Why is Sex Fun?

Why is Sex Fun? Plot Summary

Introduction

When you pause to think about it, humans have remarkably strange sexual habits compared to other animals. While our primate relatives like chimpanzees only mate when females are visibly in estrus, humans engage in sex throughout the monthly cycle. We have sex in private, form long-term pair bonds, and strangely enough, continue having sex even after a woman becomes pregnant or past her reproductive years. Most peculiarly, we often have sex purely for recreation with no intention of producing children. None of these behaviors makes immediate evolutionary sense. This exploration of human sexuality takes a fascinating journey through evolutionary biology to understand why our sexual behaviors evolved so differently from other species. By examining the unusual characteristics of human sexuality—from concealed ovulation to female menopause, from the battle of the sexes to the evolutionary role of men—we gain profound insights into what makes us human. Understanding our sexual evolution helps explain not just our bedroom behaviors, but also illuminates the development of our social structures, cultural practices, and even our intellectual capacities. The evolutionary forces that shaped our sexuality have profoundly influenced what it means to be human in ways that might surprise you.

Chapter 1: The Unusual Sexual Practices of Humans Among Animals

Human sexuality appears bizarrely out of step with the rest of the animal kingdom. Most mammals follow straightforward reproductive patterns: females advertise fertility through visual, olfactory, or behavioral signals, mating occurs in public, and sex happens only during the brief window when conception is possible. In contrast, humans engage in sex regardless of fertility status, hide their ovulation, seek privacy for sexual activity, and maintain long-term pair bonds for child-rearing. If your dog could comment on human sexual behavior, it would find it utterly perplexing that we continue having sex when a woman is pregnant, past menopause, or not ovulating. These peculiarities appear even more striking when compared with our closest evolutionary relatives. Chimpanzees live in promiscuous social groups where females mate with multiple males during visible estrus periods. Gorilla males gather harems of females but mate infrequently, only when a female is fertile. Orangutans are largely solitary, with males and females coming together briefly for reproduction. None of these great apes show our pattern of lifetime pair-bonding, concealed fertility, or recreational sex. From an evolutionary perspective, these differences demand explanation. The key to understanding human sexuality lies in recognizing it as an evolved adaptation. Just as anatomical features evolve to suit particular environments and lifestyles, so too do sexual and social systems. Sexual behaviors that might seem counterproductive can actually increase reproductive success under specific ecological conditions. For instance, the seemingly self-destructive sexual cannibalism practiced by male spiders makes perfect evolutionary sense when opportunities to mate are rare and female nutrition directly increases offspring number. What evolutionary pressures led humans to develop such unusual sexual behaviors? Our large brains and upright posture certainly played roles in our divergence from other primates, but sexuality formed the third crucial component in this evolutionary trinity. The extended childhood dependency of human offspring created new selective pressures on mating strategies. Our transition to tool use, language acquisition, and complex social structures all interconnected with our evolving sexuality. The development of recreational sex, pair bonding, and even menopause weren't evolutionary accidents but adaptations that helped our ancestors survive and thrive. Human sexual uniqueness evolved through natural selection responding to specific ecological and social challenges our ancestors faced after diverging from other primates around seven million years ago. The question is not just why we evolved sex for fun, but how this evolution shaped the very essence of humanity. Understanding our sexual evolution illuminates not just our bedroom behaviors, but our entire social fabric, from family structures to cultural institutions and even our capacity for cooperation.

Chapter 2: The Battle of the Sexes: Evolutionary Conflicts of Interest

The battle of the sexes isn't just a catchy phrase—it reflects a fundamental evolutionary reality. While both males and females share the goal of reproducing, their optimal strategies for achieving this often conflict dramatically. In most mammal species, the female's obligatory investment in offspring begins with an energy-rich egg and continues through gestation and nursing. Males, meanwhile, can potentially fertilize many females with minimal energy investment per offspring. This asymmetry creates inherent tension: females benefit from careful mate selection and parental investment, while males often benefit from multiple matings with minimal commitment. This conflict manifests differently across species depending on reproductive biology. In externally fertilizing species like many fish, eggs and sperm are released simultaneously into the water. With such minimal investment, both sexes can potentially abandon their offspring immediately. In internally fertilizing mammals, however, the female becomes "committed" to pregnancy after mating, while the male could theoretically leave to seek other mates. This creates a biological basis for the common pattern of maternal care and paternal desertion seen across the animal world. The evolutionary logic resembles a game of chicken. After conception, which parent will be the first to desert, leaving the other to provide care? The answer depends on several factors: relative parental investment, alternative reproductive opportunities, and confidence in parenthood. For internally fertilizing species, females are already committed to substantial investment through pregnancy and cannot immediately reproduce again. Males face uncertainty about paternity—they can never be 100% sure the offspring carries their genes. These asymmetries explain why maternal care predominates in mammals while paternal desertion is common. However, nature offers fascinating exceptions that illuminate the evolutionary rules. In species where males can be certain of paternity—such as fish where males immediately gather externally fertilized eggs—paternal care sometimes evolves. In certain shorebirds like phalaropes, sex roles reverse entirely: females compete for males, who then incubate eggs and care for young. This reversal occurs because shorebird chicks are precocial (born relatively developed) and the energetic cost of producing large eggs leaves females depleted. Males gain reproductive advantages by caring for offspring while females recover to produce more eggs. For humans, the battle of the sexes manifests uniquely. Unlike most mammals, human offspring require substantial biparental care to survive. Yet the fundamental conflict persists: men can potentially reproduce more by seeking multiple partners, while women maximize reproductive success through securing commitment and resources. This tension underlies many human relationship dynamics—from different attitudes toward casual sex to patterns of parental investment. The evolutionary conflict between male and female reproductive interests hasn't disappeared in humans; it has merely taken new forms within our species' unusual sexual system.

