The Uncomfortable Science Of Sex Differences - Steve Stewart-Williams
In a Nutshell
Men and women show evolved psychological and physical differences shaped by anisogamy and parental investment, with males facing higher reproductive variance and females higher per-offspring costs; these produce modest average gaps in traits like aggression, sociosexuality, people-things interests, and neuroticism, yet massive overlap within sexes. Evidence for innate contributions includes early emergence, cross-cultural universality, hormonal correlates, and persistence despite egalitarian pressures, while the gender-equality paradox shows larger differences in freer societies. Denying these patterns leads to misdiagnosed causes of gaps and ineffective policies; acknowledging them enables realistic expectations without justifying coercion or excusing behavior.
These notes were generated by AI and may contain inaccuracies.
The book on sex differences would have been received more negatively six years earlier due to a more charged cultural climate. Discussion of sex differences remains controversial primarily because of the long history of sexism against women, including problematic statements from 19th-century science. A notable example comes from Gustave Le Bon, who claimed that intellectually accomplished women are as rare as two-headed gorillas and therefore need not be considered.
This historical context makes people nervous that discussing evolved sex differences could reopen doors to old forms of sexism. The science of sex differences has actually disproved many of those earlier prejudicial claims. Men and women can be treated respectfully and equally even when average differences exist across traits.
Science serves as the solution to early sexism rather than its continuation. The history of science on sex differences parallels how the Nazi Party relates to behavioral genetics.
Studying sex differences is important for multiple reasons. The primary motivation is that the topic is deeply fascinating, providing a sense of insight similar to understanding the phases of the moon as a child—suddenly everything clicks into place and makes sense.
Beyond intellectual interest, understanding sex differences helps steer society in positive directions by improving understanding of interactions between men and women. The guiding philosophy is to let people be themselves: if individuals fit traditional gender roles, that is acceptable; if they do not, that is equally acceptable.
Evolutionary origins of traits do not determine whether those traits are good or bad. The fact that something is natural or evolved is morally neutral—it is up to society to decide its value.
A common worry is that evolutionary explanations might excuse problematic behavior or justify repressive social norms. This concern is valid in the sense that people might falsely assume evolutionary explanations carry normative implications. When discussing sex differences, it is important to emphasize that most psychological differences are modest, that there is enormous variation within each sex and substantial overlap between sexes, and that no normative conclusions follow from evolutionary origins.
When multiple modest differences are stacked together, they can produce large overall separation between groups, sometimes described as multivariate sex differences. However, this statistical approach requires caution. Randomly dividing people into two groups by coin flip would also produce small differences across many traits; when stacked, these would appear massively discrepant under multivariate analysis. The same approach could exaggerate differences between New Zealanders and Australians or between the British and the French.
Sex is defined by gamete size. In sexually reproducing species, males produce small gametes (sperm) and females produce large gametes (eggs). This binary holds across the vast majority of species. The definition is definitional rather than purely empirical: males are defined as the sex that produces smaller gametes.
Most sexually reproducing species exhibit anisogamy—two different gamete sizes. A small number of simple organisms (certain bacteria) exhibit isogamy, producing identical gametes. Isogamy is thought to represent the ancestral state before sexual reproduction evolved toward anisogamy.
Isogamy tends to be evolutionarily unstable. When gametes are the same size, selection favors variants that load extra resources into gametes to improve offspring survival. This pushes some gametes to become larger. Once larger gametes exist, selection favors smaller gametes that seek out and "ride on the coattails" of the larger ones. This dynamic drives the two sizes apart. Medium-sized gametes are disadvantaged because they neither survive as well as large gametes nor can be produced in the high numbers possible with small gametes.
Daily human sperm production ranges from 100 to 300 million per healthy adult male. Global daily totals are estimated at 200 quadrillion sperm versus 70 million eggs.
