting ability, the psychological di mensions investigators have proposed to sup port sex differences in hunting and gathering in the literature are mostly cognitive, e.g., spa tial rotation and object memory, rather than re

lated to emotions or personality, in keeping with

an emphasis on cognitive changes in human evolution (Kaplan et al. 2000).

The role of the reproductive endocrine sys tem in human sex differences has been assessed

using several techniques. For concurrent ef fects in children and adults, concentrations of sex hormones can be measured in the blood

or saliva. For prenatal effects, several indirect

techniques have been used, including compar ison of children with congenital adrenal hy perplasia with controls, concentrations of sex hormones in amniotic fluid, and the relative

lengths of the index and ring fingers, abbre viated as 2D:4D (Cohen-Bendahan et al. 2005, Mclntyre 2006).

SOME EVIDENCE FROM ADULT MEN AND WOMEN Some evidence indicates at least a small role of

the reproductive endocrine system (especially androgens, like testosterone) in the ongoing maintenance of adult sex differences in em

pathy and dominance striving. For example, Deady et al. (2006) found a negative association of basal testosterone concentrations with ma

ternal ambitions in women, and Hermans et al.

(2006b) found evidence that an exogenous dose

of testosterone reduces empathy as assessed by unconscious facial mimicry. However, levels of testosterone in women vary over the course of

the menstrual cycle and even over the course of several days (Sellers et al. 2007). A number of studies have found associations between

basal or exogenous levels of testosterone and behaviors or attitudes associated with

dominance striving in men and women (Dabbs 1997, Wirth & Schultheiss 2007) and women alone (Cashdan 1995, Grant & France 2001, Hermans et al. 2006a).

However, as noted by O'Carroll (O'Carroll 1998), the interpretation of these results is

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complicated by the interesting, and better es tablished, observation that men's testosterone levels also fall in response to failures in domi nance contests of various kinds (Archer 2006, Dabbs & Dabbs 2000, Elias 1981, Mazur & Booth 1998), especially for men who strive more for dominance (Schultheiss et al. 2005). Archer (2006) proposed that this response is part of a primitive, evolved system by which men's willingness to enter dominance contests is informed by their previous record of success.

Recent evidence suggests that willingness to enter new contests is influenced by basal testos terone level (Mehta et al. 2008) and/or testos terone response to winning or losing (Carre &

McCormick 2008), and the effect is probably mediated by subtle physiological, rather than psychological, shifts (van Honk et al. 2004). Of course, hormones also have many other nonpsychological functions, including the reg ulation of muscle mass, which could be evolu

tionarily meaningful (Bribiescas 2001). In keeping with the view that male parental

investment increased during human evolution, a number of studies have identified possible suppressive effects of romantic relationships,

marriage, or fatherhood on testosterone lev els in men from several societies and, surpris ingly, lesbians (Gray 2003; Gray et al. 2002, 2004, 2006, 2007; Mazur & Michalek 1998;

Mclntyre et al. 2006; van Anders & Watson 2006, 2007). Many of these studies have re vealed interesting interactions suggesting that social and psychological factors might play sub

tle roles in regulating the suppression of testos

terone and mating effort.

In the case of coalitional aggression, little evidence indicates that hormones play a major role in sex differences. Burnham (2007) found

that men with higher testosterone reject low of

fers in an economic experiment called the ulti matum game. This could be interpreted simply as a reaction to a perceived threat to personal status or dominance. However, he also noted

a nonsignificant trend for men with higher testosterone to make larger offers in the game.

Together these trends might suggest a role for testosterone in the establishment of reciprocal

86 Mclntyre Edwards

relationships through moralistic aggression. However, as we noted, social dominance orien tation is the most established measure of group

level affiliation and a recent study found no association of social dominance orientation

with either testosterone or 2D:4D (Johnson et al. 2006, Mclntyre et al. 2007).

There are a number of established sex dif

ferences in the performance of Western adults

on a number of cognitive tests, including men tal rotation of shapes on which men perform better and verbal and object memory on which women perform better (Kimura 1999). How ever, Ecuyer-Dab & Robert (2004) have noted that there are two competing evolutionary in terpretations of these differences. They may be part of the primitive systems supporting sex dif

ferences in ranging and mate seeking (Gaulin & FitzGerald 1986, Jones et al. 2003), or they may be derived specifically to support hunting by men and gathering by women (Silverman & Eals 1992). A sex difference in throwing and targeting ability might be more recently derived

in response to male hunting (Westergaard et al.

