The Uncomfortable Science Of Sex Differences - Steve Stewart-Williams - #1120

Biological Basis of Sexual Dimorphism Arises From Differences in Male and Female Reproductive Capacity and Investment

Across nearly every sexually reproducing species, a binary exists: individuals produce either sperm or eggs. While phenotypic variation occurs within each sex, the fundamental division is based on gamete size—males produce many small, mobile sperm, while females produce fewer, resource-rich eggs. This universal mechanism underpins sexual dimorphism.

Sex Based On Gamete Size: Males With Smaller Sperm, Females With Larger Eggs Across Species

Species originally began with isogamy, producing same-sized gametes. Evolutionarily, isogamy is unstable because there is selection pressure for some individuals to pack gametes with extra resources to increase offspring survival, causing some gametes to increase in size. As soon as larger gametes appear, selection favors smaller, more mobile gametes that seek out and fertilize the bigger ones. The spectrum pushes towards two extremes: large gametes that provide more resources and small gametes that are optimized for mobility and numbers. Those producing medium-sized gametes are outcompeted: their offspring are less viable than those from large gametes, and they are outnumbered by the producers of many small gametes.

Sex Differences in Reproductive Variance: Males Produce More Offspring Due to Higher Female Investment

Because females invest much more into each offspring (from producing sizable eggs, to pregnancy, birth, and caring for offspring), the maximum number of offspring a female can have is limited by these investments. Males, producing tiny and numerous sperm, could theoretically father many more offspring. Across species with equal sex ratios, the average number of offspring per male and female balances out, but reproductive variance is greater in males. A few successful males can produce many offspring, while many males have few or none at all, whereas reproductive success is more evenly distributed among females.

Parental Investment Influences Sex Differences in Psychology, Behavior, and Physiology Across Species

Parental investment drives the evolution of sex differences across physiology, psychology, and behavior.

Female Mammal Offspring Investment Limits; Male Reproductive Variance Drives Selection For Aggression, Risk-Taking, Status, and Mating Strategies

In mammals, females must undergo a long pregnancy, give birth, and nurse their young, which demands significant energy and time and limits the number of offspring they can produce. Males face strong selection pressures for traits that increase chances of reproductive success, such as aggression, risk-taking, status competition, and a variety of mating strategies. Males of many species compete for mates, sometimes directly through aggression or indirectly by displaying showy features like a peacock’s tail, or through seeking multiple mates.

Because females invest so much, mating with multiple males gives them few advantages, so they tend to be choosier in mate selection. For males, reproductive success is often about accessing more mates, and selection shapes behaviors accordingly.

Reduced Sex Differences in Humans due to Biparental Care, Aligning With Birds Rather Than Mammals

Humans, however, are unique among mammals in our high levels of biparental care. Both men and women typically invest heavily in raising offspring, with strong pair-bonding and mutual mate choice. This pattern is rare in mammals (found in only about 5-10%) but common among birds (about 90% have pair bonds and biparental care). As Dr. Steve Stewart-Williams notes, in our reproductive behavior, humans resemble birds more than other mammals.

This biparental care lowers the reproductive skew between human males and females. While human males still have higher reproductive variance than females, the difference is far smaller than in most mammals. Correspondingly, traditional sex differences in aggression, sexuality, and parenting are present in humans but are less pronounced or “muted” compared to species where males do little to no parenting.

Bateman's Principle: Greater Reproductive Variance in Males due to Parental Investm ...

The discussion on sex differences between men and women draws on evidence from developmental timing, hormonal influence, and notable cross-cultural consistency, challenging conventional sociocultural explanations and suggesting roots in biology and evolutionary pressures.

Sex Differences Appear Early Before Socialization Could Account For Patterns

Steve Stewart-Williams states that sex differences in aggression and risk-taking emerge far earlier than cultural socialization could realistically act. Boys, even as young children, are disproportionately represented in emergency rooms for injuries related to aggression and risk-taking before significant cultural gender norms are imposed. Stewart-Williams highlights that parents and teachers actually work to suppress male aggression from early childhood, not because they disapprove less of aggression in girls, but because boys exhibit it more, requiring stricter discouragement.