A sex binary exists in nearly all sexually reproducing species. Substantial variation occurs within each sex in phenotypic characteristics, including levels of masculinity and femininity, and degree of sexual size dimorphism versus monomorphism. Intersex conditions and hermaphroditism exist but do not disprove the binary, as individuals are still categorized by whether they produce sperm or eggs.
Evolutionary psychologists acknowledge that socialization substantially shapes the expression of sex differences across cultures and historical periods. However, multiple lines of evidence indicate an innate contribution beyond socialization alone.
Six lines of evidence support innate contributions:
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Sex differences often appear very early in development, before extensive socialization is plausible. The aggression and risk-taking differences emerge as soon as children can move and engage in those behaviors, with boys appearing more frequently in emergency rooms due to risk-taking from early childhood.
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Many differences intensify at puberty across cultures, consistent with biological activation rather than solely cultural pressure.
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Differences often appear despite cultural efforts to suppress them. Most cultures attempt to reduce male aggression and risk-taking post-puberty, yet these behaviors increase anyway.
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Sex differences in mate preferences and career-related interests remained stable throughout the 20th century in the United States despite strong social pressure to increase women in traditionally male roles.
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Fetal brain differences detectable via fMRI appear as early as three months gestation (sometimes earlier).
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Differences persist over long periods despite changing social conditions.
Parents do not treat sons and daughters differently in several important respects in Western research: they are equally likely to encourage success and independence in both. The second law of behavioral genetics indicates that identical twins reared together become no more similar by adulthood than identical twins reared apart, suggesting parental treatment has less lasting impact than commonly assumed.
Social roles theory and patriarchy theory predict larger sex differences in more patriarchal societies with stricter gender roles. The gender equality paradox contradicts this prediction: sex differences in career preferences, personality, and other traits are often larger in more gender-equal societies with less rigid roles.
In societies that are less patriarchal, larger sex differences are often found rather than smaller ones. This finding runs in the opposite direction predicted by social role theory. An evolutionary perspective initially led to the prediction that patriarchy and stricter sex roles would increase sex differences, but the reverse pattern was surprising.
Rebuttals to sociocultural explanations for the gender equality paradox include the claim that in more patriarchal societies, men and women have little interaction with each other. This leads them to hold exaggerated pictures of the opposite sex. When completing personality inventories, individuals compare themselves only to members of their own sex and therefore perceive themselves as more similar to their comparison group. A counter-rebuttal notes that the pattern appears not only in self-report measures like personality but also in cognitive abilities such as spatial ability and in physical traits such as height.
The sex difference in throwing velocity and accuracy is substantial. Research shows that the difference at age eight is comparable in magnitude to adults' ability to detect sex from facial features alone, reaching approximately 90 percent accuracy. A 1970s study used a tennis ball serving machine to test spatial awareness by firing balls at participants; no males were hit. Such projectile-based methods would no longer receive ethics approval.
The largest sex differences are physical. Reproductive anatomy shows near-complete separation, with virtually all males possessing penises and all females possessing vaginas. Upper-body strength exhibits very large differences with minimal overlap between the sexes. Voice pitch constitutes another large difference; this sex difference is larger in humans than in other apes, including species more dimorphic in body size such as gorillas and orangutans.
Psychological sex differences are generally more modest, yet one stands out as nearly as large as major physical differences: the sex difference in the primary sex to which each sex is attracted. The vast majority of men are primarily attracted to women, and the vast majority of women are primarily attracted to men. This difference receives relatively little mention in summaries of evolved sex differences despite its clear evolutionary rationale.
Interest in casual sex and sexual variety, measured as sociosexuality, shows a sizable sex difference. Men score higher on average. The effect size is approximately Cohen's d = 1.0, indicating roughly one standard deviation separates male and female means. When randomly selecting one man and one woman, the man scores higher in approximately two-thirds to 70 percent of pairings. Substantial overlap remains; a significant minority of cases show the woman scoring higher.