Chapter 3: The Male Role: Providers, Show-offs, and Genetic Strategy

The traditional view of human males as "providers" bringing home meat to feed their families seems so obvious that anthropologists long took it for granted. However, recent research challenges this simplistic narrative. Kristen Hawkes and colleagues studying the Ache people of Paraguay made a surprising discovery: men hunting large game actually brought home fewer average calories than women gathering plant foods. More puzzlingly, successful hunters shared meat widely with the entire group rather than prioritizing their own families. This contradicted the basic premise of the provider hypothesis—if men hunt to feed their families, why choose a less reliable food source and then share it with everyone? The paradox deepened when researchers observed similar patterns across multiple hunter-gatherer societies. Hadza men of Tanzania spend considerable time hunting despite succeeding only once every twenty-nine days on average. New Guinea men persist in hunting despite very low yields. This apparently irrational behavior demanded explanation beyond simple provisioning. If men truly aimed to maximize calories for their families, they would adopt more reliable gathering strategies like women. The provider model simply didn't fit the evidence. An alternative explanation emerged: the "show-off" hypothesis. When men bring home occasional large kills, they gain substantial social benefits beyond mere nutrition. Successful hunters receive public acclaim, political influence, and—crucially—sexual attention from women. Studies among the Ache revealed women named good hunters more frequently as lovers and potential fathers of their children. Rather than maximizing daily calories, men appear to be maximizing reproductive opportunities through display hunting. This creates a fascinating evolutionary dynamic. Women benefit most from pairing with reliable "providers," but they also benefit from occasional access to resources and genes from successful "show-offs." Men, meanwhile, face competing strategies: provide consistently for fewer offspring with greater certainty of paternity, or display hunting prowess to attract multiple mating opportunities with less certainty of paternity. Mathematical models suggest that under many ecological conditions, the show-off strategy yields higher reproductive success despite bringing fewer resources to a man's primary family. This evolutionary perspective illuminates modern male behavior patterns. While many contemporary men are devoted providers, the show-off strategy persists in various forms—from expensive status symbols to risk-taking behaviors that demonstrate genetic quality. Time-budget studies consistently show women in industrialized nations spend significantly more hours on childcare and household responsibilities than men. Even in modern contexts, the evolutionary tensions between male mating effort and parental investment remain visible. The male role in human society reflects not a simple division of labor but a complex evolutionary compromise between competing reproductive strategies.

Chapter 4: Female Menopause: An Evolutionary Advantage

Female menopause represents one of the most puzzling features of human sexuality from an evolutionary perspective. Most wild animals remain fertile until death or nearly so. Even human males can potentially father children into their nineties. Yet women experience a complete cessation of fertility around age fifty, often living decades beyond their reproductive capacity. This poses a theoretical paradox: how could natural selection—which maximizes reproductive output—favor genes that shut down reproduction entirely? On the surface, menopause seems to contradict evolutionary logic. Traditional explanations that menopause merely reflects recent increases in human lifespan fail scrutiny. If menopause were simply an artifact of modern longevity, we would expect all bodily systems to fail simultaneously near the end of our "natural" lifespan. Yet only female reproduction shuts down early while other systems continue functioning for decades. The proximate mechanism—depletion of eggs—doesn't explain why evolution didn't provide women with more eggs or eggs that remain viable longer, as in other long-lived animals like elephants and whales. The evolutionary solution lies in understanding how stopping reproduction can paradoxically increase reproductive success. Human children require exceptionally long periods of care and learning before becoming independent. A mother's death during childbirth or while caring for dependent offspring jeopardizes her existing children's survival. As women age, the risks of maternal mortality increase while the chances of successful pregnancy decrease. At some point, the genetic payoff from continued reproduction becomes outweighed by the risk to existing offspring—one's established genetic investment. This "grandmother hypothesis" gains support from studies of traditional societies. Kristen Hawkes observed that post-menopausal Hadza women spent more hours gathering food than any other age group. These industrious grandmothers directed their harvests toward grandchildren and adult children, effectively converting their energy into support for relatives sharing their genes. By ceasing reproduction and redirecting resources to existing kin, older women could have greater impact on their genetic legacy than by producing additional high-risk offspring. An often-overlooked benefit of post-reproductive longevity emerged in pre-literate societies: the value of accumulated knowledge. In traditional societies without writing, elderly individuals served as repositories of crucial information about rare environmental events like famines, floods, or cyclones that might occur only once in several decades. A seventy-year-old grandmother might be the only person who remembered survival techniques from the last major drought fifty years earlier. This "library" function made elderly women essential not just to their immediate descendants but to entire communities of genetic relatives. Human female menopause thus represents not an evolutionary accident but a sophisticated adaptation that increased genetic legacy by protecting previous reproductive investments and enhancing group survival through intergenerational knowledge transfer.

