Much remains to be learned about the nature and origins of various sex differences, but more is known than most people realize. Much of the current confusion is generated by activists who suppress, attack, and distort information on sex differences in order to reinforce their preferred ideological narratives. These ideology-driven distortions are helpfully illustrated by a recent New York Timesessay by Chelsea Conaboy, which announces that the maternal instinct is a “myth”—a social construct generated and upheld by the patriarchy to impel women to raise children and keep them out of the workforce.
Maternal instinct and the patriarchy
Conaboy’s goal, apparently, is to undo 200 million years of mammalian evolution, which produced maternal investment in offspring. She correctly points out that, in the past, Western societies discouraged and often excluded women from entering higher education and professional jobs. But while this continues to occur in many parts of the world, in highly developed Western societies women now outnumber men in higher education. Jerry Coyne has provided a valuable rejoinder on this point and several others in Conaboy’s essay, to which I will add a few more here.
The first relates to the construction of the parental brain, about which Conaboy writes:
New research on the parental brain makes clear that the idea of maternal instinct as something innate, automatic and distinctly female is a myth, one that has stuck despite the best efforts of feminists to debunk it from the moment it entered public discourse.
According to this view, the parental brain is essentially a blank slate filled with experiences chiefly dictated by social expectations for women and men. In other words, women’s and men’s parental brains and associated behaviors would be the same with the right social mores and behavioral expectations. But this claim fails to consider that males do very little parenting in the vast majority of mammals, especially those (including humans) in which males compete intensely for status.
As it happens, humans are among the exceptions to this pattern. Men invest considerably more in their children than do the males of our closest relatives—chimpanzees and bonobos—who are uniformly deadbeats. Nevertheless, the sex difference in the direct care of human children is found throughout the world, especially in infancy. Fathers among the Aka—a nomadic people indigenous to the Democratic Republic of the Congo—provide more direct care to their infants and children than fathers in any other society hitherto studied. Yet Barry Hewlett’s observations indicate that when in camp, “the father would on average hold his infant for a total of 57 minutes while the mother would hold the infant [for] 490 minutes.”
The sex difference here and elsewhere is related in part to maternal suckling of infants that can last for several years. But it persists past infancy, and follows more general patterns. Female primates not only gestate and suckle young but are consistently more sensitive to and behaviorally attentive to offspring than are males. This is not to say that these differences are all hormonally or genetically determined—experiences do matter, as Conaboy argues. But nature has not left engagement in the associated behaviors to chance. Nor has it made maternal and paternal brains equally responsive to the associated experiences. In a review of brain imaging studies conducted while parents look at images or film clips of their children, Feldman noted that there is:
greater amygdala activation [associated with emotions] in mothers and greater cortical activation in fathers, suggesting that the hormones of pregnancy may chart a unique limbic path to parenting in mothers, which in fathers is constructed via cortical networks and active caregiving behavior.
The tendency to see results like these as categorical reflections of women’s and men’s responses to their children only adds to the confusion. These are general trends that are not applicable to each and every mother or father. Within-sex variation in parental behavior is found in both sexes, and in this case, more in men than in women. The variation in maternal and paternal attentiveness and responsiveness to children is partly heritable, but it is also related to the characteristics of parents’ individual children, to their past experiences, and to wider social mores (for instance, marriage rules). This is why, as Conaboy points out, some women describe parenthood as a fabulous and happy experience, while others find it physically and emotionally draining. There is nothing exceptional or unexpected about this variation. Biology will produce female-typical and male-typical biases and behaviors, including in parenting, as well as within-sex variation and cross-sex overlap.
But the argument that social forces in highly developed Western nations are directing women to maternal activities and restricting their opportunities in the workforce does not survive scrutiny. Sex differences associated with being career-focused versus family-focused are well-documented and show that women are more variable than men when it comes to these trade-offs. A nationally representative survey of adults in the United Kingdom found that 14 percent of women were work-focused (most men are work-focused), 16 percent were home-focused, and the remainder had a mixed home- and work-preference.
