Even in our supposedly enlightened times, “Is it a boy or a girl?” is still the first question asked of nearly every newborn — and the answer continues to shape how the child is raised. Research shows that from infancy, boys and girls are touched, comforted, spoken to, and treated differently by parents and caregivers. These early experiences may reinforce sex-typical patterns of behavior that often persist into adulthood.
People are intrinsically fascinated by psychological sex differences — the average differences between men and women in personality, behavior, and preferences. Psychologists have studied this topic systematically for decades, beginning with landmark works like The Psychology of Sex Differences (1974) by Eleanor Maccoby and Carol Jacklin. That book helped spark a wave of research that continues to this day. Since then, increasingly sophisticated methods have enabled researchers to detect subtle but consistent differences in how men and women think, feel, and act.
Men and women use language and think about the world in broadly similar ways. They experience the same basic emotions. Both seek kind, intelligent, and attractive romantic partners, enjoy sex, get jealous, make sacrifices for their children, compete for status, and sometimes resort to aggression in pursuit of their interests. In the end, women and men are more alike than different. But they are not identical.
To be sure, sociocultural influences play a role in creating those differences. But environmental factors don’t act on blank slates. To understand young men and young women, we must consider not only cultural context but also evolved sex differences. We are, after all, biological creatures. Like other mammals, we share similar physiology and emotional systems, so it’s not surprising that meaningful differences exist between human males and females.
To understand why psychological and behavioral sex differences evolved, the key concept is parental investment theory, developed by evolutionary biologist Robert Trivers in 1972. The basic idea is straightforward: the sex that invests more in offspring tends to be more selective when choosing a mate. This selectivity follows basic evolutionary logic: those with more to lose are more cautious and risk-averse. To put the stakes in perspective: raising a child from birth to independence in a traditional, preindustrial society requires an estimated 10 million to 13 million calories — the equivalent of about 20,000 Big Macs. For women, reproduction is enormously expensive.
Men also incur reproductive costs, though of a different kind. On average, they have about 20 percent more active metabolic tissue — such as muscle — that fuels their efforts in competition, courtship, and provisioning. While pregnancy requires a large, immediate investment from women, men’s reproductive effort is more gradual, spread out over a lifetime. In evolutionary terms, both sexes pay a price for reproduction, but in different currencies — women through gestation and caregiving, men through physical competition and resource acquisition.
Yet while nature can inform our understanding of human behavior, it does not dictate how we ought to live. A clearer grasp of sex differences can help guide our decisions. It cannot define our values.
Rob Henderson, “Sex Differences Don’t Go Away Just Because You Want Them To”, Rob Henderson’s Newsletter, 2025-08-03.
November 10, 2025
QotD: “Is it a boy or a girl?”
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