Additional evidence includes hormonal correlates. Prenatal testosterone exposure correlates with later aggression, risk-taking, and reduced parental inclinations, regardless of the individual's sex. Women with congenital adrenal hyperplasia, exposed to elevated prenatal testosterone, display more male-typical traits, including reduced interest in dolls, marriage, and children, and increased interest in thing-oriented professions. They are also more likely to identify as bisexual or lesbian.
Cross-cultural universality provides further support. Key differences appear in every culture or the vast majority: men are more aggressive, commit the overwhelming majority of homicides in every nation with data, exhibit greater risk-taking and accidental fatalities, and women show greater involvement in parental care.
Sex differences also appear in other species subject to comparable evolutionary selection pressures. Most sex differences originate from the sex difference in maximum offspring number. In most species, males can potentially produce more offspring than females. Although average offspring numbers are equal due to the requirement of one male and one female per offspring, variance is typically greater among males. Some males sire many offspring while others sire none.
Reproductive variance is greater in males than females in most species; this is known as Bateman's principle. Robert Trivers refined the explanation by emphasizing parental investment rather than anisogamy alone. Females invest more per offspring through gestation, parturition, and nursing, lowering their maximum offspring ceiling. Males can, in principle, invest only in one sex act per offspring and therefore can mate with multiple females.
Parental investment is the primary driver of the sex difference in maximum offspring number, though ecological factors also influence reproductive variance. In solitary species, limited opportunities for additional matings can equalize maximum offspring numbers despite unequal investment.
Estimates suggesting 40 percent of ancestral human males and 80 percent of females reproduced likely overstate reproductive skew in humans. Human males frequently provide substantial parental care, reducing maximum male offspring number and bringing it closer to female levels. This biparental care produces lower reproductive skew than found in most mammals.
Because females invest more per offspring, they must be choosier about mates. Mating with many males confers less advantage to females than to males. Ecological constraints and the demands of offspring production together shape the strength of sexual selection on each sex.
In ancestral environments, approximately 50% of children born survived to reproduce. This reflects the high infant mortality rates that characterized most of human history. Civilization has dramatically reduced this number close to zero, which represents one of the major reductions in human misery achieved through modern development.
Charles Darwin lost three of his ten children before they reached adulthood. The death of his daughter at age 12 or 13 was particularly psychologically painful.
Grief intensity tracks the reproductive value of the deceased individual. Grief levels increase as children move from vulnerable early infancy through childhood, peaking a couple of years after puberty when reproductive value is highest, then declining as people age. This pattern has been replicated in research.
The evolutionary explanation is that parents detect when a child reaches the life stage where they could begin functioning as a grandparent-optimizing machine. The death of a child on the cusp of reproductive capability is evolutionarily the most costly outcome.
A key distinction exists between proximate and ultimate motivations for behavior. People eat because food tastes good (proximate), but the evolutionary reason is survival (ultimate). People have sex because it feels good (proximate), but the evolutionary reason is reproduction (ultimate). These are different levels of explanation.
Evolutionary explanations describe why psychological drives exist rather than claiming people consciously think about reproductive outcomes. The grief response operates as a built-in tendency rather than requiring explicit calculation of reproductive value.
Socio-cultural explanations for behavior are not inherently less bleak than evolutionary ones. Being at the mercy of cultural memes and trends decided by others can be viewed as equally deterministic. Denying genetic influences on life outcomes creates a world where individuals bear sole responsibility for all failures, which places an enormous burden on personal effort and parental investment.
Natural selection has produced both less noble traits and positive capacities including the ability to fall in love, love children, and maintain friendships.
Humans are unusual among mammals in exhibiting high levels of pair bonding and biparental care, occurring in only 5-10% of mammal species. In contrast, around 90% of bird species form pair bonds with biparental care. In primary reproductive behavior, humans resemble the average bird more than the average mammal.
