Sculpted by Evolution

by David Schmitt, Ph.D., Peter Jaret

It’s time for a grown-up conversation about sex. No, not that one. The other one, about the minds of men and women. Converging lines of empirical evidence—from developmental neuroscience, medical genetics, evolutionary biology, cross-cultural psychology, and new studies of transsexuality—along with our evolutionary heritage, all point to the same conclusion: There are psychological differences between men and women. And they affect matters as trivial as sensitivity to smelly socks and as significant as susceptibility to disorders such as depression and autism. In addition, women and men appear to experience pain differently. The male hormone testosterone may protect against pain, some research suggests. The female hormone estrogen, on the other hand, may intensify pain. And when they’re in pain, men and women seem to react differently, too. Studies suggest that women have greater sensitivity to pain than men and are more likely to dwell on it. Women are more likely to be diagnosed with painful conditions such as rheumatoid arthritis, multiple sclerosis, irritable bowel syndrome, cystitis, migraines, and fibromyalgia. A large study that looked at the medical records of more than 11,000 patients found that women reported more pain than men across a wide range of medical conditions.

Then again, men may simply be less willing to admit that they’re in pain—toughing it out because they think that’s the manly thing to do. Yet when men do complain about being in pain, they may be taken more seriously than women. In an older (2001) review article in the Journal of Law, Medicine & Ethics, for example, researchers found that women were less likely to receive aggressive treatment when diagnosed with a painful condition. They were also more likely than men to have their pain dismissed as “emotional” or “all in their heads.” And a 2008 study in Academic Emergency Medicine found that women who visited emergency rooms with abdominal pain were less likely than men to be given high-strength pain medications for abdominal pain.

The dramatic physical and behavioral differences between men and women, including strength and size, pubertal timing, consistent patterns around the world of hunting versus gathering and childrearing, as well as pervasive differences in risk-taking, mortality, and reproductive requirements, attest to the likelihood that evolution sculpted adaptations into men and women that make us somewhat different creatures. Psychologically, this sculpting by evolution has left men and women with particular approaches to life and love built upon a common core of human nature.

Ironically, just as the evidence is mounting that psychological sex differences are real, denial of differences has become rampant. Attempts at respectful and productive conversations about biological sex differences often end with name-calling (genetic determinist!) or outright cancellation of events—not to mention the very public firing of a Google software engineer for writing a memo on the topic.

One reason that conversations about sex differences run aground is the widespread lack of foundational knowledge about sex and gender. Then there’s the huge array of influences, from both inside and out, that shape all our emotions, thoughts, and behaviors. Still, denying men’s and women’s different psychologies is not merely a denial of reality; it has serious health consequences for significant segments of the population.

Sex vs. Gender Differences

It’s most logical to term the differences between men and women sex differences, not gender differences. After all, our species has biological sexes—typically defined by gamete size, genital morphology, the presence or absence of a Y chromosome, and normative sex hormone levels. For sure, there are atypical (and uncommon) variations in sex chromosomes and in pivotal hormonal experiences during sexual development that can make defining one’s sex unclear. The International Olympic Committee has struggled for decades to define biological sex—and it’s still struggling.

Whether you identify as a man or a woman is your sexual identity. When they study differences in the way self-identified men and women think (such as how they read a map), feel (the degree to which they experience empathy), and behave (say, their likelihood of committing homicide), psychologists are said to be investigating psychological sex differences.

Gender, or gender psychology, according to the American Psychological Association, reflects the attitudes, feelings, and behaviors that a given culture associates with biological sex. The term “gender identity” is often conflated with sexual identity, but “gender” refers to whether a person is typically masculine and/or feminine as defined by their local culture. (Sometimes this is called gender-role or sex-role orientation or gender expression. I know, it’s confusing, which is why so many find it difficult to be clear when discussing sex differences.)

Some psychologists contend that we should call most differences between men and women gender differences, not sex differences—because they feel that such differences are culturally constructed and the term “sex” should be reserved for differences that are primarily biological in origin. But this is a dangerous game to play, as social psychologist Alice Eagly has pointed out: It presupposes the ultimate source of observable differences between men and women.

There are other variations in sex-related identities that complicate discussions. For instance, people’s sexual orientation can come in many forms across varieties of sex and gender, including androphilia (finding male bodies erotic), gynephilia (finding female bodies erotic), bisexuality, asexuality, and more. Sex and gender and orientations come in many varieties. But to compare the psychologies of self-identified men and women is to discuss psychological sex differences—whatever their origins.

