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4 Biology and Behavior

The data on how parents treat boys and girls reviewed in chapter 2 and

on how children learn about sex differences reviewed in chapter 3 raise

the question of whether all the sex differences we see are differences that

adults construct. What about built-in sex differences? In this chapter and

the next I summarize what we know about intrinsic differences between

males and females and consider the implications of those differences. One of the problems involved in discussions of sex differences is that

many people view biological influences as all-powerful and final. If a sex difference-such as mathematical ability-can be shown to have a bio-

logical component, it seems immutable and eternal. Our perception of

immutability stems, I think, from an inaccurate understanding of what it

means for a characteristic to have a biological basis. We interpret a bio-

logical sex difference as a difference that is a direct result of having a

certain set of sex chromosomes. Since we cannot change the set we have,

we are stuck with the differences that go along with being XX or XY.

This everyday interpretation of biology, however, is radically uninformed. Although chromosomal differences may be qualitative, none of the ensu-

ing differences are. Biology leaves us a lot of room to manoeuvre. Biology

is not necessarily destiny. In a way, it is odd that we should interpret sex differences as immutable,

when we do not accept biology as destiny in other aspects of human exis-

tence. For example, biology sets limits on the human life span, but we need not and do not for that reason accept a short average life span as

o~r fate. As a society, we put forth great efforts to understand the mecha- nisms of health and to cure disease and illness. We practice good hygiene,

we 11 westigate the roles of diet and exercise, we sterilize, we vaccinate, we

imx:ulate, we medi1:ate. Although we agree that no one can live forever,

wt· su1:1:essfully invest tremendous resources in trying to live longer. Our

su~·1:ess is evidence of the !ability and complexity of the processes that

underlie life and death and of humans’ ability to intervene in those

pro1:esses. I propose that we adopt the same attitude toward biological sex differ-

en1:es. Biologi1:al sex differences arise through the actions of sex hor-

mones operating in our physical and social environments. Like the

pro1:esses relevant to life and death, those relevant to sex differences are

intrirnte and susceptible to change. Biology sets limits, but we need not

for that reason accept the differences we see as immutable. We have good

evidence from cultural, situational, and temporal differences that the dif-

ferences arc not immutable. In this 1:hapter and chapter 5, I have applied my own interpretation to

the resear1:h findings on physical, behavioral, and cognitive differences,

highlighting some findings the researchers themselves did not and deem-

phasizing others. Many different points of view on these data exist: while

some people deny the existence or importance of a biological influence,

others refer all differences to biology. My own position is that biological

influcn1:es exist and are important, but are only part of the story.

Sex Hormones

When we talk about “biological” sex differences, we are talking about

the influence of the hormones responsible for differentiation of the sexes.

The sex chromosomes themselves (XX for females and XY for males) do

not have an automatic and rigid set of consequences. They do not act like

on-off switches. Nor are they, even, the immediate agents of sex differen-

tiation. They exercise their influence through the differing hormonal de-

velopments they set into motion. Those developments in turn have their ‘ ‘

effects within one or another context and are inherently variable. The

same thing is true of genetic effects in general (see discussion in Neisser

et al. 1996). So, although we cannot change our chromosomes, they are, in a sense, irrelevant.

Biology and Behavior 69

Mammals with XX chromosomes develop ovaries that secrete one set

of hormones; mammals with XY chromosomes develop testes that secrete

the same set of hormones but in different amounts. The action of those

hormones underlies further physical sex differentiation. The three main

types of sex hormones-androgens, estrogens, and progestins-occur in

both sexes; the sexes differ, sometimes dramatically, in the amount of each hormone they produce. For example, testosterone levels in college-

age males and females are very different. The male-to-female ratio has

been reported to be as high as ten to one, with no overlap between the two groups (Udry & Talbert 1988). Average testosterone-concentration

levels in saliva have been reported to be about three times as high in males

as in females, again with no overlap between the two groups (Gouchie & Kimura 1991 ). Within each sex, however, there is a considerable range of

secretions of each hormone.