Despite this exception, the same evolutionary principles apply to humans as to other species. Men have always had greater reproductive variance than women, though this difference is relatively low compared to most mammals due to biparental investment.
In most insects and many species, females are larger than males because larger females can produce more eggs. The main driver of sexual dimorphism in these cases is egg production capacity rather than reproductive variance. Mammals and other large vertebrates represent the exception where males are typically larger, driven by intersexual selection favoring fighting ability in males.
Sex differences in interest in casual sex, sexual variety, and cheating stem from the male strategy of seeking multiple partners to maximize offspring number. Because humans have biparental care, male reproductive variance is constrained compared to most mammals, which correspondingly reduces the magnitude of sex differences in sexuality, aggression, and parenting investment.
The existence of evolutionary explanations for behavioral tendencies does not excuse those behaviors any more than socio-cultural explanations would. All behavior has causes, and the institution of holding people responsible remains necessary regardless of identified causes.
Men place greater weight on physical attractiveness in mates because youthfulness correlates more strongly with fertility in women due to menopause. This contributes to men being more avid consumers of pornography than women.
Women show greater interest in romance novels despite men and women being similarly interested in long-term committed relationships. Research indicates men may actually be more romantic in some respects, falling in love more quickly, declaring love sooner, and suffering more after breakups.
In sexual fantasies, men on average cycle through four to six different partners, while women tend to focus on fewer or a single partner. Romance novels and films typically reflect this pattern, centering on one or two romantic interests rather than multiple partners.
Men are more likely to falsely infer sexual interest from women than women are from men. This represents a failure of cross-sex mind reading and follows the smoke detector principle: over-detecting potential mating opportunities carries lower costs than missing genuine opportunities.
Men face asymmetric costs in interpreting sexual interest: missing an opportunity with an available woman carries greater reproductive cost than risking an awkward rejection. This explains average male tendencies to overinterpret signals rather than underinterpret them. The framework does not excuse inappropriate workplace behavior such as unsolicited advances toward receptionists or colleagues, but it accounts for why misread signals occur more frequently from men on average.
High-profile cases, such as the Goldman Sachs incident involving a very senior male executive accused by another man, elicited widespread skepticism because the alleged behavior was extremely atypical for men. Observers intuitively recognized the statistical rarity, highlighting how sex differences shape expectations around such events.
Gay men engage in more casual sex than straight men, while lesbians engage in less casual sex than straight women. Because same-sex couples do not need to compromise with the opposite sex's typical inclinations, their behavior provides a clearer view of underlying male and female sexual motivations.
Straight men compromise by having less casual sex than they would prefer because women act as gatekeepers. Straight women compromise by having more casual sex than they would prefer when measured against lesbian patterns. Within marriages, men typically report wanting roughly twice as much sex as they receive, whereas women report satisfaction with current frequency, indicating an ongoing compromise where female preferences set the baseline.
Men perform a form of desire containment in long-term relationships that is not symmetrically required of women. This compromise is often invisible to women because sexual entitlement is frequently expressed in unattractive ways. More dead bedrooms occur in lesbian relationships than in heterosexual ones.
Men and women show substantial overlap in long-term mate preferences. Both value intelligence, kindness, mutual love and attraction, physical attractiveness, and emotional stability. However, average differences exist in weighting: men place greater emphasis on physical appearance in long-term partners, while women place greater emphasis on resources and status.
Neither looks (for men) nor resources/status (for women) rank as the single most important trait. Kindness consistently tops or equals these preferences for both sexes.
Two primary explanations account for why women value resources and status in long-term mates. The first is biparental care: males who control resources and maintain group standing can better invest in offspring. The second frames resource acquisition and status climbing as a human equivalent of the peacock's tail—a signal of good genes that will be passed to sons.
Both mechanisms likely operate. Resource and status displays function simultaneously as direct fitness benefits and as indicators of genetic quality.