Importantly, most women were able to achieve these preferences. Four out of five (or 82 percent of) well-educated and work-focused women had full-time careers, whether or not they had children: “[P]atriarchal values have very little impact, and child care responsibilities have no impact at all on work rates among work-centered women.” If anything, the home-focused women were less able to realize their preferences, as many of them had to work to contribute to family finances. This type of variation among women (and men) follows from sexual reproduction and is a natural part of life.
There are two sexes
Sexual reproduction is a milestone in the evolution of planetary life and it is found in one form or another in nearly all eukaryotes (organisms whose cells contain a nucleus). The most fundamental mechanisms that support sexual reproduction are evolutionarily conserved (found across species) and emerged at least 1.5 billion years ago. Early eukaryotes were unicellular organisms, typically with two mating types that produced gametes of about the same size. Competition to merge with the gametes of the other mating type set the stage for the evolution of smaller (sperm) and larger (egg) gametes. These emerged under what biologists call disruptive selection, which favors large or small gametes rather than those in between.
With sperm, you’re buying thousands of lottery tickets—most will be losers but there’s a good chance of hitting a few jackpots. The sperm are built for movement and speed and so don’t carry large caloric or nutritional reserves. If two of them combine there are insufficient reserves to support post-fertilization growth. The larger eggs generally stay put and have the cache of reserves needed to support this growth. Their size means they are more costly to make than sperm and so there are fewer of them. Moderate-sized gametes, meanwhile, are outgunned on both sides. They have some reserves but are not as mobile as sperm and there aren’t as many of them. Even if they were fertilized, they cannot support the same level of post-fertilization growth as larger eggs. No doubt they were around following the evolution of sexual reproduction but they were evolutionary dead ends. The result was the evolutionary emergence of individuals who either produced small gametes (sperm, males) or large ones (eggs, females).
In other words, sexual reproduction has independently evolved in different groups of plants and animals and has always arrived at the same solution—two sexes distinguished by the size of the respective gametes they produce. In some species, sex can change depending on ecological or social conditions, changes that typically improve their reproductive success. Some reef fish, for example, might transition from female to male as they grow larger, if large males can monopolize breeding territories and thus reproduce with lots of females. But this type of sex change does not occur in mammals (or birds). Sex is therefore a categorical binary.
The existence of two sexes does not mean that males always behave in evolved male-typical ways and females in female-typical ways. The picture is complicated by human self-awareness and by psychological and social issues that can arise from engaging in sex-atypical behaviors. Engagement in these activities (such as doll play for boys) can undermine acceptance by peers and result in a higher risk of psychological issues like depression, especially for boys. Engagement in behaviors that are more typical of the other sex might result in preferring the other sex as playmates or friends, at least at times, but this is not the same as wanting to physically become the other sex.
Most people (more than 99 percent, in fact), including those with interests more common to the opposite sex, psychologically identify as the sex of their birth. A tiny minority of people with gender dysphoria desire to be seen as the opposite sex and will medically transition, but this is not as common as implied by activists (or the attention currently devoted to the issue), nor would we expect it to be common from a biological perspective.
The origins of sex differences
There are, of course, many similarities in the behaviors and other traits of males and females in most species, but there are also well-understood differences that are common across species. Differences are typically associated with different approaches to reproduction, as detailed by Charles Darwin more than 150 years ago. These approaches generally turn on relative contributions to parenting, with the higher investing sex being choosier when it comes to mates and the other sex competing for access to mates. This typically manifests as male-male competition and female choice, although there are species in which females are more competitive and males invest more in parenting. Female parenting and higher investment in offspring are built into the biology of reproduction for mammals, with internal gestation and postpartum suckling. Males provide considerable care in some mammals, but it’s not the norm. Instead, males generally attack one another or compete in other ways to gain social status with which to attract females.
