This episode features evolutionary psychologist Steve Stewart-Williams discussing his book on sex differences, exploring why the topic is controversial, what the science actually shows, and why acknowledging differences matters for understanding human behavior, health, and society.
Why Sex Differences Are Controversial and Why They Matter
The controversy stems largely from a long history of scientific sexism (e.g., 19th-century claims that intellectually accomplished women are “as rare as a two-headed gorilla”), making people wary that discussing evolved differences will reopen the door to discrimination.
Stewart-Williams argues science has disproven those old ideas and that recognizing average differences does not justify unequal treatment; his core philosophy: “let people be themselves” — whether they fit traditional roles or not.
Studying sex differences is important because it provides deep explanatory power (like understanding moon phases), helps us understand ourselves and each other, and allows society to steer toward desired outcomes rather than ignoring root causes of behavior.
Defining Sex Biologically
Sex is defined by gamete size: males produce small gametes (sperm), females produce large gametes (eggs) — a near-universal binary across sexually reproducing species.
This definitional truth reflects an evolutionary stability: isogamy (equal gametes) is unstable because selection favors either larger, nutrient-packed gametes or smaller, numerous gametes that seek them out.
Intersex conditions, hermaphroditism, and within-sex variation do not disprove the binary; they are variations within the two gamete-producing categories.
Human sperm production is ~100–300 million per day per male; global daily sperm production is ~200 quadrillion vs. ~70 million eggs released.
Six Lines of Evidence for Innate Contributions to Sex Differences
Early emergence: Differences in aggression and risk-taking appear as soon as children are mobile; more boys than girls visit ERs from risk-taking from early childhood.
Puberty amplification: Many differences (e.g., aggression) skyrocket at puberty across cultures, suggesting hormonal activation rather than sudden cultural pressure.
Resistance to cultural suppression: Cultures typically try to suppress male aggression, yet the sex difference persists and even grows post-puberty.
Persistence across time: Sex differences in mate preferences (men prioritize looks, women prioritize resources/status) and career interests (things vs. people) remained stable throughout the 20th century despite strong social pressure to change them.
Hormonal correlates: Prenatal testosterone exposure predicts male-typical traits (aggression, risk-taking, things-oriented interests, lower parental inclination) regardless of sex; girls with CAH (high prenatal testosterone) show masculinized preferences and higher rates of bisexual/lesbian orientation.
Cross-cultural universality: Key differences (male aggression, homicide perpetration, risk-taking, female parental care) appear in every or nearly every culture studied.
Non-human parallels: Similar differences appear in other species facing comparable evolutionary pressures, pointing to shared selection mechanisms.
The Gender Equality Paradox
Social role theory predicts sex differences should shrink in more gender-equal, less patriarchal societies; instead, many psychological and physical differences (personality, cognitive abilities, height) are larger in more egalitarian nations.
This paradox surprised even evolutionary psychologists, who expected patriarchy to amplify differences.
Rebuttals (e.g., men and women in patriarchal societies compare themselves only within sex, not across) are weakened because the pattern appears in objective measures like spatial ability and height, not just self-reports.
Major Sex Differences: Physical and Psychological
Largest physical differences: Reproductive anatomy, upper body strength (near-zero overlap), voice pitch (unusually large compared to other apes).
Largest psychological difference: Sexual orientation — the vast majority of men are primarily attracted to women, and vice versa; this difference is nearly categorical and has a clear evolutionary rationale (without it, reproduction fails).
Sexuality differences: Men show greater interest in casual sex and sexual variety (sociosexuality, d ≈ 1.0, ~70% of random male-female pairs the man scores higher); both sexes desire long-term commitment equally, but men add a short-term mating interest.
Visual vs. narrative arousal: Men are more aroused by visual sexual stimuli (driving porn consumption); women are more drawn to romance novels (relationship narratives), even though men report falling in love faster and suffering more after breakups.
Overperception bias: Men overestimate women’s sexual interest (projecting their own); women underestimate men’s commitment interest — a “smoke detector” asymmetry where missing a mating opportunity is costlier for men, false alarms costlier for women.
Evolutionary Foundations: Parental Investment and Reproductive Variance
Bateman’s principle: Reproductive variance is greater in males; Trivers’ refinement: Parental investment (gestation, lactation, care) is the main driver — females invest more per offspring, lowering their maximum offspring number.
In humans, biparental care reduces male reproductive variance relative to other mammals (likely less extreme than the often-cited 40%/80% ancestral reproduction ratio), muting but not eliminating sex differences.
Fisherian equilibrium: Sex ratio stabilizes at 50/50 because if one sex becomes rarer, selection favors producing that sex.
Ecological factors (e.g., solitude limiting mating opportunities) can also constrain variance.
Mate Preferences: Similarities and Differences
Overwhelming similarity: Both sexes prioritize kindness, intelligence, mutual attraction, emotional stability, and good looks in long-term partners.
Consistent differences: Men weight physical attractiveness more (linked to youth/fertility cues, amplified by menopause); women weight resources and status more (linked to biparental investment capacity and/or “good genes” signaling).
Short-term context: The attractiveness gap evaporates; women become at least as focused on looks as men, because only genetic quality is at stake.
