Jared Mason Diamond

Why is sex fun?

To Marie, My best friend, coparent, lover, and wife.


The subject of sex preoccupies us. It's the source of our most intense pleasures. Often it's also the cause of misery, much of which arises from built-in conflicts between the evolved roles of women and men.

This book is a speculative account of how human sexuality came to be the way it now is. Most of us don't realize how unusual human sexual practices are, compared to those of all other living animals. Scientists infer that the sex life of even our recent apelike ancestors was very different from ours today. Some distinctive evolutionary forces must have operated on our ancestors to make us different. What were those forces, and what really is so bizarre about us?

Understanding how our sexuality evolved is fascinating not only in its own right but also in order to understand our other distinctively human features. Those features include our culture, speech, parent-child relations, and mastery of complex tools. While paleontologists usually attribute the evolution of these features to our attainment of large brains and upright posture, I argue that our bizarre sexuality was equally essential for their evolution.

Among the unusual aspects of human sexuality that I discuss are female menopause, the role of men in human societies, having sex in private, often having sex for fun rather than for procreation, and the expansion of women's breasts even before use in lactation. To the layperson, these features all seem almost too natural to require explanation. On reflection, though, they prove surprisingly difficult to account for. I'll also discuss the function of men's penises and the reasons women but not men nurse their babies. The answers to these two questions seem utterly obvious. Within even these questions, though, lurk baffling unsolved problems.

Reading this book will not teach you new positions for enjoying intercourse, nor will it help you reduce the discomfort of menstrual cramps or menopause. It will not abolish the pain of discovering that your spouse is having an affair, neglecting your joint child, or neglecting you in favor of your child. But this book may help you understand why your body feels the way it does, and why your beloved is behaving the way he or she is. Perhaps, too, if you understand why you feel driven to some self-destructive sexual behavior, that understanding may help you to gain distance from your instincts and to deal more intelligently with them.

Earlier versions of material in some chapters appeared as articles in Discover and Natural History magazines. It is a pleasure to acknowledge my debt to many scientist colleagues for discussions and comments, to Roger Short and Nancy Wayne for their scrutiny of the whole manuscript, to Ellen Modecki for the illustrations, and to John Brock-man for the invitation to write this book.


If your dog had your brain and could speak, and if you asked it what it thought of your sex life, you might be surprised by its response. It would be something like this:

Those disgusting humans have sex any day of the month! Barbara proposes sex even when she knows perfectly well that she isn't fertile-like just after her period. John is eager for sex all the time, without caring whether his efforts could result in a baby or not. But if you want to hear something really gross-Barbara and John kept on having sex while she was pregnant! That's as bad as all the times when John's parents come for a visit, and I can hear them too having sex, although John's mother went through this thing they call menopause years ago. Now she can't have babies anymore, but she still wants sex, and John's father obliges her. What a waste of effort! Here's the weirdest thing of all: Barbara and John, and John's parents, close the bedroom door and have sex in private, instead of doing it in front of their friends like any self-respecting dog!

To understand where your dog is coming from, you need to free yourself from your human-based perspective on what constitutes normal sexual behavior. Increasingly today, we consider it narrow-minded and despicably prejudiced to denigrate those who do not conform to our own standards. Each such form of narrow- mindedness is associated with a despicable “ism”—for instance, racism, sexism, Eurocentrism, and phallocentrism. To that list of modern “ism” sins, defenders of animal rights are now adding the sin of species-ism. Our standards of sexual conduct are especially warped, species-ist, and human-centric because human sexuality is so abnormal by the standards of the world's thirty million other animal species. It's also abnormal by the standards of the world's millions of species of plants, fungi, and microbes, but I'll ignore that broader perspective because I haven't yet worked through my own zoo-centrism. This book confines itself to the insights that we can gain into our sexuality merely by broadening our perspective to encompass other animal species.

As a beginning, let's consider normal sexuality by the standards of the world's approximately 4,300 species of mammals, of which we humans are just one. Most mammals do not live as a nuclear family of a mated adult male and adult female, caring jointly for their offspring. Instead, in many mammal species both adult males and adult females are solitary, at least during the breeding season, and meet only to copulate. Hence, males do not provide paternal care; their sperm is their sole contribution to their offspring and to their temporary mate.