The sex difference in emphasis on physical attractiveness disappears in short-term mating contexts. Women place equal or greater weight on male physical attractiveness when commitment is low. In short-term encounters, the primary benefit is genetic; therefore, traits signaling good genes become paramount because no ongoing investment is expected.
Men and women both display status and attractiveness cues in short-term contexts (fast cars, Rolex watches, beautification) because these signals are immediately legible when time for assessment is limited. Cultural variation exists in how status is advertised, but the underlying logic remains consistent across societies.
When women select sperm donors, removing courtship and seduction requirements reveals the same mate preferences observed in normal social interactions: emphasis on intelligence, health, and other heritable traits.
In the Clark and Hatfield campus study, attractive confederates approached opposite-sex strangers and asked one of three questions: go on a date, come to my room, or go to bed tonight. For the date request, acceptance rates were roughly equal (approximately 50%). For the room request, roughly 67% of men agreed versus 6% of women. For the sexual request, 75% of men agreed versus 0% of women.
Men who declined were typically polite and sometimes requested rain checks. Women who declined reacted with strong negative affect and no apologies. The magnitude of the difference reflects both average sex differences in interest in casual sex and women's greater physical risk when leaving with an unfamiliar man.
Women sometimes respond negatively to approaches from much lower-value men because the approach itself challenges their self-perceived mate value. Men are less likely to interpret approaches from lower-value women as insults and more likely to view them as flattering given the lower baseline rate of female initiation.
Men show higher rates of direct, physical aggression. The difference is modest for verbal aggression (effect size around 0.5) but increases with severity. Men predominate in low-level physical aggression, serious violent crime, and especially homicide.
Across all societies with available data, men commit over 90% of homicides and constitute 70-80% of victims. The pattern is nearly identical in chimpanzees: males commit 92% of lethal aggression and are 73% of victims. The size of the sex difference in lethal aggression is virtually the same between humans and chimpanzees.
The evolutionary driver is greater male-male competition for mating opportunities. Males compete for status and resources that translate into reproductive access. Females experience weaker selection pressure for same-sex physical aggression because their maximum reproductive output is physiologically capped; the costs of combat outweigh potential gains.
Male physical traits such as brow ridges and larger hands reflect historical selection for male-male competition. Some women are attracted to physically formidable men who can direct aggression protectively toward family defense rather than indiscriminately. Training in controlled violence (for example, Krav Maga) emphasizes learning to regulate aggressive responses rather than simply increasing them.
Physical strength and aggression present a mixed blessing for women in partner selection. Traits that enable a man to protect a family can also be directed against the partner in domestic disputes. Aggression itself does not increase attractiveness, but protectiveness does. Muscularity reliably improves attractiveness and represents the most easily modifiable trait for men.
A replicated study found that women lose more attraction when a partner fails to protect them during a physical altercation than when he cheats. The key factor is the willingness to protect rather than raw ability. A man may be physically imposing yet unwilling to intervene, rendering his size irrelevant.
Women's strong preference for taller men aligns with the protectiveness cluster of traits. Height and muscularity function together as indicators of protective capacity. This preference persists regardless of the woman's own height. A tall man dating a shorter woman does not gain or lose protective utility based on her stature.
The "tall girl problem" illustrates this dynamic. Taller women often express frustration when tall men pair with much shorter partners, yet the protective value of height remains constant irrespective of the woman's size. Height compensation effects are minimal above approximately 5'10".
Men engage in stalking at substantially higher rates than women. This behavior represents an extreme manifestation of male mate-seeking strategies and risk-taking tendencies. When individuals become fixated on non-reciprocating partners, the condition resembles limerence—inappropriate romantic fixation. Men's greater propensity for aggression converts this fixation into surveillance and stalking behaviors more frequently.
Male parental uncertainty drives surveillance behaviors. No woman who gives birth questions whether the child is hers, but men face inherent uncertainty due to concealed ovulation. This uncertainty creates evolutionary pressure for mate guarding and surveillance to ensure paternity.