Cultural “peacock’s tails”: Displays of status (Rolex, VIP booth) and beautification are human equivalents of the peacock’s tail — culturally variable but functionally similar.
Sperm donor preferences: When seduction is removed, women’s stated preferences align with the same evolved criteria (health, intelligence, status).
Aggression and Violence
Direct aggression: Consistently male-biased across cultures; effect size grows with severity (verbal d ≈ 0.5 → physical → homicide 90%+ male perpetrators in every nation with data).
Chimpanzee parallel: Male chimps commit 92% of lethal aggression, are 73% of victims — virtually identical to human ratios.
Evolutionary logic: Male-male competition for status/resources/mates selects for aggression; female aggression is less rewarded because offspring ceiling is lower.
Female attraction to formidable men: Protectiveness is attractive; aggression per se is not. Women face a trade-off: a man capable of violence can protect but may also direct it inward. Willingness to protect matters more than ability alone.
Sexual Violence and Jealousy
Sexual violence: Men perpetrate the vast majority (effect size ~2.0); evolutionary logic combines male interest in casual sex with willingness to use force — seen across species.
Female distress: Rape is deeply upsetting because it circumvent evolved female choice, which is critical given higher per-offspring investment.
Jealousy differences: Men more distressed by sexual infidelity (paternity uncertainty); women more distressed by emotional infidelity (risk of partner desertion and resource loss). These reactions can persist even when evolutionary stakes are gone (e.g., 30-year-old affair revealed via DNA test).
Personality Differences (Big Five)
Neuroticism: Women higher (d ≈ 0.2–0.5) — linked to greater self-protectiveness, the flip side of male risk-taking.
Agreeableness: Women higher (d ≈ 0.2–0.5) — linked to lower aggression and different social bonding styles.
Other traits: Small or inconsistent differences in extraversion, conscientiousness, openness.
People vs. Things: One of the largest psychological differences (d ≈ 1.0+), extremely cross-culturally consistent (53/53 and 80/80 nations), stable for 100+ years, linked to prenatal testosterone, and resistant to social engineering (gender equality paradox: gap widens in egalitarian nations).
Cognitive Abilities
General intelligence (IQ): No average sex difference; tests not rigged for equality show same means. Male variance slightly higher (more extremes both ways), but effect is small.
Specific abilities: Men average higher in spatial rotation; women average higher in verbal ability and are more likely to be “double threats” (high math + high verbal) at the extreme right tail.
Overall: Sexes far more similar than different in cognition; differences smaller than in personality or interests.
Physical Health and Lifespan
Men: More cardiovascular disease (until very old age), most cancers (except reproductive), die younger in nearly all cultures — even where maternal mortality is high.
Women: More autoimmune disorders, pain conditions.
Testosterone cost: Eunuchs (castrated pre-puberty) lived longer than intact males of same status; testosterone drives strength/aggression but shortens lifespan.
Mortality impact: Equalizing male mortality to female rates would save more life-years than curing cancer (Randy Nesse).
Risk trade-off: 88% of Darwin Award winners (stupid self-inflicted deaths) are men; 90% of Carnegie Hero Awards (risking life to save strangers) are also men — risk cuts both ways.
Mental Health Differences
Women higher: Depression, anxiety (clinical and subclinical) — likely downstream of neuroticism difference; adaptive at mean (self-protection), maladaptive at extremes (spandrel).
Men higher: Antisocial personality, psychopathy, schizophrenia (slight), autism, ADHD — latter two linked to male-typical activity/inattention distribution.
Bipolar disorder: No sex difference — “least sexist disorder.”
Costs of Denying Sex Differences
Health: Overlooking female heart attack symptoms (shortness of breath vs. classic arm pain); underdiagnosing autism in girls (different presentation, fewer repetitive behaviors).
Social problems: Assuming intimate partner violence is unidirectional (male→female) misses bidirectional low-level aggression and female→male verbal abuse; severe violence remains male-predominant.
Policy: Misdiagnosing occupational gaps as purely discrimination leads to ineffective/coercive interventions, resentment, reverse discrimination, and deterring women from fields falsely labeled hostile.
Costs of Exaggerating Sex Differences
Gender straightjackets: Pushing people into traditional roles (or reverse roles) ignores within-sex variation and the modest size of most differences.
Health blind spots: Assuming cardiovascular disease is “male” misses it in women; assuming depression is “female” misses it in men (~1/3 of cases).
Social problems: Exaggerating male perpetration of harassment/violence overlooks male victims and female perpetrators.
Ideological irony: Gender progressives often minimize differences except for aggression/harassment, where they maximize them.
Practical Philosophy: Truth, Barriers, and Freedom
Tell people all the facts: Explain that risk-taking cuts both ways (more success and more failure); let men and women choose with full information.
Remove bias and barriers aggressively: Discrimination is real and must be fought.
Respect choices: After barriers fall, remaining gaps may reflect preferences — not injustice. Forcing equality of outcome creates new injustices and backlash.
Soft bigotry of male expectations: Valuing male-typical paths (STEM, full-time career) over female-typical ones (care, part-time, motherhood) is itself sexist; both deserve respect.