FMRI scans detect brain structure differences between male and female fetuses as young as three months in the womb, a stage at which postnatal socialization or cultural influence is impossible. Stewart-Williams and Chris Williamson both emphasize the implausibility of prenatal socialization, arguing that the observed early differences are almost certainly biological rather than cultural in origin.

Innate Drives Resist Socialization: Sex Differences Persist Despite Cultural Pressure

Stewart-Williams points to increased male aggression at puberty, noting that, despite society's heightened efforts to discourage it after childhood, aggression rises sharply among boys. The fact that societies actively clamp down on male aggression—which intensifies after puberty—undermines claims that such behaviors are purely or primarily culturally inculcated.

There is also remarkable stability in career interest sex differences: men, on average, show greater interests in things and thing-related jobs, while women prioritize people and people-related careers. These patterns persist despite the 20th-century US's strong push towards integrating women into traditionally male-dominated fields. Even substantial social pressure, including public campaigns to boost female participation in STEM, has not reversed these trends.

The Israeli kibbutz experiment further demonstrates the persistence of these preferences; when traditional family and childcare structures were deliberately dismantled in favor of egalitarian communal child-rearing, it was the mothers, more than the fathers, who strongly objected and ultimately forced a return to maternal caregiving. This occurred even under an intentional effort to eradicate gender-specific roles, suggesting a deeply rooted, possibly intrinsic, maternal preference for direct involvement in childcare.

Hormone Exposure During Key Developmental Periods Correlates With Later-Life Sex-specific Traits, Independent of Socialization

Stewart-Williams describes a strong line of evidence involving prenatal [restricted term]. Fetuses—regardless of sex—exposed to higher levels of [restricted term] in utero show higher subsequent rates of aggression, risk-taking, and lower parental inclinations later in life. Notably, women with congenital adrenal hyperplasia (CAH)—a condition that causes heightened early [restricted term] exposure—are less interested in dolls, less inclined to marry or seek children, display greater interest in thing-related professions, and exhibit higher rates of same-sex attraction.

These within-sex differences demonstrate that hormonal exposure can override cultural gender role messages, pointing to biology as a primary driver. Importantly, sex differences in aggression and related behaviors expand dramatically at puberty—a pattern witnessed across all cultures—further supporting the idea of biological activation in adolescence.

Sex Differences Show Cross-Cultural Consistency Suggesting Universal Biological Roots Over Cultural Constructs

Meta-analyses covering hundreds of thousands of participants across 53, then 80, nations consistently document sex differences in people-versus-things career interests in every country analyzed. This cross-cultural uniformity is rare in social sciences and strongly suggests a universal, likely biological, basis.

Male aggression, in particular, predominates universally. In every society with available data, males commit over 90% of homicides. Stewart-Williams notes this mirrors the patterns seen in chimpanzees, where males commit 92% of lethal attacks—a near-identical percentage—indicating evolutionary continuity.

Furthermore, cross-culturally, women consistently show higher involvement in parental care, while men are consistently overrepresented in risk-taking, accidental fatalities, and aggressive acts.

Sex differences in mating preferences, parental roles, and leadership also appear with similar direction and magnitude across diverse societies, suggesting these patterns are not mere artifacts of localized culture.

Sex Differences Under Evolutionary Pressures Show Homology Over Human-Specific Cultural Construction

Similar sex differences appear not only in humans but in other species subjected to comparable evolutionary selection pressures. For example, male chimpanzees exhibit similar sex-skewed aggression and aggression victims. Stewart-Williams notes that while chimps are generally ...

