Why is Sex Fun

Why do humans have sex for fun, even when it doesn't lead to children? In Why Is Sex Fun?, Jared Diamond challenges you to reconsider everything you think you know about intimacy, gender roles, and reproduction. Diamond argues that our sexual behaviors—so natural to us—are, by biological standards, spectacularly weird. Why do women conceal ovulation, experience menopause, and have breasts that exist long before lactation? Why do men stay with partners and participate in child-rearing when few other male mammals do? These peculiar human traits reveal deep evolutionary strategies that shaped our species and civilization itself.

Human Sex: The Exception, Not the Rule

Most mammals mate only for procreation during brief fertile windows, driven by biological needs rather than recreation. Humans, in contrast, engage in sex privately, year-round, and often without reproductive purpose. This behavior, Diamond notes, was crucial to our evolutionary success—it fostered pair bonding, cooperation, and the stable parenting required to raise helpless human babies. By viewing humans through the eyes of a hypothetical talking dog, Diamond humorously shows just how bizarre our behavior would appear to other species: private sex, constant desire, and continued intimacy even through pregnancy and after menopause.

Why Our Sexuality Made Us Human

Diamond contends that our sexual distinctiveness is not a random accident but a critical evolutionary adaptation. He explores how concealed ovulation and long-term pair bonding may have evolved to ensure paternal involvement—the glue of the human family. Our ancestors required cooperation between males and females to raise slow-developing children, and recreational sex became a mechanism for maintaining that bond. In this sense, sex for pleasure encouraged social stability, allowing culture, language, and technology to develop.

The Broader Purpose of the Book

More than an exploration of sex, Why Is Sex Fun? serves as an inquiry into human nature. Diamond applies evolutionary biology and comparative zoology to explain why our species stands alone in its intimacy, monogamy, and emotional complexity. He asks questions that connect biology with everyday human experience: Why don't men breastfeed? Why did female menopause evolve? Why do we show sexual signals that seem unnecessary, like large breasts and deep voices? Each answer, he suggests, reveals a balance between biology’s demands and the cultural structures that emerged to manage them.

Why It Matters for You

Understanding how and why sex became 'fun' sheds light on broader human dilemmas—desire, jealousy, fidelity, and cooperation. Diamond invites you to see your instincts not as isolated emotions but as evolved mechanisms with deep biological roots. In doing so, he helps you appreciate that your relationships, conflicts, and pleasures are part of an ancient story of survival—one that made humans the most distinctive animals on Earth.

Diamond frames male and female relationships as an enduring evolutionary negotiation rather than simple partnership. He explains that natural selection operates differently on men and women, producing built-in conflicts between reproduction and survival. Understanding this tension helps you see why differences in sexuality and parental behavior persist even today.

Investment and Opportunity

For females, reproduction involves enormous investment—pregnancy, childbirth, and lactation—which limits their opportunities for further offspring. Males, on the other hand, invest minimally in fertilization yet can pursue many potential mates. This imbalance creates an evolutionary 'game of chicken'—each sex trying to maximize genetic success while minimizing personal cost. In species like humans, this tension led to the evolution of cooperation: males staying to help rear young because the offspring’s survival required it.

Confidence in Parenthood

Unlike females, who can always be certain of maternity, males face uncertainty. Paternity paranoia has fueled restrictive cultural practices—from virgin bride prices to chastity laws and even female infibulation. These reflect ancient biological anxieties about investing resources in non-biological offspring. Diamond contrasts these behaviors with examples from the animal kingdom, where paternity assurance shapes whether males participate in child care, as in fish and frogs with external fertilization.

From Animal Behavior to Human Society

By comparing species, Diamond shows that the human family—male partnership, monogamy, and shared parenting—is rare in nature. It evolved because helpless infants demanded cooperation. He argues that understanding this biological logic clarifies modern gender conflicts about fidelity, responsibility, and sexual freedom. Even our moral codes are echoes of evolutionary strategies that maximized ancestral survival.

Diamond’s exploration of male lactation reveals how small physiological possibilities become shaped—or inhibited—by evolution. Technically, men can be coaxed to lactate through hormones or stimulation, and certain mammals even do so naturally. But humans didn’t evolve this capacity, because biology follows usefulness, not fairness. The question isn’t “can men lactate?” but “why would evolution favor it?”

Physiological Potential vs. Evolutionary Commitment

Men have the hormonal machinery for milk production, but Diamond explains that our species has committed to a design where females handle lactation. He uses analogies—like trucks vs. airplanes—to illustrate “evolutionary commitment”: once evolution builds complex systems (like pregnancy and breastfeeding), they become hard to reverse. Birds evolved one set of reproductive commitments (egg laying), mammals another (internal gestation and milk production). These paths define which traits are possible.

Why Mother’s Milk Became Exclusive

Because mammals evolved with internal fertilization and long pregnancies, females already invest heavily in offspring. Lactation extended that investment. For men, whose evolutionary advantage lies in seeking multiple partners, shared nursing offered too little genetic payoff. Over millions of years, evolution reinforced this division of labor, not out of fairness, but efficiency.

Could Male Lactation Still Evolve?