Size Matters

It is a common, but erroneous, notion that men and women differ because of the simple presence (or absence) of a Y chromosome and its accompanying effects on testosterone levels. The fact that there are many degrees of difference between men and women contradicts that belief. But what makes any sex difference worth talking about in the first place is the size of the difference.

When analyzing data from large numbers of previous studies—a procedure called meta-analysis—psychologists have found several sex differences in the realm of emotions. They have shown that women are somewhat more empathic than men, whereas men tend to more powerfully experience the emotion of sexual jealousy. In the cognitive realm, men tend to be better able to rotate a dimensional object in their mind and to recognize, say, an upside-down character, whereas women excel at locating an object in a visual field and remembering exactly where Big Ben is on a map of London. Behaviorally, men are more physically aggressive and homicidal with same-sex others than women are, and women tend to choose older and wealthier partners for marriage compared with men. What kind of difference, how big a difference, and how pervasive a difference must there be to support a claim that men are psychologically different from women?

Take one of the most obvious and noncontroversial differences between men and women. The typical man is much taller than the typical woman. But the sex difference in height (or any other sex difference) is not a statement about individual men and women; it pertains only to group averages.

Therein resides a key problem in discussing sex differences: Research almost always approaches sex differences as group averages, but most people think about individual men and women. When researchers talk about the average man being taller than the average woman, they are not implying that every member of the group “men” is taller than all members of the group “women.” They are talking about height differences as distributed across many individuals from both groups.

If you were to line up 100 women and 100 men and peruse the heights of the two groups, you would easily notice differences in their group distributions of heights. In the United States, the average adult woman is only about as tall as the average 14-year-old boy. Too often, when scientists claim that there exists a sex difference in height, it gets translated by others as meaning that the feature of height is biologically essential to being a man (taller) or a woman (shorter). The tendency to misconstrue as a necessary difference what shows up only as a group difference is a huge barrier to respectful and productive conversations about sex differences.

One way to clarify discussions about differences in group averages is to put a specific number to them. Psychologists often use a precise number to express the size of sex differences, referred to as an “effect size,” with the most common usage being the dstatistic. A positive d value typically indicates that men are higher on a particular attribute; a negative value indicates that women are higher. The size of the d value establishes exactly how big the average sex difference is.

d value near zero means that the sex difference is trivial. Once a d value reaches +/- 0.20, psychologists take notice. A d value of -0.20, for instance, indicates that 58 percent of women are higher than the average man on a psychological trait. These are considered “small” effect sizes. Sex differences in interpersonal trust, conformity, and general verbal ability reside in this range.

d value of +0.50 is considered “moderate” and indicates that 69 percent of men are higher than the average woman on a particular attribute. Sex differences in spatial rotation skills, certain mathematics abilities (3-dimensional geometry and calculus), and task-oriented leadership (focusing on accomplishing a group goal rather than maintaining harmony within the group) reside within this size range.

d value of -0.80 is considered “large” and indicates that 79 percent of women are higher than the average man. Sex differences in tender-mindedness, being interested more in people than in things, and lack of interest in casual sex reside in this size range.

Larger d values are less common in psychology, but a value  of +1.00 indicates that 84 percent of men are higher than the average woman. Sex differences of this magnitude include differences in height, in expressing interest in engineering as an occupation, and in absence of sexual disgust (such as not feeling grossed out when hearing the neighbors having sex).

d value of +2.00 indicates that 98 percent of men are higher than the average woman in a trait, about as close as researchers can get to finding a truly dimorphic difference. Sex differences in throwing ability, grip strength, and voice pitch are in this range.

No matter how big or small a sex difference, there is almost always significant overlap across distributions of men and women. Some women are able to throw farther than some men. Psychological sex differences are about group distributions, not dichotomous binaries of all men versus all women.

Biology Matters

Neurologically, all humans start as female. During prenatal development, the Y chromosome of human males initiates a series of masculinizing events of both body (during the first two months) and brain (after the first trimester). One of the most critical periods in the second trimester of gestation occurs when male brains, but typically not female brains, are permanently altered by exposure to androgens.

According to the organizational hypothesis of sexual differentiation, the prenatal exposure to androgens masculinizes the brain in ways that influence psychological sex differences. There is substantial evidence that these effects are real. For instance, prenatal androgen exposure within normal levels predicts sex differences in postnatal play preferences (rough-and-tumble play), personality traits (thrill-seeking, aggression), and cognitive abilities (mental rotation ability).