Within-Sex Variability It is in the realm of behavior that the variable effects of sex hormones within each sex are clearest. The effects vary depending on the social-

psychological context they occur in, for nonhuman animals as well as humans (Buchanan et al. 1992; Collaer & Hines 1995). Hormonal ef- fects, in other words, are context-dependent. Even in rats, the effects of

sex hormones differ, depending on the sort of handling the animals re- ceive, the type and amount of stimulation provided by their environment,

and the kind of maternal care they receive. (See Collaer & Hines 1995, 1

for a summary of these and other effects of gonadal steroids.) Although we commonly speak of the environment modifying or moder-

ating or influencing hormonal effects on human behavior, I find that ter-

minology misleading. It suggests a primacy and a univocality for the action of sex hormones that do not exist. It would be similarly inappro- priate to label environmental effects on human behavior as primary, and speak of hormones as modifying or moderating those environmental

effects. Rather, hormonal and environmental effects act together-they coact-to jointly influence people’s and animals’ traits and behaviors

(Moore 1985). Within the realm of behavior, there is no such thing as a

pure hormonal effect, because there is no such thing as a zero or neutral

70 Chapter 4

environment. Equally, there is no such thing as a pure environmental ef­

fect, because there is no such thing as zero or neutral hormones. To under­

stand any behavior, it is necessary to understand the contribution of both

hormones and the environment. We can get a small idea of the complexity of the interactions between

sex hormones and the environment in adult humans by looking at how

time of year and level of circulating testosterone affects men’s ability to

rotate objects mentally (Kimura & Hampson 1994). In the fall, North American males have higher levels of testosterone than they have in the

spring; they also have lower scores on spatial rotation tests than they do in

the spring. As individuals, males who are below the average in circulating

testosterone also score higher on spatial rotation tests than those whose

levels are higher than average. Men’s cycles occur on a daily basis as well.

When testosterone levels are higher, in the morning, men perform more

poorly on tests of spatial rotation. (See Kimura 1996 for a summary of

this and other experiments involving hormonal effects.)

Males’ spatial rotation scores are linked to testosterone level in ways

that our gender schemas would not predict. Gender schemas represent

testosterone as contributing to masculinity and label certain kinds of

skills-like map reading and mental rotation-as masculine. But the data

show that, for mental rotation, having less testosterone is better than hav­ ing more-up to a point.

Being below the median is better than being above it, but for males

being very far below it is worse than being somewhat below. There ap­

pears to be a curvilinear relationship between testosterone level and

scores on mental rotation tests. Aging males, who have very low testoster­

one levels, have lower scores on tests of mental rotation than college-age

males whose scores are below the median (Kimura 1996). Women’s tes­ tosterone levels are very low.

The data on men’s variability show that our notions of people’s abilities

are oversimplified. It is obvious that the component of spatial ability that

is due to knowledge of spatial relations cannot fluctuate over a twenty­

four-hour period. Still, spatial ability undoubtedly involves more than

knowledge. It also requires registering the spatial properties of objects,

storing that information, and then mentally rearranging the objects to see

71 Biology and Behavior

what they would look like from different angles. Those processes appear

vulnerable to changes in testosterone level.2 Researchers do not know whether male-female differences in spatial abilities are due to underlying

differences in knowledge or to processing differences. There are at least some spatial tasks where males perform better than

females that seem unrelated to testosterone levels. For example, although male homosexuals and heterosexuals have comparable testosterone levels,

homosexuals generally perform the task of throwing a ball at a target less

well than heterosexuals do (Hall & Kimura 1995; Kimura 1996).3

Female scores on spatial-orientation tests fluctuate somewhat across

the menstrual cycle and show some individual variability. When estrogen

levels are very high, females score worse on some-but not all-tests of

spatial ability than they do when estrogen levels are moderate or low (Hampson 1990a, 1990b). For women, there is also some indication of a

curvilinear relation between estrogen and spatial skills, just as there is a

curvilinear relation between men’s testosterone levels and spatial perfor­

mance (Hampson 1990b). At the same time, female monthly fluctuations are considerably smaller

than male seasonal variations. The hormonal effects in women are rela­ tively small and do not show the other patterns we would expect to see