Sex Differences in Sexual Desire and Mating Preferences Due to Reproductive Variance and Parental Investment

One of the largest and most robust sex differences relates to interest in casual sex and sexual variety—so-called sociosexuality. Steve Stewart-Williams notes this sex difference has a significant effect size (Cohen’s d of about 0.8 to 1). On average, if you select a random man and a random woman, the man will more often score higher in interest in casual sex, but around one-third of women surpass the average man. Nevertheless, a large proportion of men would be interested in sex with female friends, while far fewer women report reciprocal interest, a pattern also influenced by sexual overperception bias—men tend to overestimate female sexual interest.

The landmark Clark-Hatfield study tested sexual openness by having attractive men and women approach strangers for a date, to come to their room, or for sex. About half of both genders accepted a date. For "come to my room," 67% of men and 6% of women agreed. For direct sexual proposals, 75% of men accepted, while 0% of women did. Male refusals were generally polite or even asked for a “rain check,” while women were more likely to respond with offense, highlighting stark differences in sexual motivation.

Gay men report more casual sex than straight men, while lesbians have even less than straight women. This demonstrates that, without having to compromise with the opposite sex’s preferences, men are more oriented toward casual encounters and women less so, supporting deep motivational differences that are not easily explained by cultural scripts alone.

Men are especially attuned to visual sexual stimuli and consume more pornography, with male sexual fantasy tending to involve multiple partners (cycling through 4–6 per fantasy), while female fantasy involves fewer. In romance novels and movies, female protagonists rarely pursue numerous partners, further illustrating these divergent motivations. Men have a neurological and evolved predisposition to respond even to subtle or abstract visual sexual cues, whereas women’s arousal is less triggered by the visual alone.

When seeking long-term mates, both sexes value physical attractiveness, but for men, looks and youth matter more, as these signal fertility. Women show stronger preferences for status and resource acquisition, explained by evolutionary models linking a mate’s resourcefulness or social standing to better offspring support—as in pair-bonding species and via “fitness indicator” effects. However, in short-term mating, women’s preference for physical attractiveness equals or even surpasses that of men, since in that context “good genes” (signaled by looks) are paramount and resource potential is less relevant.

Studies of sperm donor selection further support these tendencies: when seduction or courtship is removed, women’s optimization for desired traits in offspring remains similar to those expressed for actual short-term mates. Displays of wealth or physical beautification observed in nightlife are explained by these underlying mating strategies, with men signaling resources/status and women maximizing attractiveness.

Sex Differences in Aggression Intensify, Especially In Extreme Violence

Sex differences in direct aggression are consistent across cultures. Verbal aggression shows a modest male advantage (effect size ~0.5 standard deviations), with men outscoring women two-thirds of the time. In physical aggression, especially more severe or violent forms, the sex difference is much more pronounced. Globally, over 90% of homicides are committed by men, and most violent crime, especially severe, is perpetrated by males, particularly against other men.

Evolutionary selection pressures explain this: reproductive variance is greater for males, incentivizing risk-taking and status competition. In many species, including humans, same-sex male competition is intense, since the payoff (mating opportunities) can be extremely high for successful competitors, while for females, reproductive ceilings are lower, discouraging risky physical contests. Male physicality (brow ridge, larger hands, muscularity) reflects selection for this role, and training (e.g., Krav Maga) often focuses men on controlling, not inciting, aggression.

Sexual violence shows an even greater sex difference—about two standard deviations—driven by men’s higher interest in sexual variety and their greater likelihood of using force. This risk is not only a product of evolved motivations but also of failures in cross-sex mind reading: men often underestimate how distressing sexual aggression or harassment is to women, while women’s evolutionary vigilance reflects the very real risks.

Women Consistently Do More Direct Parenting Across Societies

Across cultures, women almost always do more direct parenting. They invest more in childcare than men, regardless of typical gender roles or cultural efforts toward equal parenting. These patterns persist even when culture pushes the opposite way—women are often observed actively seeking these roles, demonstrating stronger parental inclinations that seem intrinsic rather than externally imposed.