Evidence supporting the organizational effects of androgen also arises from studies of male and female children who, for various reasons, have atypical hormonal profiles. Studies of girls prenatally exposed to male-typical levels of androgens during the second trimester of gestation display more male-typical psychology (in play preferences, personality traits, and cognitive abilities) compared with their unaffected sisters. Studies of infants as young as a few months consistently find that some psychological sex differences emerge before extensive gender socialization has taken place. Exhibit A is spatial rotation ability, as seen in preverbal, five-month-old infants rotating and recognizing mirror-image objects.

Studies of adults experiencing hormone treatments, gender dysphoria, and transsexualism similarly indicate that to some degree, biology contributes to psychological sex differences between men and women. For instance, several studies have found that male-to-female transsexuals show signs of feminine psychological and physical traits before they transition. After undergoing hormone treatments to reduce their testosterone, more pronounced feminine thoughts, feelings, and behaviors are evident.

Development Matters

Biological predisposition to develop a masculine or feminine psychology in no way implies that men’s and women’s psychologies form a simple binary; the overlap in distribution of men’s and women’s traits refutes such dichotomous thinking. Nor does the existence of biological predispositions mean that sex differences are fixed and unchangeable after birth—a kind of genetically deterministic thinking. Nearly all biological mechanisms that have arisen over the course of history have been designed to be responsive to key elements in the environment, developmentally sensitive to features of family, social structures, and local ecologies. And the input from the environment almost always affects the degree of sexual differentiation.

Many psychological sex differences emerge long after prenatal experiences. Sex differences appear during puberty or other critical periods when genes become sensitive to activation by major maturational events, such as sexual debut, parenting, and menopause. Sex differences in the personality trait of neuroticism, including sensitivity to negative emotions and vulnerability to stress, do not reach their full adult form until around age 14, for example, suggesting that pubertal factors influence their development. In other words, some psychological sex differences are specially designed by evolution to arise developmentally and only after particular milestones.

Besides the early organizational effects and the later activational ones, some psychological sex differences result from direct effects of genes—apart from the sex chromosomes and their related hormones—functioning differently in men’s and women’s brains. For example, psychologist Janet Hyde and her colleagues have found that a serotonin-transporter gene, 5-HTTLPR, which exists in short and long versions, the short version being associated with higher negative emotionality, is more closely linked to the emergence of neuroticism-related traits in women than in men. The researchers identify gene variants affecting other neurotransmitters that are similarly disproportionately expressed in women. And they cite several mechanisms—such as X-linkage, or the involvement of genes found on the X chromosome, and estrogen-induced gene expression—that may play key causal roles in the emergence of one of the most notable psychological sex differences: the higher prevalence of depression among women than men.

Even if they originate from direct genetic effects, psychological sex differences do not take precisely the same form or manifest to the same degree across all cultures. Human psychology is highly sensitive to developmental and socioecological contexts. Such environmental factors as the threat of malaria and other infections, the number of men relative to the number of women in a local population, the degree to which men and women must compete in order to find a mate and reproduce—all magnify or minimize sex differences.

For instance, across all cultures men more than women tend to favor physical attractiveness in potential marriage partners, especially such cues to youth and fertility as a small waist and curvy hips. However, when pathogens are prevalent and diseases such as malaria are a constant danger (say, in India and Brazil as opposed to New Zealand and Germany), both men and women tend to emphasize physical attractiveness in a potential mate—possibly because attractiveness is generally a reliable indicator of good health. In these environments, the amplification of desire for physical attractiveness is more pronounced among women, leading to smaller sex differences in mate preference for attractiveness in societies with higher pathogen burdens.

Culture Matters (But Not How You Think)

Fact: As a percentage of enrollment, there are more female science majors in Burma, Oman, and Morocco than in the countries of Scandinavia.

Fact: American women are 15 percent less likely to reach a managerial position in the workplace than are men—but in Sweden women are 48 percent less likely, in Norway 52 percent, in Finland 56 percent, and in Denmark 63 percent.

Whatever the differences in men’s and women’s psyches—empathy, jealousy, cognitive abilities, mate preferences—many theories in psychology assume that they result primarily from direct gender socialization by parents, media, and societal institutions. As a result, it is often expected that sex differences will be smaller in cultures with higher levels of gender-related egalitarianism, as in Scandinavia, where socialization and roles are more balanced between men and women and sociopolitical gender equity prevails.

Surprisingly, several large cross-cultural studies have found this is not at all the case. Whether scientists measure Big Five personality traits, such as neuroticism; Dark Triad traits, such as psychopathy; or self-esteem, subjective well-being, or depression, empirical evidence shows that most sex differences are conspicuously larger in cultures with more egalitarian gender roles—as in Scandinavia.