Women face a different risk: partner desertion. A woman depends more heavily on male investment for offspring survival, creating incentive to monitor partner commitment. Both sexes therefore evolved jealousy, though for different ultimate reasons—paternity certainty for men and resource security for women.
Even when discussing third parties, cross-sex mind reading failures persist. Approximately 50% of men report willingness to sleep with female friends, while far fewer women express interest in male friends. Men exhibit sexual overperception bias, assuming female friends share their level of interest. This misreading extends to understanding opposite-sex motivations in social and sexual contexts.
Men perpetrate sexual violence at dramatically higher rates. The effect size for extreme forms reaches approximately two standard deviations between male and female means. This pattern partly stems from the combination of men's greater interest in casual sex and their higher willingness to use violence. Males in many species attempt sexual coercion; human patterns fit this broader biological context.
Sexual coercion is particularly distressing to women because it directly undermines female choice. Females evolved greater choosiness due to higher per-offspring investment. Rape bypasses this evolved decision-making process, explaining the depth of the negative response beyond socialization alone.
Men systematically underestimate how upsetting sexual harassment and non-consensual acts are to women. This represents another failure of cross-sex mind reading. While men may find the prospect of prison rape frightening, they often fail to appreciate the intensity of women's distress regarding lower-level sexual boundary violations.
Advocates denying sex differences sometimes imply that equal violence rates would represent progress. The logical extension—that women should commit as many homicides as men—is rarely stated explicitly. Reducing male violence improves outcomes, but achieving statistical equality by increasing female violence would worsen conditions. Equality is not inherently valuable; the reduction of violence itself is the desirable goal.
Women perform more direct parenting and parental care across all cultures with reliable data. Men invest more than male chimpanzees or walruses but consistently less than women. These patterns appear to reflect average differences in parental inclinations rather than solely external coercion.
Resistance to acknowledging this difference often stems from concern that men will use it as justification to avoid childcare. The underlying assumption—that greater female parental inclination is negative—reflects a soft bigotry of male expectations, treating male behavior patterns as the default standard. If women are more parentally inclined by nature, devaluing that inclination constitutes sexism against female-typical traits.
Israeli kibbutzim attempted to eliminate traditional family structures by housing children communally and assigning childcare to dedicated group members rather than biological parents. The goal was to replace bourgeois biological investment with collective care. The arrangement collapsed when parents—particularly mothers—agitated to have their own children living under their roof.
This outcome demonstrates that certain behavioral patterns emerge despite cultural pressure rather than because of it. Women actively sought the primary caregiver role even when male leaders promoted egalitarian alternatives.
Women frequently report distress when missing developmental milestones due to work demands. Maternity leave policies differ substantially between countries, with the UK providing nine months compared to the minimal provisions in the United States. Cultural messaging that devalues being "just a mom" creates pressure against extended parental investment, yet many women experience genuine loss when absent from early childhood moments. Both sexes face work-family tensions, though the conflict appears more acute for women on average.
Women often address work-family balance challenges by taking career breaks when children are young, then returning once children reach school age.
Both men and women experience jealousy, but the focus differs. Men are more distressed by sexual infidelity than emotional bonds alone. Women are more distressed by emotional infidelity and a partner falling in love with someone else than by sex alone.
Men's greater concern with sexual infidelity stems from paternity uncertainty. No woman giving birth has ever questioned whether the child is hers, whereas men face a non-zero risk that a child is not biologically theirs. Women face the greater risk of being left to raise a child alone, which reduces their capacity to have additional children and imposes fitness costs.
Emotional involvement by a partner is particularly costly for women because it typically leads to relationship dissolution and loss of resource provisioning.
A person discovered through 23andMe or ancestry.com in their thirties that they had a new half-sister. The father had a one-night stand 32 years earlier; the child never met the father. This raises questions about how sexual jealousy, intimacy concerns, and resource-provisioning fears operate when infidelity occurred decades earlier, the family remained intact, and the dependent child is now independent.