For primates, a sex difference in physical size is a good indicator of the extent to which males focus on competing for mates rather than investing in offspring. If males are on average larger than females, this is generally associated with a polygynous mating system, whereby dominant males have offspring with many females and many low-status males never reproduce at all. Sometimes, these males provide protection to offspring (for example, gorillas) but females provide most of the direct parental care. When males provide social protection or other resources, females compete to develop relationships with them, as is found in several species of Savanah baboon. In these cases, we see female-female competition and male choice, along with male-male competition and female choice.
Human sex differences in physical size and many associated traits related to intense physical competition fit nicely with this general pattern. In fact, larger males than females date back at least four million years in our ancestry. Some have argued that these patterns are the result of a sexual division of labor and monogamy, whereby males provisioned females and offspring. But this is unlikely, given the general pattern in primates and that intense male-male competition and polygyny are the norm in traditional societies. In any event, the fact that human sex differences are real and have a strong biological basis does not imply genetic determinism. Sex differences are not socially constructed from whole cloth, but their expression can be exaggerated, suppressed, or deflected to be expressed in different ways by local conditions and social rules such as marriage laws or customs.
Still, sex differences in physical traits are well-documented and cannot simply be argued away. Hyde acknowledges some large differences, such as throwing distance, but argues that these are exceptions and that most differences are small. Archer later pointed out that there are quite a few individual traits with large sex differences, including very large differences in homicide rates, which can be 30 to 40 times more common among males (typically killing male rivals) than females, as well as large differences in fearfulness in real-world contexts (85–90 percent of girls and women are more fearful than the average boy or man) or pain tolerance (almost 90 percent of boys and men have a higher pain threshold than the average girl or woman). As Del Giudice and colleagues have argued, the real differences are found in the pattern of related traits.
It’s not that male-male fighting produced the evolution of taller men; it selected for a suite of correlated and co-evolving traits, including height, cardiovascular capacity, skeletal structure, bone density, lean muscle mass, as well as some less obvious traits. As with other primates with a history of physical competition, these include sex differences in brainareas involved in sensorimotor integration and aggression that support deft behavioral and emotional reactions to physical attacks. In traditional contexts and historically, male-male competition included the use of blunt force and projectile weapons. The latter favors strong throwing accuracy and velocity, as well as enhancement of the brain and cognitive systems that support the tracking of objects moving through space and integration of these systems with those that support throwing accuracy and the ability to dodge projectiles. Boys and men have advantages in all these individualareas. The sex difference for each individual trait ranges from small to quite large. The critical point is that their combination is integrated into a uniquely male suite of correlated traits.
The same is true for girls and women, who typically don’t physically fight but do engage in relational aggression that includes manipulating social information in ways that undermine the reputation of competitors and disrupt their social-support networks and access to would-be mates. Among other things, competence at relational aggression and skill at detecting and avoiding it is supported by a host of social-cognitive competencies (sometimes called emotional intelligence). These skills are also used to build cooperative friendships and include (among others) aspects of language, reading facial expressions, body posture, and gestures, and drawing inferences about the thoughts and feelings of others (called theory of mind).
Girls’ and women’s advantages in these individual areas are generally small to moderate, but in the real world, they work in combination. Here the differences are large; almost nine out of 10 women outperform the average man on tasks that involve their integration. In traditional contexts, female-female relational aggression is common, especially among women in polygynous marriages, and women who are skilled at navigating these relationships generally have more and healthier children.
Sex differences in the brain
The extent of sex differences in the brain, cognition (for example, spatial abilities), and behavior (for example, personality) continue to be vigorously debated. But minimalists tend to focus on individual behavioral and psychological traits when making their case. Hyde, for instance, argues that most of these differences (such as self-esteem) are small or close to zero. But while single psychological traits are interesting and useful to study, they do not stand alone in the real world. Like physical traits, they are components of more complicated and integrated systems, and sex differences in these suites of traits are much larger than those found for the individual components.
