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
_l___
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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