2000), but the developmental trajectory of these abilities is obviously complex and includes fac tors such as size and strength, which are often ignored (Jones & Marlowe 2002). Some of the effects of androgens on mental rotation tasks may not be related to cognitive ability (Hooven et al. 2004), and these associations vary across

cultures (Yang et al. 2007). Given the limitations of evidence coming

from adult sex differences, it is useful to con

sider the role of biological factors in the ear lier development of sex differences in infancy and childhood. Researchers with both biolog ical and sociocultural perspectives have turned to studies of children to reduce the complex problem of personal life histories, which result

from the continuous transaction of physical, fa milial, and sociocultural processes with the de

veloping individual. However, we would take this a step further and argue that a better un

derstanding of biosocial interactions over the life course also provides valuable insights into how biological systems affect sex differences, al

lowing for the formulation of hypotheses about

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how sex differences might develop in a vari ety of sociocultural systems, including ones that

no longer exist (and ones that might someday exist).

THE DEVELOPMENTAL COURSE OF GENDER Edwards (1993) noted several differences be

tween the activities of boys and girls observed in many human societies:

1. From age three, girls spend more time working, whereas boys spend more time in play.

2. When playing in groups, children self segregate by sex, in addition to age.

3. Boys begin to spend more time than girls away from home and their mothers.

4. Girls engage in more infant contact and care.

5. Boys engage in more rough-and-tumble play than girls do.

6. Boys engage in more practice play with weapons and vehicles than girls do.

7. Girls engage in more grooming (real and play) than boys do.

Some cases, such as patterns of rough-and tumble play in boys, the tendency for play groups to segregate by sex, and the high fre quency of infant care by girls, demonstrate ap parent similarities to patterns observed in other

primates (Fagan 1993, Fairbanks 1993). Sex-different patterns of behavior begin

to emerge clearly in young children, during a period when biological sex differentiation is minimal, long before puberty and the develop ment of important secondary sex differences. Patterns of sex segregation, in which children play in same-sex groups, which accompany dif

ferences in the types of games played, emerge by five years old and often earlier in many so

cieties (Munroe & Romney 2006; Waiting & Edwards 1973, 1988). The psychologist Eleanor Maccoby (1998, 2002) has argued that this pattern of sex segregation plays a key role in the development of adult gender. Unfortunately the causes of sex segregation

remain poorly understood because most of the proposals have found limited support (Maccoby et al. 1984).

Adults play only a small role in directly en couraging sex segregation in Western societies (Aydt & Corsaro 2003, Maccoby 1998, Thorne 1993), and their role appears to be even more limited in many other societies in which chil

dren are under less supervision (Edwards 1993,

2000; Whiting & Edwards 1973, 1988). Even when adults try to encourage cross-sex play groups, children resist and quickly return to same-sex partners when adult supervision is re

duced (Serbin et al. 1977). These findings are generally supported by twin studies of the heri

tability of individual variation in gender-related

behaviors. Heritability studies allocate varia tion among three categories (genetic, shared environmental, and nonshared or other envi ronmental variation) based on differential sim ilarities between identical twins, fraternal twins,

and other siblings. The role of socialization by parents should mostly appear as shared envi ronmental. Studies of variation in adult and

adolescent gender role (as with most other personality dimensions) find moderate genetic effects (25%?50%) and large nonshared envi ronmental effects (30%?75%) with little room

for substantial effects of family-based social ization (Cleveland et al. 2001, Loehlin et al. 2005). Although recent studies in young chil dren have found larger shared environmental effects, especially in boys (Iervolino et al. 2005,

Knafo et al. 2005), this difference might be ex plained by their use of parent reports about their children's gendered behavior. Rather than

finding variation in gender-related behavior attributable to parental influence, they may have found variation in parental attitudes to ward their children's gender (a type of rater bias). This is especially likely because effects were stronger in boys, and American parents are more concerned, and have stronger views,

about their sons' than their daughters' gender appropriate behavior (Fagot 1977, Langlois & Downs 1980).