Evolutionary explanations for jealousy involve ultimate causes such as loss of partner investment and resources. However, the proximate mechanism may be a simple gut-level emotional reaction to a partner becoming close to someone else. This reaction can persist even decades later when the original evolutionary costs no longer apply, because the evolved response does not discriminate fine details of timing or current circumstances.
The Big Five personality traits are openness to experience, conscientiousness, extraversion, agreeableness, and neuroticism. Two differences stand out consistently:
- Neuroticism: Women score higher on average (proneness to negative emotions such as anxiety and depression plus emotional volatility). Effect size is medium, typically 0.2 to 0.5 standard deviations.
- Agreeableness: Women score higher on average. Effect size is also medium-small to medium (0.2 to 0.5 standard deviations).
Smaller or less consistent differences appear in the remaining traits: women sometimes score slightly higher on conscientiousness, extraversion, and openness, but findings vary across studies.
The neuroticism difference may reflect greater female self-protectiveness, the counterpart to male risk-taking. Risk-taking evolved in men partly because of higher maximum reproductive output and the need to climb status hierarchies. Anxiety functions analogously to a turtle shell by promoting protection.
The agreeableness difference may be the counterpart to male aggression and status-striving. Women evolved to form social bonds that emphasize compassion and understanding of others.
This is one of the larger psychological sex differences, with an effect size of approximately one standard deviation or greater. Men show greater interest in things; women show greater interest in people. The difference is highly consistent across cultures and has persisted for over 100 years (first documented in 1911).
Evidence against purely socialization explanations includes:
- Persistence despite cultural efforts to increase women in male-dominated fields
- Association with prenatal hormonal exposure
- Presence in 53 of 53 nations (200,000 participants) and 80 of 80 nations (500,000 participants) in large cross-national studies
The difference appears in career interests that are evolutionarily novel, yet remains robust.
Attempts to close the people-things gap through deliberate interventions have not substantially altered it. In more gender-equal nations, the gap in interests and occupational choices tends to widen rather than shrink, with fewer women entering things-oriented roles.
Public efforts focus heavily on increasing women in STEM while giving less attention to increasing men in female-dominated fields. Interest, rather than pay differentials alone, drives many choices. Some individuals develop interests transactionally for resource acquisition, a pattern more common among men in wealthy societies.
In high income-inequality environments, women engage in more self-sexualization and beautification on social media, interpreted as maximizing resource-provisioning prospects when both upward mobility and downward risk are salient.
The environmental security hypothesis proposes that men prefer larger women during economic hardship and slimmer women during prosperity. Evolutionary plausibility is debated because hunter-gatherer populations rarely accumulated sufficient surplus for body-size differences to exert long-term selection pressure before approximately 13,000 years ago.
There is no meaningful average sex difference in general intelligence or IQ. Some IQ tests were constructed to equalize averages, but representative samples on other tests also show essentially identical means.
Men exhibit slightly greater variance in cognitive ability, producing more individuals at both extremes. Apparent small male advantages in some samples may partly reflect under-sampling of very low-IQ males.
Women show average advantages in verbal abilities. Among mathematically gifted individuals, women are more likely than men to also possess high verbal ability, creating a "double threat" profile.
Specific cognitive differences exist but are smaller than personality differences and much smaller than differences in interests. Spatial abilities tend to favor men on average. Most differences are small, and overall cognitive profiles show substantial overlap.
Men have higher rates of cardiovascular disease and most cancers (except sex-specific reproductive cancers). Women have higher rates of immune-system disorders and pain disorders.
Men die younger than women in the vast majority of cultures, even those with high maternal mortality. Castration before puberty eliminates the average lifespan gap, indicating testosterone's direct biological cost.
Lowering male mortality rates to female levels would save more years of life than curing cancer.