if estrogen level alone controlled behavior. For example, undergraduate

women who major in science score better on tests of spatial ability than do women who major in other fields. If estrogen level predicted spatial ability well, women in science would have estrogen levels different from

other women, but they do not (Hampson 1990b). Females’ levels of testosterone, however, are related to scores on at least

some tests of spatial ability, even though those levels are much lower than males’ levels. Females with above-average testosterone levels for females

score as well on spatial tests as males with below-average levels for males (Gouchie & Kimura 1991). That is so even though the testosterone levels of these women are only half as high as the male levels. A similarly low level in males would be associated with poor spatial performance. Per­

haps because of differences in brain structure, a very low testosterone

level in males-a level too low to mediate good spatial performance-is

adequate for females to attain good spatial performance.

72 Ch<1pter 4

The resean:h on hormonal influences on behavior thus contradicts any

notion we might have that sex chromosomes determine our destiny, or

that hormones act independently of the environment in which they are

embedded. Hormones are relevant influences on our traits and behaviors,

but their effects are labile. Evaluation of the effects of hormones on the

specific physical and behavioral traits discussed in the remainder of the

chapter confirms this general conclusion.

Variation in Trait Expression

We can illustrate the relations between biology and trait expression with

an example unrelated to sex differences. Although some hearts are intrin-

sically less efficient pumps than others, diet and exercise can increase their

efficiency, just as a poor diet and lack of exercise can decrease it. The

environment influences how well a heart does its work. For most traits,

biology establishes a continuum of values rather than a specific value. In

the case of hearts, biology establishes the usual range within which they

operate, rather than a specific efficiency value that holds for all hearts.

In the same way, some sex differences change in value from one situa-

tion to another, while others are relatively invariant over an individual’s

adult lifetime. Variability that occurs within an individual is variability

that can be altered.

Reproduction

Most people’s everyday experience is that there are two, clearly different

sexes. Male and female genitalia differ markedly. (Ambiguous genitalia

exist but are rare.) A concomitant functional difference is as marked as

the anatomical difference: females are capable of birth and lactation and males are not.<

Unlike other sex differences that we pay attention to, anatomical differ-

ences hold to the same degree across all cultures, across all situations, and

across time. Cognitive and behavioral sex differences, on the other hand,

exist along a continuum, with a great deal of overlap between the sexes

and a great deal of variability within each sex. Only in the reproductive

area are the differences qualitative. All other differences between the

73 Biology and Behavior

sexes are quantitative and inherently variable, though the type and size of

the variability differs from trait to trait. In all the physical and behavioral

characteristics we consider here, the variability within each sex is greater

than the (average) difference between the sexes.

Height In all cultures, males are on average taller than females-a difference that

has persisted for thousands of years. Nonetheless, even with height, some

types of variability do exist. Within each sex, some people are taller than others. Some females are taller than others and are also taller than some

males. The difference between the extreme heights within each sex is

larger than the difference in the average height between the sexes. Height also varies from individual to individual as a function of nutri-

tion-which is environmentally determined-and development. Among

children born in the same place at the same time, those who are well

nourished grow taller than those who are poorly nourished. In old people,

height decreases because of bone loss. Changes in the social environment, however, do not affect adult height.

People are not, for example, taller at work than they are at home. Even

here, though, perceptions of height can be manipulated: people may wear

shoes that increase their apparent height, or they may, by slouching, re-

duce it. Compared to other characteristics we consider later in the chap-

ter, however, individual height has only limited variability.

Voice Pitch Other physical sex differences show much more mutability within an indi-

vidual. Consider voice pitch. Like height, it has a underlying biological

basis: on average, males have larger and more muscular larynxes, and

larger and more resonant throats. The result is that, within any culture,

males’ voices tend to be deeper in pitch than females’. Unlike height, how-

ever, an individual’s adult voice pitch is not fixed. We all possess a fairly

~ide range of possible pitches. Pitch is responsive, for example, to emo-

tional stress. If the expression of a trait can vary within an individual from one mo-

ment to the next, it follows that all other types of variability can also

l,

1.