While it is sometimes controversial to attribute higher female parental interest to biology, Stewart-Williams and Williamson argue that rejecting this as a possibility often reveals a cultural devaluation of caregiving, prioritizing male-typical standards and roles, and perpetuating biases.

Sex Differences in Personality Show Consistent Cross-Cultural and Longitudinal Patterns

Two of the most consistent sex differences in personality are neuroticism and agreeableness. Women score higher on neuroticism (proneness to anxiety, depression, and emotional volatility, with an effect size of 0.2–0.5 SD), likely as an adaptive form of self-protection evolving in response to higher risk aversion due to lower reproductive ceilings ...

Distinguishing Between Discrimination and Preferences in Career Interests and Occupational Distribution

Steve Stewart-Williams and Chris Williamson emphasize the necessity of distinguishing discrimination from true sex-based preferences when analyzing career gaps. The "gender equality paradox" demonstrates that in societies with greater equality, career sex differences actually widen, suggesting that women’s underrepresentation in STEM is not just a product of discrimination, but is also shaped by preferences. Attributing all gender gaps solely to discrimination can also generate resentment—particularly among men—because it implies mistreatment where other factors may be at play. This approach often leads to costly and ineffective interventions and, when results do not materialize, can result in coercive policies that merely reverse the direction of discrimination, causing further injustice and backlash.

Stewart-Williams warns that exaggerating discrimination as the main cause can also deter women from entering certain fields, even when these fields might align with their interests. Conversely, minimizing or ignoring existing sex differences risks pushing everyone into a “unisex” or reversed gender straightjacket, instead of genuinely respecting diversity in choices. He advocates for providing clear information and respecting individual choices, rather than manipulating ratios or enforcing predetermined distributions. The optimal path is to do everything possible to remove barriers and bias and then accept people’s authentic vocational preferences—even if this results in uneven gender distributions.

Account For Sex Differences in Diagnosis and Treatment to Avoid Overlooking Serious Conditions

Acknowledging real sex differences is also vital in medicine and mental health. Stewart-Williams notes that cardiovascular issues often present differently in women, with symptoms like shortness of breath instead of the classic male-associated chest pain. Ignoring such differences, or assuming certain diseases are masculine, can lead to misdiagnosis and women’s symptoms being overlooked.

A similar problem exists in neurodevelopmental and psychiatric conditions. While autism is more frequently diagnosed in boys, it may be underdiagnosed in girls because their symptoms—such as diminished repetitive behaviors—are less obvious. Likewise, emphasizing that depression primarily affects women obscures the fact that about one-third of the most severe cases are in men, which can result in men not receiving appropriate help for depression or anxiety. Exaggerating or ignoring these sex differences in diagnosis and treatment disadvantages both women and men.

Intimate Partner Dynamics: Sex Differences in Jealousy, Guarding, and Surveillance

Chris Williamson and Stewart-Williams discuss how intimate partner dynamics and jealousy also reflect evolved sex differences. Men’s jealousy tends to be more strongly triggered by sexual infidelity, a pattern linked to paternity uncertainty—an evolutionary issue unique to males due to concealed ovulation. Women, by contrast, are more distressed by emotional infidelity, as the risk of their partner leaving or redirecting resources and support is the greater threat to their reproductive interests.

Mate-guarding and partner surveillance are typical in both sexes, though motivated by different concerns—paternity for men, desertion for women. When it comes to sexual dynamics, studies show that men in marriages typically desire sex more frequently than women, while women are usually content with the current frequency, resulting in a marital compromise that more often aligns with women’s preferences. This is not presented as oppression, but simply as the typical resolution of differing desires.

Cross-sex mind reading is often poor: men and women display great insight into the tricks and motivations of their own sex but fail to fully grasp the preferences of the other, often misjudging what is upsetting or attractive to their partners. For instance, men may underestimate how distressing sexual harassment is for women, as they are less likely to be as affected by it themselves.

Recognizing Bidirectional Patterns In Intimate and Social Violence

The discussion extends to social probl ...