The same holds true for cognitive attributes, including mental rotation and location ability, objectively measured on tests, as well as for physical traits such as height and blood pressure (both greater in men). And among such differences as preferring physically attractive mates, some of the largest psychological variances of all occur among the most progressive people: Scandinavians. The phenomenon is called the gender equality paradox.

Culture matters in explaining psychological sex differences, but not in the way most people think. It’s not harsher gender socialization by parents and media, stringent societal gender roles, or institutional sociopolitical forces that widen the differences between men and women in the most progressive nations in the world. When you treat everyone the same, as in the Nordic countries, it’s only genetic predispositions that produce the most observable individual differences. Extremes of sexual freedom beget larger psychological sex differences. Or as explained by Israeli psychologists Shalom Schwartz and Tammy Rubel-Lifshitz, it may be that having fewer gendered restrictions in a culture allows “both sexes to pursue more freely the values they inherently care about more.”

Dials, Not Switches

If your sexual identity is “I am a man,” it is likely that you also have a deeper voice and a stronger sex drive than most women. But not all women. Evolution in sexually reproducing species allows for a lot of variation along sex and gender dimensions. Think about sex differences for myriad traits as dozens of interconnected sex/gender dials with endpoints ranging from extreme male-typicality/masculine to extreme female-typicality/feminine. Our psychological sex difference dials do not all need to be turned up to 11 for men and women to be significantly different from one another and for evolution to have played a role in producing human sexual diversity.

Viewing sex differences as dimensional sex/gender dials makes it clear that there is not one simple sex adaptation that gives rise to male and female psychologies. Rather, there are likely dozens, if not hundreds, of evolved functional mechanisms generating physical sex differences in height, strength, voice, and hirsuteness and psychological differences in personality, play preferences, mate selection, erotic desires, personal values, and cognitive abilities. Each adaptation turns the respective sex/gender dials of men and women in oblique, context-sensitive ways; each contributes a small part in generating the panoply of sex differences seen in our species all around the world.

It is ironic that just when science is rapidly improving its fundamental understanding of sex differences and documenting the sometimes subtle ways that biology and culture interact, the progress has come under assault. The uproar over the Google memo was just one example. Another manifestation is the recent decision by England’s Royal Society to name as science book of the year Testosterone Rex: Unmaking the Myths of Our Gendered Minds by Cordelia Fine. Healthy backlash points out the dangers of wholly biologizing our sexual selves, but Fine repudiates psychological sex differences altogether. Perhaps significantly, the judges were primarily media members, not scientists.

Despite Fine’s increasingly vocal perspective, substantial evidence attests to the existence of many psychological sex differences. But even a difference clearly stemming from prenatal hormone exposure—say, preference for rough-and-tumble play—does not imply genetic determinism; the feature is still modifiable by future developmental experiences.

And sometimes we will want to do all that we can to modify sex differences. It is critical to acknowledge that biological sex differences are not necessarily morally good or justifiable. There are sex differences whose development society needs to actively redress, such as the greater risk of severe autism in males and depression in females. There is only one way to develop the tools necessary for subduing such undesirable developments—understanding their biological provenance. And that starts with recognizing their existence.

Divergent Thinking

Some traits are more prevalent in women, some in men; listed by magnitude of sex difference.

WOMEN

SMALL:

  • Conformity
  • General verbal ability
  • Indirect aggression (gossip)
  • Interpersonal trust
  • Sensitivity to negative emotions
  • Spatial location ability

MEDIUM:

  • Tendency to smile

MEDIUM/LARGE:

  • Preference for status in mate
  • Tender-mindedness

LARGE:

  •  Body fat
  •  Cooking, among foragers
  •  Early onset of puberty
  •  Empathy
  •  Interest in people over things
  •  Preference for female-typical toys
  •  Preference for taller mate
  •  Primary caretaker of children, among foragers
  •  Sexual disgust
  •  Vulnerability to depression

MEN

SMALL/MEDIUM:

  •  Impulsivity
  •  Sexual jealousy

MEDIUM:

  • 3-D geometry ability
  • High blood pressure
  • Risk-taking
  • Sex drive
  • Task-oriented leadership

MEDIUM/LARGE:

  • Mental rotation ability
  • Physical aggression
  • Preference for physically attractive mate

LARGE:

  • Deep voice pitch
  • Early mortality
  • Grip strength
  • Height
  • Likelihood of homicide
  • Preference for rough-and-tumble play
  • Throwing ability
  • Upper-body strength
  • Vulnerability to psychopathy

Further Reading

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