74 Chapter 4

exist. Again, voice pitch is a good example. The size of the sex difference

varies from culture to culture. In Italy, for instance, the difference is less

than it is in the United States (McConnell-Ginet 1983).

Unlike height, voice pitch could be, on average, almost the same in

males and females, because of the variability within an individual, and

because of the responsiveness of pitch to social and cultural factors. Fe­

males could speak nearer the lower end of their range and males nearer

the upper end. (British Prime Minister Thatcher reportedly profited from

lessons in lowering her voice pitch so that she would appear more author­

itative.) By the same token, the average difference in pitch between the

sexes would be extremely high if females spoke at the upper end of their

range and males spoke at the lower end of theirs. Thus, even for traits

with a clear underlying physiological connection, societies can “choose”

how extreme the behavioral differences will be. Traits whose expression

is inherently variable-like voice pitch-rather than more rigid-like

height-allow for the largest cultural and gender differences.

In essence, what matters in evaluating sex differences is not the existence

of a biological connection but the inherent variability in the expression

of a trait. Height and voice pitch both have a clear biological connection,

but adult height cannot be directly influenced by culture, whereas voice

pitch can. If a trait is inherently variable, like voice pitch, the expression of it is malleable. The variability makes it correspondingly difficult to as­

sess the influence of biology. For some sex differences, biology limits the range of a behavior; further, the midpoint of the range for one sex may

be higher or lower than the midpoint for the other sex. The differences in

the midpoints may or may not have practical significance, depending on

how great the range is for each sex. To assess the sources of sex differ­

ences we need to know both whether a given behavior has a direct biologi­ cal connection and, if it does, whether the proper analogy is to height or

to voice pitch. Yet we often do not know. For the traits that could be

relevant to professional achievement, voice pitch seems a better analogy,

because those traits are responsive to cultural, social, and psychological influences.

When I speak of influences, I am not suggesting that the environment

shapes or molds children or adults. Rather, people form nonconscious

75 Biology and Behavior

hypotheses based in part on the data they receive from the environment. Those hypotheses about sex differences-gender schemas-then guide

people’s behavior. When a trait is malleable, gender schemas can affect

its expression.

Behavioral Traits and Professional Achievement

None of the traits discussed so far have any direct connection to people’s

intellectual or professional abilities. Height and voice pitch have nothing to do with competence, even though we tend to think of competent people as tall and having a low voice. Other differences, however, have

more potential significance.

Activity Level Males are more active than females, a difference that appears to increase throughout childhood and into early adolescence (Eaton & Enns 1986).

6

Since few studies have examined activity differences after the age of fif- teen, we do not know whether the differences persist throughout the life span or begin to diminish at some point. Higher activity levels could indi- rectly cause more exploration and, even more indirectly, be related to

achievement. But that is speculation. The existence of infant sex differences in activity level has not gone

unquestioned. Relatively few studies have looked at infant sex differences; those that have found no differences in global activity level, although there is some suggestion that infant boys’ movements are more vigorous than girls’. One careful study comparing girl and boy babies at two and a half months and at five months found almost no differences either in activity level or in vigor of movement ( Cossette, Malcuit, & Pomerleau 1991). The few differences observed were small and could have been due

to chance. Even fewer studies have looked at prenatal activity levels, but one or

two have found differences. If the existence of in utero sex differences were confirmed, that would suggest very strongly that activity differences

are indeed hormonally initiated, as male and female fetuses receive no differential social stimulation (Eaton & Enns 1986). The fact that sex differences in activity increase as development proceeds is compatible

76 Chapter 4

with two possibilities: (1) that high activity is encouraged in boys and

Jisrnuraged in girls; or (2) that activity levels mature and follow different

paths of biological development in boys and girls. One indication that differences in activity level are initiated by hor-

monal differences comes from data on children who experienced overly

high levels of androgens in utero. In a relatively rare genetic disorder (oc-

curring approximately once in every ten thousand births), an enzyme de-

ficiency leads to overproduction of androgens by the fetus’s adrenal

glands. The disorder, commonly called congenital adrenal hyperplasia

(CAH), is usually visible in chromosomal females soon after birth, be-

cause the genitals are masculinized. As newborns, the girls typically re-

ceive genital surgery and hormonal treatment to counteract further effects

of the androgens. Both boys and girls with CAH have very high levels of

circulating androgens. Detection at birth is more difficult in boys, and

solid data on the effects on boys are not available.