Older children clearly use cognitive ideas about gender (some of which may come from

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cultural norms) in their play; much of the cur rent thinking about sex segregation focuses on the importance of gender-related sociolinguis tic categories to children (Bandura & Bussey 2004, Kyratzis 2004, Martin & Ruble 2004) and argues that children choose playmates on the basis of their categorical understanding of their own gender and that of other children (Powlishta et al. 1993, Serbin et al. 2001). These

arguments follow Kohlberg's (1966) ideas about the importance of cognitive knowledge about gender, such as knowledge of its constancy. For

example, Martin & Ruble (2004) regard chil dren as young as four years old as "gender de tectives" who are actively trying to discover ex

aggerated stereotypes about men and women by listening to and observing adults and often make amusing errors. Children are motivated first by the knowledge that they are boys or girls

and that this will not change (gender constancy)

and second by a desire for in-group dominance.

This knowledge would imply a fascinating and very radical evolutionary change in which sex differences in adult behaviors, like empathetic

parenting by women and dominance striving by men, which appear similar to sex differences ob

served in many other species, nevertheless de velop in a completely novel way via cultural and cognitive processes with limited input from the

reproductive endocrine system. However, there has been some disagree

ment about whether cognitive knowledge about gender is necessary, especially in younger chil dren. Differences among children in their cog

nitive understanding of gender are unrelated to the sex of their play partners (Munroe & Romney 2006, Serbin et al. 1994). Munroe & Romney (2006) further argue that the term sex

aggregation should be used instead of segre gation because larger groups of boys, which may or may not include a few girls, break out to engage in rough-and-tumble play. Children who do not join these groups (mostly girls) tend to play alone, in dyads, or in smaller groups. This occurrence implies a primary role for differences between boys and girls in the types of games played rather than in the pre ferred sex of the play partners. Nevertheless, the

88 Mclntyre Edwards

limited evidence that exists (all from Western

children) about the relationship between play type preferences and sex-of-partner preferences has been mixed (Alexander & Hines 1994, Hoffmann & Powlishta 2001, Moller & Serbin 1996).

To the extent that segregation or "border work," as described by Barrie Thorne, is ac tively undertaken, scholars have debated its im portance and source. Thorne has documented the importance of borderwork in American preschools, and girls seem to play a more important role than boys do. That is, spaces are more likely to be declared off-limits to boys

than off-limits to girls (Aydt & Corsaro 2003, Thorne 1993). This observation also makes sense from Munroe & Romney's aggregation perspective if sex segregation is being driven partly by refusal on the part of some girls to par

ticipate in large-group, rough-and-tumble play

THE ROLE OF BIOLOGICAL SEX DIFFERENCES Secondary sex differentiation in mammals, which has usually been conceived as includ ing behavior, is guided primarily by sex hor

mones produced in the fetal gonads, especially androgens, and sex differentiation in humans (Hughes 2001) and other primates (Wallen 2005) is thought to entail similar processes. As such, human researchers employing a biological approach to studying behavioral gender in chil dren have focused primarily on prenatal andro gens (Cohen-Bendahan et al. 2005, Mclntyre & Hooven 2009). The evidence that childhood

sex differences are directly shaped by effects of sex hormones on the brain remains somewhat

weak, despite substantial research, but, to be fair, also in the face of substantial methodolog

ical limitations (Mclntyre & Hooven 2009). In particular, it is difficult, for technical and eth ical reasons, to directly measure prenatal hor

mones of fetuses in carefully designed studies. However, the possibility that sex segregation is driven partly by sex differences in play prefer

ences provides an opportunity for biological sex differences to influence gender development in

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subtler ways. The construct of temperament

(Kagan 2003, Rothbart 1989) might be one av enue for biological influences.

Infant boys and girls show small but consis tent differences in dimensions of temperament.

In particular, girls show higher fear when confronted with a novel stimulus, expressed as shorter latency to or threshold of crying (Else-Quest et al. 2006, Martin et al. 1997). Boys show a higher motor activity level (Campbell & Eaton 1999, Else-Quest et al. 2006). Some evidence indicates that individual

(and perhaps sex) differences in reactive fear (DiPietro et al. 2008) and especially motor activity (Almli et al. 2001, Eaton & Saudino 1992, Groome et al. 1999) begin to develop in utero. Infant boys also show greater at tention to mechanical crib mobiles than girls do (Connellan et al. 2000), but girls show greater attention to faces by 12 months old (Lutchmaya & Baron-Cohen 2002). These at tentional biases have been linked with toy pref

erences and characterized as a primitive mascu line attentional bias to movement and feminine

attentional bias to people (Alexander 2003). Infant temperament has been further linked

with measures of personality later in life that are

salient to the dominance/empathy paradigm. Infants with greater fear reactivity develop both greater empathy and social anxiety as toddlers (Spinrad & Stifter 2006). Although infant tem perament has not been studied in relation to later dominance orientation per se in humans,

male rhesus monkey infants with higher activ ity levels rise higher in dominance hierarchies later in life (Weinstein & Capitanio 2008), and human infants displaying lower fear reactivity (Burgess et al. 2003) and physical activity levels

(Canals et al. 2006) display more externalizing behavior as children.