Even most social mammal species, such as lions, wolves, chimpanzees, and many hoofed mammals, are not paired off within the herd/pride/pack/band into male/ female couples. Within such a herd/pride/et cetera, each adult male shows no signs of recognizing specific infants as his offspring by devoting himself to them at the expense of other infants in the herd. Indeed, it is only within the last few years that scientists studying lions, wolves, and chimpanzees have begun to figure out, with the help of DNA testing, which male sired which infant. However, like all generalizations, these admit exceptions. Among the minority of adult male mammals that do offer their offspring paternal care are polygynous male zebras and gorillas with harems of females, male gibbons paired off with females as solitary couples, and saddleback tamarin monkeys, of which two adult males are kept as a harem by one polyan-drous adult female.

Sex in social mammals is generally carried out in public, before the gazes of other members of the troop. For instance, a female Barbary macaque in estrus copulates with every adult male in her troop and makes no effort to conceal each copulation from other males. The best-documented exception to this pattern of public sex is in chimpanzee troops, where an adult male and estrous female may go off by themselves for a few days on what human observers term a “consortship”. However, the same female chimpanzee that has private sex with a consort may also have public sex with other adult male chimpanzees within the same estrus cycle.

Adult females of most mammal species use various means of conspicuously advertising the brief phase of their reproductive cycle when they are ovulating and can be fertilized. The advertisement may be visual (for instance, the area around the vagina turning bright red), olfactory (releasing a distinctive smell), auditory (making noises), or behavioral (crouching in front of an adult male and displaying the vagina). Females solicit sex only during those fertile days, are sexually unattractive or less attractive to males on other days because they lack the arousing signals, and rebuff the advances of any male that is nevertheless interested on other days. Thus, sex is emphatically not just for fun and is rarely divorced from its function of fertilization. This generalization too admits exceptions: sex is flagrantly separated from reproduction in a few species, including bonobos (pygmy chimpanzees) and dolphins.

Finally, the existence of menopause as a regular phenomenon is not well established for most wild mammal populations. By menopause is meant a complete cessation of fertility within a time span that is much briefer than the previous fertile career, and that is followed by an infertile life span of significant length. Instead, wild mammals either are still fertile at the time of death or else exhibit gradually diminishing fertility with advancing age.

Now contrast what I have just said about normal mammalian sexuality with human sexuality. The following human attributes are among those that we take for granted as normal:

1: Most men and women in most human societies end up in a long-term pair relationship (“marriage”) that other members of the society recognize as a contract involving mutual obligations. The couple has sex repeatedly, and mainly or exclusively with each other.

2: In addition to being a sexual union, marriage is a partnership for joint rearing of the resulting babies. In particular, human males as well as females commonly provide parental care.

3: Despite forming a couple (or occasionally a harem), a husband and wife (or wives) do not live (like gibbons) as a solitary couple in an exclusive territory that they defend against other couples, but instead they live embedded in a society of other couples with whom they cooperate economically and share access to communal territory.

4: Marriage partners usually have sex in private, rather than being indifferent to the presence of other humans.

5: Human ovulation is concealed rather than advertised. That is, women's brief period of fertility around the time of ovulation is difficult to detect for their potential sex partners as well as for most women themselves. A woman's sexual receptivity extends beyond the time of fertility to encompass most or all of the menstrual cycle. Hence, most human copulations occur at a time unsuitable for conception. That is, human sex is mostly for fun, not for insemination.

6: All women who live past the age of forty or fifty undergo menopause, a complete shutdown of fertility. Men in general do not undergo menopause: while individual men may develop fertility problems at any age, there is no age-clumping of infertility or universal shutdown.

Norms imply violation of norms: we call something a “norm” merely because it is more frequent than its opposite (the “violation of the norm”). That's as true for human sexual norms as for other norms. Readers of the last two pages will surely have been thinking of exceptions to the supposed generalizations that I have been describing, but they still stand as generalizations. For example, even in societies that recognize monogamy by law or custom there is much extramarital and premarital sex, and much sex that is not part of a long-term relationship. Humans do engage in one-night stands. On the other hand, most humans also engage in many-year or many-decade stands, whereas tigers and orangutans engage in nothing except one-night stands. The genetically based paternity tests developed over the last half-century have shown that the majority of American, British, and Italian babies are indeed sired by the husband (or steady boyfriend) of the baby's mother.

Readers may also bristle at hearing human societies described as monogamous; the term “harem,” which zoologists apply to zebras and gorillas, is taken from the Arabic word for a human institution. Yes, many humans practice sequential monogamy. Yes, polygyny (long-term simultaneous unions between one man and multiple wives) is legal in some countries today, and polyandry (long-term simultaneous unions between one woman and multiple husbands) is legal in a few societies. In fact, polygyny was accepted in the great majority of traditional human societies before the rise of state institutions. However, even in officially polygynous societies most men

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