Most studies have found that girls with CAH have higher activity levels

than normal girls, levels that are similar to those of normal boys (see dis-

cussion in Collaer & Hines 1995). Rough-and-tumble play, which is much more characteristic of boys than of girls, is also frequent in girls

with CAH. A study comparing three- to eight-year-old girls with and

without CAH found that the girls with CAH spent more time playing

with toys associated with males, such as vehicles and construction

toys, than the others did (Berenbaum & Hines 1992). A study of older children produced similar results (Berenbaum & Snyder 1995). The girls with CAH spent the same amount of time with masculine toys as boys

without CAH did. Toys typically associated with boys may lend them-

selves to high-activity play more easily than do toys typically associated with girls.

To summarize, activity level seems like a good candidate for a hormonally

influenced sex difference. Data both from boys and girls with normal pre-

natal hormonal development and from girls who experience excess an-

drogens in utero suggest hormonal involvement. But interactions with social-psychological factors cannot be ignored. We know that parents

treat boys and girls differently; parents of girls with CAH may be more

F

Biology and Behavior 77

tolerant of active play than the parents of girls with no history of atypical

hormonal production (see also chapter 5, n. 8).

There is no evidence that a high activity level or rough-and-tumble play

is either necessary or sufficient for later achievement. If either were im- portant, changes in the child’s social environment could increase girls’

participation.

Aggression Another candidate for an intrinsic sex difference in behavior is hostile

physical aggression-defined as touching another person with the inten-

tion of inflicting harm, with or without the desire to obtain some goal.

There is a higher incidence of such hostility in boys, although the sex

differences appear to develop later-around the age of three-than dif- ferences in activity level (see review in Berk 1994).

Sex differences in aggression increase throughout adolescence, then di-

minish, apparently because society looks less and less favorably upon

physical aggression as children become adults. Overall, studies reliably

show males to be more physically aggressive than females, with little or no difference from one generation to the next (Eagly & Steffen 1986; Knight, Fabes, & Higgins 1996). Cultures vary in how often people ex- press hostile physical aggression, but there is usually a sex difference, with

males being more active and aggressive. Females of one culture may well be more physically active or aggressive than the males of another culture,

but they are likely to be less physically aggressive than males of their own culture. Even that difference, however, is not universal, as Mead demon- strated (1935).

Although males are usually more aggressive than females, the actual level of individual aggression is inherently highly variable. As with other

characteristics, the range of aggression within each sex is larger than the average difference between the sexes. Interestingly, boys seem less prone

to aggression if they have had experience looking after younger children (see Maccoby & Jacklin 1980; Tieger 1980). Hostile behavior can appar- ently be reduced by practicing nurturant behavior.

Most girls in most cultures either have actual child care experience, or the surrogate experience of playing with dolls. Unlike boys, girls may thus

hr Jiwrtnl from aggrrssion throughout their childhood. If more boys \\’t”rt’ giwn thr opportunity to take care of young children, reduced ag-

grc~s1on might rrsult. Again, thrrr is no evidence that a tendency to physical aggression is

cithrr nrcrssary or sufficient for later achievement. I emphasize physical

aggrt·ss1011 hrcause that is where sex differences are most marked. Aggres-

siw thoughts anJ words, on the other hand, are more equally shared be-

twt·rn the sexes. StuJirs of white college-age males from the North or South of the

UnitrJ States Jemunstrate that a tendency to aggressive solutions is in-

tluenceJ by an individual’s subculture. In a series of experiments, re-

st·archers arranged for the young men to be mildly insulted by a peer

(Cohen, Nisbett, Bowdle, & Schwarz 1996). After the insult, the students participated in various tasks designed to measure their aggression. North-

rrners and southerners responded differently in those follow-up tasks. In

one task, the students were asked to complete a story in which a male

studrnt’s girlfriend complained to him that a male acquaintance had tried

to kiss her. Southerners who had been insulted completed the story more

aggressively than northerners who had been insulted, as well as more ag-

gressively than southerners or northerners who had not been insulted.