Increasing evidence shows that variation in infant and childhood temperament is influ enced by genetic and hormonal factors. Greater attention to faces has been associated with lower

prenatal testosterone concentrations measured in amniotic fluid (Lutchmaya et al. 2002). Girls

with congenital adrenal hyperplasia show a preference for male-typed toys such as trucks

(Meyer-Bahlburg et al. 2004, Pasterski et al. 2005).

These temperamental differences or differences in their effects on later gender development might also result, at least in part, from differential parental treatment of infant boys and girls. However, evidence to date about differential treatment of infant

boys and girls has come largely from Western societies and yielded mixed results. The body of findings does not present a strong case for the effect of infant sex or gender label per se on parental treatment, particularly in younger infants (Biringen et al. 1999, Jacklin et al. 1984,

Lytton & Romney 1991, Robinson et al. 1993, Stern & Karraker 1989). In American infants, individual variations in infant temperament are

also almost entirely explicable by genetic vari ation (Goldsmith et al. 1999), and the presence

of analogous sex differences in nonhuman pri mates (Alexander & Hines 2002, Hassett et al.

2008, Herman et al. 2003) probably argues against a major role of socialization. Many findings of caregiving variations in treatment of girl and boy infants do not remove variance

contributed by what the infants themselves elicit on the basis of their activity levels, capac ity for mutual gaze, emotional expressiveness, or other temperamental differences. When child characteristics are included, gender differences in maternal behavior are reduced.

For instance, Moss (1967) found that mothers of three-week-old infants were observed to

hold, look at, arouse, and stimulate physically sons more than daughters; however, sons were

more irritable and when infant irritability was covaried in the analyses differences in holding and looking dropped out. Some researchers (e.g., Donovan et al. 2007) have attempted to remove child-temperament effects by devel oping experiments in which adults respond to a stranger infant (or photographs), but these studies have the weakness of measuring parental behavior in a nonnatural situation in

which they are struggling to read the (often ambiguous) signals of an unknown child and, hence, are in a situation in which they would be expected to be most guided by expectations and

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stereotypes rather than by meaningful child cues. In sum, although it is widely assumed that adult perceptual sets and gender stereotypes influence caregiver behavior toward male versus female infants, independent of the child's characteristics, such differences have

not been clearly demonstrated. The issue is complicated even further by the fact that infant

girls and boys might react differently to the same caregiver behaviors in light of their own individual differences, such as differences in

temperament. For example, boys tend to react more strongly than girls do to differential ma ternal sensitivity (Biringen et al. 1999, Warren & Simmens 2005, Weinberg et al. 2006), particularly if the infant is difficult (Warren & Simmens 2005) or in challenging social contexts (Weinberg et al. 2006). This pattern of transactions between mother and infant

could lead to complex amplification of initially small differences in either infant or caregiver behavioral variation, which might also be influenced by the social context, for example, by the extent to which fathers and siblings are involved in care of the child. Bornstein et al.

(2008) found that mothers from metropolitan regions were more emotionally available than were those from rural regions, and sons, but not daughters, from metropolitan regions were more responsive than were those from rural regions. These findings suggest that key developmental systems are highly sensitive to sociocultural and/or economic factors, which

as Beatrice Whiting (1976) suggested are complex "packaged variables" that need to be broken down and analyzed in terms of compo nents that really matter. Developmental studies

about how biological sex differences in children operate in varied social contexts will continue to be informative, particularly as societies are radically transformed by globalization.

REFOCUSING BIOLOGICAL STUDIES OF HUMAN GENDER

We suggest that biological studies of gender can be benefited by paying more attention to (a) infancy and childhood and (b) broader

po Mclntyre Edwards

social processes. Ours is certainly not the first call for complex descriptions of biocultural in teractions (Edwards 1993; Mclntyre & Hooven 2009; Worthman 1993, 1995). Biocultural in teractions happen repeatedly over the course of

life to subjects that are themselves the products of previous interactions. Therefore, we should not be tempted to think that even the bod ies of infants are, so to speak, all biology and no culture. Paying attention to broader social processes is a more difficult proposition and will force evolutionary anthropologists and psy chologists to gently set aside the reconstructed

Paleolithic society in favor of the actual soci eties in which they work.