Men account for 88% of Darwin Award winners (deaths from spectacular self-inflicted misjudgment) and 90% of Carnegie Hero Awards (risking one's life to save a stranger). The same risk-taking propensity that produces more male business successes also produces more male failures.
Encouraging greater female risk-taking would increase both success and failure probabilities. The recommended approach is to present complete information about trade-offs and allow individuals to choose.
Presenting full information rather than partial narratives reduces later resentment arising from decisions made under incomplete information. Attributing all occupational gender gaps to discrimination alone creates unnecessary resentment and leads to costly interventions that ignore preference differences. Effective policy requires examining both bias/barriers and genuine differences in preferences.
Costly interventions targeting gaps often fail because they address non-primary causes. When these fail, societies shift toward more coercive measures. In some cases, this inadvertently produces anti-male discrimination while attempting to eliminate gaps. Rather than removing discrimination, the approach reverses its direction, which adds to the total sum of injustices instead of correcting past ones. This creates backlash among men.
Exaggerating discrimination as the cause of gaps, such as in STEM fields, can deter women and girls who might otherwise pursue those paths. Framing STEM as riddled with sexism ironically produces the opposite effect of what the messaging intends.
Women show higher rates of neuroticism, leading to greater day-to-day depression and anxiety as well as higher rates of clinical depression and clinical anxiety disorders. Men show slightly higher rates of schizophrenia and much higher rates of antisocial personality disorders, sociopathy, and psychopathy. Bipolar disorder shows no sex difference in frequency across cultures and is described as the least sexist disorder on record.
The depression and anxiety differences are viewed as side effects of the neuroticism sex difference. Moderate levels of anxiety proneness are adaptive for women because they encourage protective behaviors and risk avoidance, which aligns with lower maximum reproductive output. However, clinical levels of depression and anxiety are not considered adaptive; they represent maladaptive extremes or byproducts of distributions where moderate levels confer benefits.
Schizophrenia shows higher prevalence in men, but no established theory currently explains this pattern. Autism and ADHD both occur more frequently in boys. ADHD is interpreted as an extreme manifestation of average male traits involving greater activity, lower attentiveness, and higher distractibility. These traits are not problematic within normal ranges but become clinically significant at the tails of the distribution.
Minimizing or denying sex differences carries distinct problems. One risk involves attempting to push people out of traditional gender roles, replacing one form of constraint with a unisex or reversed straightjacket. Another issue concerns explanations for occupational gender gaps, where denying evolved or biological contributions leads to misdiagnosis of causes and ineffective policy responses.
Physical and mental health outcomes also suffer. Cardiovascular symptoms present differently in women, with shortness of breath more common than classic arm pain; underappreciating sex differences leads to missed diagnoses. Autism presents differently in girls, with lower rates of repetitive behaviors, resulting in underdiagnosis when clinicians lack awareness of sex-specific patterns.
Intimate partner abuse is bidirectional at lower levels, including verbal abuse and pushing or shoving. In Western samples, verbal abuse may occur more frequently from women toward men, though patterns differ across cultures. Severe physical aggression remains male-predominant even in Western data, though not exclusively so. Assuming unidirectional patterns leads to underestimating female-to-male aggression.
Exaggerating differences encourages forcing individuals into traditional gender roles and pathologizing those who deviate, despite the existence of within-sex variation and modest average differences. Overstating male predominance in cardiovascular disease can cause clinicians to overlook symptoms in women. Overstating female predominance in depression can cause clinicians to miss cases in men, despite roughly one-third of depression cases occurring in males.
Exaggerating male perpetration of sexual harassment and intimate partner abuse leads to overlooking female perpetration in both directions. An observed irony is that gender-role progressives, who typically minimize sex differences, often reverse this stance when discussing violence and harassment, instead exaggerating the differences in those specific domains.
The book discussed is titled Billion Years of Sex Differences. The author maintains a Substack newsletter featuring monthly link fests that compile 40 to 50 studies and insights.
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