Brains are a mosaic of integrated regions that in most areas are more similar than different across boys and girls and men and women. At the same time, we would expect sex differences in the brain to be distributed across regions that are integrated into functional systems (areas that work in concert), such as those that support physical fighting or simultaneously processing and responding to different forms of social information (language, gesture, and so on). The result would be small to moderate differences in some areas, but potentially large differences in whole-brain patterns.
These patterns are the key to fully understanding sex differences. For individual personality traits, such as emotional stability (about seven out of 10 men are more stable than the average woman) or social agreeableness (about three out of five women are more agreeable than the average man), there are small to moderate differences. These are interesting and important, but if we look at the entire structure of personality, including things like risk-taking, openness to new ideas and experiences, conscientiousness, and so forth, the differences are now two to five times larger than they are for these individual traits.
One study examined the patterns of grey and white matter in the brains of nearly 10,000 boys and girls and asked whether the sex of the child could be determined by these patterns. They can. The sex of 93 percent of the children was correctly identified. Histograms of the numbers of boys and girls with male-typical and female-typical brain patterns are shown in the figure below.
As can be seen, most boys (left side of figure) and girls (right side) have grey and white matter patterns that are sex-typical (highly like other members of the same sex) and substantively different than those of the opposite sex. There are boys and girls with brain patterns that are in between male- and female-typical patterns and some with patterns found in the opposite sex, in keeping with natural within-sex variation.
We don’t know if brain patterns that are more like those of the opposite sex are associated with behaviors and interests that are more common in the opposite sex, but it seems likely that they are. Girls with prenatal exposure to male hormones engage in more male-typical behavior than other girls and are less interested in infants and more likely to be work-focused. Men with a cellular insensitivity to testosterone have female-typical brain activity patterns during spatial tasks, suggesting a more feminine brain.
This is not an anomalous finding. Brain patterns can be used to correctly identify whether the owner is a man or a woman with 93 to 96 percent accuracy. By analogy, the human face is also a suite of traits with overlap across females and males in the size of core features (for example, the area of the eyes) but the sex of the individual is easily determined by most people. At the same time, studies like these ignore areas where boys and girls and men and women are similar and use crude computer algorithms to identify areas that best discriminate one sex from the other, without considering whether these differences are components of integrated systems.
One way to assess the latter is to examine patterns of spontaneous activity that often reflect the synchronized activation of integrated and distributed brain networks that support functional systems, such as language comprehension and production. These types of studies can now be conducted prenatally and reveal distinct brain networks in four- to six-month-old fetuses. Synchronized activity across different brain areas helps to build and strengthen these systems, and there are sex differences even at this age. At least some later sex differences in brain organization appear to have their roots in prenatal development. A recent study found that the sex of 83 percent of eight- to 23-year-olds could be correctly identified based on differences in the spontaneous activity patterns of different brain networks that in turn were associated with sex differences in patterns of gene expression. Some of these functional brain-network differences have been linked to sex differences in social information processing, although much remains to be learned.
Sex differences in anatomical and functional brain networks have not yet been fully integrated with what we know about sex differences in behavior and cognition, nor fully integrated within an evolutionary perspective. But there has been considerable progress. Despite these current gaps in our knowledge, the findings noted here and in many other neuroscience studies are not consistent with 52 genders, nor are they consistent with the social construction of gender or gender/sex differences. To be sure, there are social and cultural influences on the expression of these differences, as noted, but the argument that they are purely (or mostly) the product of social forces is simply untrue, and that includes Chelsea Conaboy’s claims about parenting.
The bottom line
The claims made in a virtual world of internet algorithms populated by ideological social media pundits, journalists, and gender studies professors contradicts common sense and rational analysis of real-world phenomena. This is a world of words and ideas fraught with wishes and desires that are not always tethered to reality, including many far-fetched beliefs about the number of sexes and the origins and malleability of any associated sex or gender differences. Much remains to be learned about these differences which leaves plenty of room for legitimate debate. But there is no scientific room for the nonsensical idea that boys and girls and men and women are infinitely malleable and merely socially constructed products of the patriarchy or some other social system.