In environments where some males achieve much higher reproductive success than others (high variance), investment in sons produces higher return than investment in daughters—a son in good condition can father many offspring; a son in poor condition fathers few or none. But in poor environments, a son has low reproductive prospects regardless of parental investment, while a daughter's reproductive prospects (with contraception aside) remain relatively constant.1
The Trivers-Willard hypothesis predicts that parents in good condition (high resources, high status) should bias investment toward sons; parents in poor condition should bias toward daughters. This maximizes reproductive success by allocating heavy investment to whichever sex produces higher return given the parent's condition.2
The mechanism is condition-dependent sex allocation: parents adjust the sex ratio of offspring they produce (or the investment they allocate to each sex) based on their own condition—wealth, status, health.3
Human and primate studies show evidence consistent with the hypothesis:4
Wealth-Based Allocation: High-status/high-wealth males in some human societies have more sons than daughters, and sons have higher reproductive success. High-status females have more sons and invest more in sons. Low-status parents show the opposite pattern or no preference.5
Health-Based Allocation: Parents in good health at conception show slight bias toward sons; parents in poor health show slight bias toward daughters.6
Paternity Confidence: Fathers allocate more investment to sons when paternity confidence is high (ensuring the son is genetically theirs). When paternity is uncertain, investment allocation to daughters increases (daughters are genetically certain).7
Yet the effects are modest—the bias is typically 5-15% shifts in sex ratio or investment allocation, not dramatic reversal. The hypothesis is directionally supported but the effect sizes are smaller than predicted.8
The mechanism isn't explicit reproductive calculation—parents don't consciously think "I should have sons." Rather, condition affects factors that influence sex allocation:9
Hormone Levels: Parental testosterone and other hormones influence sex ratio, possibly through effects on the sex chromosome of sperm or through uterine environment. High-condition parents show different hormone profiles than low-condition parents.10
Parental Preference: Parents might unconsciously allocate more care to sons when in high condition and more to daughters when in low condition, without conscious awareness of the pattern.11
Sex-Specific Responses to Investment: Sons might be more responsive to paternal investment (sons benefit more from resources in high-resource environments), while daughters are less variable in reproductive outcome regardless of investment.12