Chapter 5: Body Signals: Truth in Advertising and Sexual Selection

Humans rely heavily on body signals to communicate age, sex, health, and reproductive status. When we enter a room of strangers, we instantly assess who attracts us based on physical features—faces, body shapes, hair, skin—often without conscious awareness. These rapid assessments might seem superficial, but they reflect deeply ingrained evolutionary responses to signals that historically conveyed important biological information. Just as animals evolved specific markings, colors, or structures to communicate with potential mates, humans developed their own signaling systems. Three competing theories explain how such sexual signals evolve. Fisher's "runaway selection" model suggests that once females begin preferring males with certain traits, a feedback loop develops where both the trait and the preference for it become increasingly exaggerated, like a peacock's tail growing more elaborate over generations. Zahavi's "handicap principle" proposes that costly, seemingly detrimental traits serve as honest advertisements of genetic quality—only truly superior individuals can afford such handicaps. The "truth in advertising" model suggests that costly signals are honest indicators directly linked to important survival traits, like deer antlers signaling nutritional status and fighting ability. Human sexual signals often function as honest advertisements of reproductive value. Men's muscular development serves as a truthful indicator of physical condition, nutritional status, and hormone levels that couldn't be easily faked before modern supplements. Similarly, facial beauty correlates with health, symmetry, and resistance to parasites and disease—especially important in pre-modern societies. Women's body fat distribution in breasts and hips honestly signals nutritional reserves crucial for pregnancy and lactation, while its specific pattern may indicate estrogen levels and fertility. Not all human signals carry deeper meaning. The pubic and axillary hair that appears in both sexes during puberty functions as a simple, arbitrary marker of reproductive maturity, similar to the red spot on a gull's bill that triggers feeding behavior in chicks. These signals carry no deeper information about genetic quality but serve as clear markers of developmental status. Male beards and voice deepening likewise function primarily as arbitrary age indicators rather than quality advertisements. One particularly intriguing case is the human male penis, which is significantly larger than necessary for reproduction compared to our great ape relatives. This suggests possible sexual selection through female preference, male competition, or both. When New Guinea highland men create enormous decorative penis sheaths up to two feet long, they essentially demonstrate what the human penis might look like if not constrained by practical limitations. Whether primarily aimed at impressing females or intimidating other males remains debated, but the signal function seems clear. Like other aspects of human sexuality, our body signals reflect our evolutionary history as social primates while also showing unique adaptations that distinguish us from our closest relatives.

Chapter 6: Concealed Ovulation and Its Impact on Human Society

Unlike most female mammals who advertise fertility through visual, behavioral, or olfactory signals, human women conceal their ovulation. A female chimpanzee develops a conspicuous pink swelling when fertile, attracting males and mating only during this brief period. Human females, by contrast, show no reliable external signs of ovulation and remain sexually receptive throughout their menstrual cycle. This concealment represents a remarkable evolutionary deviation from the mammalian norm and has profoundly shaped human sexuality and social organization. Concealed ovulation creates an information asymmetry with significant consequences. Without clear fertility signals, men cannot determine when a woman is most likely to conceive. This uncertainty encourages males to remain sexually engaged throughout the female cycle rather than abandoning females between fertile periods. In species with advertised ovulation, males often compete intensely for access to females during fertile periods but show little interest at other times. Human males, lacking reliable fertility cues, must maintain longer associations with females to ensure reproductive success. This extended association created the foundation for human pair-bonding. A male who remained with one female continuously rather than pursuing multiple females sequentially had better chances of fathering her offspring. Simultaneously, females gained benefits from establishing stable relationships with males who provided resources, protection, and parental investment. Concealed ovulation thus promoted the evolution of long-term partnerships—a reproductive strategy rare among primates but central to human social organization. Perhaps most significantly, concealed ovulation enabled the separation of sex from reproduction. With fertility hidden and sexual receptivity extended throughout the cycle, sex could serve purposes beyond conception. Recreational sex strengthened emotional bonds between partners, reinforced cooperative relationships, and provided incentives for continued male investment. This transformation of sex into a social bonding mechanism represented a revolutionary development in primate evolution. The human pattern of concealed ovulation and continuous sexual receptivity created a positive feedback loop with other uniquely human traits. Pair-bonding facilitated cooperative child-rearing needed for our species' extended juvenile dependency. Regular sex reinforced emotional attachment between partners. The separation of sex from reproduction allowed sexuality to become intertwined with complex social relationships. Together with our large brains and bipedalism, concealed ovulation helped create the conditions for human cultural evolution—from language development to complex social structures. What initially evolved as a reproductive strategy ultimately transformed human society and consciousness in ways that extended far beyond reproduction itself.