Diamond notes that changing social and technological conditions—paternity testing, fertility treatments, and shared parenting—make male lactation theoretically advantageous today. He speculates that humans might one day short-circuit evolution through medicine. But biological inertia and psychological barriers remain: men’s identity and ego are tied to non-nursing roles. Diamond concludes that our species’ greatest capacity is to make intentional, counter-evolutionary choices—to nurture by will, not biology.

Why do humans have sex all the time, even when it can’t result in conception? Diamond calls this the paradox of recreational sex. Our constant sexual receptivity and hidden ovulation make us exceptional among mammals—and these peculiarities shaped love, jealousy, and even marriage.

The 'Daddy-at-Home' Theory

According to biologists Richard Alexander and Katharine Noonan, concealed ovulation evolved to promote monogamy. If men couldn’t tell when women were fertile, they’d stay close and have sex often, increasing chances of fathering offspring—and reducing temptations to stray. Diamond notes how this contrasts with other species where visible ovulation prompts males to leave once fertilization occurs.

The 'Many-Fathers' Theory

Anthropologist Sarah Hrdy offered another view: concealed ovulation evolved to confuse males, lessening infanticide risks. In species like gorillas and vervet monkeys, males kill unrelated infants. By mating with multiple partners, females make many males uncertain of paternity—and therefore more protective or at least non-lethal toward infants. Diamond cleverly points out that both theories may be right at different stages in evolution: hidden ovulation first prevented infanticide, then reinforced monogamy.

From Evolution to Emotion

Diamond’s comparative analysis of sixty-eight primate species reveals that concealed ovulation and monogamy co-evolved repeatedly, with sexuality changing function from survival to intimacy. Our ancestors’ shift from harems to pair bonds transformed sex from reproduction into affection. What began to protect children eventually came to sustain love and family.

Are men truly providers, or are they evolutionary show-offs? Diamond examines men’s roles from hunters to modern fathers and reveals that much of male behavior—then and now—may serve self-interest more than family welfare.

The Provider vs. The Show-Off

Research on Ache and Hadza hunter-gatherers (by anthropologist Kristen Hawkes) showed that men’s big-game hunting yields less reliable food than women’s gathering. Men often share meat widely instead of bringing it home. Diamond argues this is a “show-off” strategy: hunting provides status, admiration, and extramarital opportunities. It’s a form of biological advertising—successful hunters gain prestige, attract mates, and pass on more genes, even if their families receive less benefit.

Modern Echoes

Modern men still pursue equivalent displays: careers, sports, luxury cars—symbolic hunts that demonstrate capability. Meanwhile, women remain primary caregivers. Diamond calls this a continuation of ancient reproductive economics: men trade attention for status, women trade stability for survival. Understanding this pattern helps you see relationships not as moral failures but as adaptations with lingering instincts.

The Hope for Evolutionary Redemption

Diamond suggests awareness is our salvation. Unlike Ache hunters, modern men can choose responsibility over instinct. Recognizing the “show-off” circuit isn’t shameful—it’s an opportunity to redirect it toward creativity, nurturing, and care.

Menopause seems unfair—why should women’s fertility end decades before death? Diamond shows that menopause is an evolutionary triumph, not a defect. By stopping reproduction early, women ultimately help their genes survive more effectively.

Making More by Making Less

Raising a human child demands extraordinary time and resources. Older mothers risk dying in childbirth and losing existing children who still need care. Evolution favored women who stopped bearing children earlier, investing instead in grandchildren and family stability. The 'grandmother effect,' confirmed by studies of Tanzanian Hadza women, shows that postmenopausal women often gather more food than younger women and sustain their descendants.

Menopause and Human Longevity

Humans are unique in regularly outliving fertility, a pattern shared only with pilot whales. Diamond explains that older non-reproductive individuals became vital repositories of wisdom. In preliterate societies, elders preserved survival knowledge—like food sources during famines or storms. Menopause thus extended life and culture, transforming biological limits into social strength.

An Evolutionary Balancing Act

Diamond concludes that menopause represents evolution’s trade-off between quantity and quality. By ceasing childbirth, women increase family survival, mentorship, and the continuity of culture. Menopause wasn’t nature’s cruelty—it was humanity’s strategic refinement.

Diamond ends by examining how bodies function as advertisements—honest, deceptive, and sometimes brilliantly absurd. From muscles to breasts to voices, our anatomy communicates age, fertility, and attractiveness, often unconsciously.

Signals in Nature

Drawing from animal experiments, Diamond shows how simple traits—colored spots, long tails, or antlers—signal genetic quality. He cites three models of signaling: Fisher’s runaway selection (traits exaggerate through attraction), Zahavi’s handicap principle (expensive traits prove strength), and Brown’s truth-in-advertising (traits honestly reflect fitness). These models apply not only to peacocks but to people.

Human Signals

Men’s muscles and voice depth, women’s breasts and body fat, even symmetrical faces serve as biological ads. Diamond argues these features evolved as signals of health, fertility, and strength rather than mere aesthetics. Our fascination with beauty stems from ancient instincts assessing “good genetics.”

The Penis as a Runaway Signal

In his most daring theory, Diamond suggests that the human penis might be a sexual ornament—its size exaggerated by runaway selection and retained as a status symbol among males. Like New Guinea men’s ornate phallocarps, it may signal virility to women and dominance to men. The truth, Diamond says, is that our biology is still marketing itself—proof that evolution never ceased being seductive.