As to what precisely makes some of us males and some females, that knowledge is even more recent. It wasn’t until 1990 that two teams in London, at the National Institute for Medical Research and at the Imperial Cancer Research Fund, identified a sex-determining region on the Y chromosome that they dubbed the SRY gene, for “sex-determining region Y.” After countless generations of making little boys and little girls, humans finally knew how they did it.
The Y chromosome is a curious and runty thing. It has only about seventy genes; other chromosomes have as many as two thousand. The Y chromosome has been shrinking for 160 million years. At its current rate of deterioration, it has been estimated, it could vanish altogether in another 4.6 million years. That doesn’t mean, happily, that males will cease to exist in 4.6 million years. The genes that determine gender traits would probably just move across to another chromosome. Moreover, our ability to manipulate the reproductive process is likely to be rather more refined in 4.6 million years, so this is probably not something we should lose sleep over.*2
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Interestingly, sex isn’t actually necessary. Quite a number of organisms have abandoned it. Geckos, the little green lizards that are often encountered clinging like suckered bath toys to walls in the tropics, have done away with males altogether. It is a slightly unsettling thought if you are a man, but what we bring to the procreative party is easily dispensed with. Geckos produce eggs, which are clones of the mother, and these grow into a new generation of geckos. From the mother’s point of view, this is an excellent arrangement because it means that 100 percent of her genes are inherited. With conventional sex, each partner passes on just half its genes, and that number is relentlessly thinned with each succeeding generation. Your grandchildren have only a quarter of your genes, your great-grandchildren only an eighth, your great-great-grandchildren a mere sixteenth, and so shrinkingly on it goes. If genetic immortality is your ambition, then sex is a very poor way of achieving it. As Siddhartha Mukherjee observed in The Gene: An Intimate History, humans don’t actually reproduce at all. Geckos reproduce; we recombine.
Sex may dilute our personal contribution to posterity, but it is great for the species. By mixing and matching genes, we get variety and that gives us safety and resilience. It makes it harder for diseases to sweep through whole populations. It also means that we can evolve. We can hold on to beneficial genes and discard ones that impede our collective happiness. Cloning gives you the same thing over and over. Sex gives you Einstein and Rembrandt—and a lot of dorks, too, of course.
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Probably no area of human existence has generated less certainty, or been more inhibitive to open discussion, than sex. Perhaps nothing says more about our delicacy toward matters genital than that the word “pudendum”—meaning the external genitals, particularly those of a woman—comes from the Latin for “to be ashamed.” It is next to impossible to get reliable figures about almost anything to do with sex as a pastime. How many people are unfaithful to their partners at some point in a relationship? Somewhere between 20 percent and 70 percent, depending on which of many studies you consult.
One problem, which should surprise no one, is that survey respondents are inclined to embrace alternative realities when they think their answers cannot be checked. In one study, the number of sexual partners women were prepared to recall increased by 30 percent when they thought they were hooked up to a lie detector. Remarkably, for a 1995 survey called the Social Organization of Sexuality: Sexual Practices in the United States, conducted jointly by the University of Chicago and the National Opinion Research Center, respondents were permitted to have someone else, usually a child or current sexual partner, present when they were interviewed, which is hardly likely to have resulted in fully candid responses. Indeed, it was shown afterward that the proportion of people answering that they had had sex with more than one person in the previous year fell from 17 percent to 5 percent when another person was present.
The survey was criticized for lots of other deficiencies. Because of funding problems, only 3,432 people were interviewed instead of the 20,000 originally intended, and because all the respondents were aged eighteen or older, it offered no conclusions on teenage pregnancies or birth control practices, or much else of crucial importance to public policy. Moreover, the survey focused only on households, so it excluded people in institutions—college students, prisoners, and members of the armed forces most notably. All of these made the report’s findings questionable if not entirely useless.
Another problem with sex surveys—and there is no delicate way of putting this—is that people are sometimes just stupid. In another analysis, reported by Cambridge University’s David Spiegelhalter in the wonderful Sex by Numbers: The Statistics of Sexual Behaviour, when asked to state what in their view constituted full sex, some 2 percent of male respondents said that penetrative intercourse did not qualify, leaving Spiegelhalter to wonder what exactly they might be waiting for “before they feel they have gone all the way.”
Because of the difficulties, the field of sex studies has a long history of providing dubious statistics. In his 1948 work, Sexual Behavior in the Human Male, Alfred Kinsey of Indiana University reported that nearly 40 percent of men had had a homosexual experience resulting in orgasm and that nearly a fifth of young men brought up on farms had had sex with livestock. Both figures are now thought highly unlikely. Even more dubious were the 1976 Hite Report on Female Sexuality and the companion Hite Report on Male Sexuality published soon afterward. The author, Shere Hite, used questionnaires and had a very low, nonrandom, highly selective response rate. Nonetheless, Hite confidently declared that 84 percent of women were dissatisfied with their male partners and that 70 percent of women married for more than five years were in an adulterous relationship. The findings were heavily criticized at the time, but the books were huge best sellers. (A more scientific, and more recent, U.S. National Health and Social Life Survey found that 15 percent of married women and 25 percent of married men said they had been unfaithful at some time.)
On top of all that, the subject of sex is full of statements and statistics that are often repeated but based on nothing. Two durable ones are these: “Men think of sex every seven seconds” and “The average amount of time spent kissing in a lifetime is 20,160 minutes” (that is 336 hours). In fact, according to genuine studies, men of college age think about sex nineteen times a day, roughly once every waking hour, which is about the same frequency as they think about food. College women think about food more often than they think about sex, but they don’t think about either terribly often. No one does anything at all every seven seconds other than perhaps respire and blink. Similarly, no one knows how much of an average lifetime is devoted to kissing or where that weirdly precise and durable figure of 20,160 minutes comes from.
On a more positive note, we can say with some confidence that the median time for sex (in Britain at least) is nine minutes, though the whole act, including foreplay and undressing, is more like twenty-five minutes. According to Spiegelhalter, energy use on average per sexual session is about a hundred calories for men and about seventy for women. A meta-analysis showed that for older people the risk of a heart attack was raised for up to three hours after sex, but it was similarly raised for shoveling snow, and sex is more fun than shoveling snow.