For example, some research has shifted greater focus to the individual interests and agency of the child, and adult, actors involved

in the day-to-day enactment of gender (Aydt & Corsaro 2003, Knobloch et al. 2005, Kyratzis 2004) . Recent analyses particularly from hu manistic (Montgomery 2005), but also from bi ological (Crittenden & Marlowe 2008, Hrdy 2005) , frameworks have argued for greater at tention to the economic value of girls' labor and the roles of parental power and coercion in the establishment of gendered patterns of play and work. This approach might allow us to think about the effects of infant tempera

ment in different ways. The temperaments of young girls, which are characterized by greater empathy, social anxiety, and social attention, might be considered more suitable for doing work around the house and caring for siblings, or they might be more cooperative with moth ers owing to their greater physiological matu rity and/or same-sex identification (Whiting & Edwards 1988). Boys, however, might gain agency by virtue of their high levels of physical

activity and perceived irresponsibility, freeing them from some household responsibilities.

Similarly interesting questions arise with regard to social institutions outside of the home.

Whereas evolutionary anthropologists and psy chologists have been looking for associations between testosterone and dominance striving,

Mclntyre & Hooven (2009) argue that the re ality in Western societies is far more complex

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and fascinating. Boys with high testosterone are dominant over their peers in adolescence (Tremblay et al. 1998), but as they leave the world of peers and family and come into con tact with other social institutions the trajecto ries of their lives are more often characterized

by delinquency (Rowe et al. 2004), criminal ac tivities (Archer 2006, Archer et al. 1998), lack of

education (Dabbs & Dabbs 2000), and low so cial prestige (Dabbs 1992, Johnson et al. 2007). Even if this tendency results from a mismatch between ancestral and current conditions, it is

a mismatch worthy of careful study, if for no other reason than it is likely to affect the re sults of any research that we conduct. It is not

possible to escape these questions by studying simpler societies.

Our suggestions are similar to those of Goodman & Leatherman's (1998) Biocultural Synthesis, which encourages more study of the effects of political and economic systems on hu

man biology but applied to reproductive biol ogy and sex differences, in addition to nutrition

and growth. We also echo some of Bourdieu's observations about patriarchy. "The biological

appearances and the very real effects which have

been produced, in people's bodies and in their brains, by a long collective labor of socializa tion of the biological and of biologization of the social combine to overturn the relationship between causes and effects..." (Bourdieu 1990,

p. 12). We further argue that biological work re

lying on insights from humanistic social science

research, far from being an alternative to evo lutionary explanations (Bribiescas 2001, Ellison 2003), can also make the social sciences more useful contributors to biological and evolution ary thinking about gender in other species. Recent work in a number of species, includ ing those whose biological gender can reverse during life, has highlighted the central roles of social stratification, power differences, and

coercive or violent tactics in explaining pat terns of sex or gender role change (Anthes &

Michiels 2007, Black & Grober 2003, Grober & Rodgers 2008, Rodgers et al. 2007). As these processes are particularly elaborated and var ied in humans, the human sciences might offer rich and surprising theoretical insights, even for

ichthyologists.

SUMMARY POINTS

1. For the most part, biological influences on psychological sex differences probably occur early in life via simple mechanisms such as temperament.

2. Small sex differences in temperament interact with social factors in complex ways that

might result in further psychological differentiation but not always in ways that are clearly

predicted by existing evolutionary theories.

FUTURE ISSUES

1. How do sex differences in physical maturity and social competence influence how children

interact with one another or are treated by adults, and what are the biological or social causes of these sex differences?

2. More evidence is needed about the social processes and individual differences that result

in sex segregation in children. In particular, the relative importance of gender cognitions

and activity or play-style preferences remains unclear.

3. Longer-term longitudinal studies would help us to understand the role of early processes

(including sex segregation) on the further consolidation of psychological sex differences

during puberty and adulthood.

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DISCLOSURE STATEMENT

The authors are not aware of any affiliations, memberships, funding, or financial holdings that might be perceived as affecting the objectivity of this review.

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