Chapter 7: The Evolutionary Significance of Recreational Sex

Most animals engage in sex solely for reproduction. They mate during specific fertility windows, and once fertilization occurs, sexual activity ceases until the next reproductive opportunity. Humans, remarkably, have evolved a dramatically different pattern: we engage in sex primarily for pleasure, with reproduction as an occasional byproduct rather than the primary purpose. This separation of sex from reproduction represents one of our most distinctive evolutionary innovations and has had profound consequences for human social development. Recreational sex provides several evolutionary advantages. First, it strengthens bonds between partners by providing ongoing physical pleasure and emotional connection. While many animals form pair bonds for a single breeding season, human couples can maintain relationships for decades, cooperating to raise multiple offspring to maturity. Regular pleasurable sex creates powerful incentives for partners to remain together through the challenges of child-rearing. The neurochemical rewards of sex—including oxytocin and dopamine release—reinforce attachment precisely because our ancestors who formed stable partnerships had greater reproductive success. The unpredictability of human fertility further enhances the bonding function of recreational sex. Since women don't display obvious signs of ovulation, men cannot simply mate during fertile periods and leave. Regular sex throughout the cycle increases the probability of conception while simultaneously reinforcing the pair bond. This stands in stark contrast to chimpanzees, where males show interest in females primarily during visible estrus. By making sex pleasurable regardless of fertility status, evolution ensured ongoing interaction between potential parents. Recreational sex also facilitated the evolution of human social complexity. Sex became integrated into relationship dynamics beyond mere reproduction, creating nuanced systems of courtship, commitment, and partnership. The social dimensions of human sexuality—including mate selection based on compatibility, resource sharing, and parental potential—became increasingly important. Sexual pleasure provided the evolutionary incentive for developing these complex social arrangements that ultimately enhanced reproductive success through cooperative child-rearing. Perhaps most profoundly, recreational sex helped create the conditions for human cultural evolution. By freeing sexuality from strict reproductive timing, humans could develop elaborate sexual customs, taboos, and practices that varied across cultures. The energy previously devoted to competitive mating during brief fertility windows could be redirected toward cooperative endeavors. Together with our large brains and extended childhoods, recreational sexuality helped create the distinctively human pattern of cultural transmission and technological innovation that has defined our species' extraordinary ecological success.

Summary

The unique features of human sexuality—concealed ovulation, recreational sex, male parental investment, female menopause, and complex body signals—represent sophisticated evolutionary adaptations rather than random quirks. These traits evolved as an integrated system that encouraged pair-bonding, facilitated extended childcare, and enhanced cooperation. Our sexual distinctiveness helped enable the development of language, culture, and technology that separates humans from other primates. The evolution of sex for pleasure rather than mere reproduction stands among our most significant adaptations, creating the foundation for the complex social structures that define humanity. Looking beyond our bedrooms, these evolutionary insights raise fascinating questions about human nature and modern society. How do our evolved sexual strategies manifest in contemporary relationships? What happens when modern contraception completely separates sex from reproduction? How should we understand the tensions between our evolved psychology and current social arrangements? These questions invite us to explore not just human sexuality but human potential. By understanding the evolutionary forces that shaped our intimate behaviors, we gain deeper insights into ourselves as a species uniquely positioned between our biological inheritance and our capacity for cultural innovation and conscious choice.

About Author

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Jared Diamond

Jared Mason Diamond is an American scientist, historian, and author best known for his popular science and history books and articles. Originally trained in biochemistry and physiology, Diamond is commonly referred to as a polymath, stemming from his knowledge in many fields including anthropology, ecology, geography, and evolutionary biology. He is a professor of geography at UCLA. In 2005, Diamond was ranked ninth on a poll by Prospect and Foreign Policy of the world's top 100 public intellectuals.

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