The researchers concluded that, for southern males, aggression and no-

tions of “honor” are interwoven: southerners may be easier to offend

than northerners and may also respond more aggressively to a perceived

offense (Cohen ct al. 1996). (Naturalistic data also indicate that white

southern males arc more likely to be violent in certain settings than their

northern counterparts (Cohen et al.)) For our purposes, the important

aspect of the experiment is its evidence that aggression is not just culturc- bound, but subculture-bound.

Aggression also varies from situation to situation (Eagly & Steffen 1986 ). A review of sixty-four experimental studies of aggression demon-

strates that, depending on the type of provocation to aggression, men may

be either more or less aggressive than women (Bettencourt & Miller 1996 ). Men, for example, react more aggressively than women do when

their intelligence is insulted. Women, however, react somewhat more ag- gressively than men to other types of insults.

79 Biology and Behavior

The wide variability in the expression of aggression in each sex, and

the concomitant variability in the presence and extent of sex differences

in aggression, indicate that cultures and subcultures play a major role in

determining people’s reactions to frustration or provocation. As it does

with voice pitch, the variability suggests that there is great flexibility in the

amount of aggression males and females express. No particular amount is

natural or inevitable for either sex. Males may exhibit very little aggres-

sion or a great deal; females exhibit a similar range of behavior. Given that inherent variability, I interpret the pervasiveness of a sex

difference across cultures as an indication that the social arrangements of

many cultures have something in common, something that is conducive

to the development of similar gender schemas that set sex differences in

place and maintain them. Such an interpretation does not rule out a con-

tribution from sex hormones but, rather, suggests that the hormonal con-

tribution neither dominates nor effaces the environmental influences.

Putting together the data on activity-especially rough-and-tumble

play-and on aggression, we can see how they might be related. A high

activity level may be suppressed or encouraged, directed toward objects

or toward people, intended to help others or to hurt them (see discussion

in Parsons 1982). Hostile aggression is not a necessary outcome of high

activity. Girls with CAH are not more aggressive in their behavior than

other girls, even though their activity levels and liking for rough-and-

tumble play are greater. Although girls with CAH do score somewhat

higher on questionnaires asking about aggressive tendencies, they do not

act out those tendencies (see summary in Collaer & Hines 1995). Nonetheless, high activity level may be a predisposing factor to aggres-

sion. If combined with anger and lack of cultural prohibitions against the expression of anger, high activity can result in hostile aggression. Anger

may be a natural human emotion but there is no natural way to handle . ‘ it. As the North-South differences suggest, an individual’s response is af-

fected by beliefs about what is appropriate. That hormones predispose but do not determine aggression is also ap-

parent from changes that occur during puberty, when boys’ testosterone t levels increase greatly. Although the hormone increases in all boys, moS

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1

80 Chapter 4

boys in most situations do not become more aggressive (Buchanan et al.

1992). Boys with higher levels of testosterone do seem to display more

physical aggression if they feel threatened or perceive a situation as unfair.

And boys who had behavior problems before puberty may have those

problems exacerbated by an increase in testosterone. Otherwise, however,

most boys do not show more aggression as a consequence of higher levels

of testosterone. To summarize, sex differences in aggression may be

linked to hormonal differences, but even physical aggression is strongly

influenced by social and psychological factors.

The Significance of Sex Differences

Physical and behavioral sex differences exist. Those differences include

reproductive role, height, voice pitch, activity level, and aggression. All

are affected to some degree by sex hormones, but in most cases we know

neither the extent nor exact nature of the influence.

To some degree, however, the presence of a hormonal influence on

behavior is irrelevant. Except for reproduction, sex differences are not

qualitative but average, quantitative differences. The variability in the ex­

pression of a trait both within an individual and across individuals dem­

onstrates the importance of the social environment. The variability tells

us that hormones and the social environment act together to produce be­

havior. We need not change people’s hormones to change their behavior; changing the social environment has clear effects

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