Why Do We Have Sex? Part 1

If your answer is "to make babies," you're wrong. Sexual activity among humans has nothing to do with fertilization more than 99.99% of the time.

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Typical heterosexual couples make love an average of a hundred times a year. Assuming they keep up this pace most of their adult lives, they will end up having had sex as many as four thousand times.

In addition to sex with a partner, most people seek lots of sexual relief when they're all alone. Men typically learn to masturbate in adolescence and keep up the practice daily until their twenties. The pace usually slows down as men age and often dips when they enter sexual relationships, but most men will probably masturbate ten thousand times in fifty to sixty years of sexual activity. Altogether, the average man can expect to experience fifteen thousand or more orgasms over the course of his life.

Women start off masturbating at a similar age and frequency as men. Most masturbate daily until their late teens, but slow down when they reach their early twenties to about a third the rate of men. Still, they typically enjoy sexual stimulation, either alone or with a partner, for a lifetime total of five to ten thousand sexual experiences.

Despite all that sexual activity, the population in the US and most other highly industrialized nations is fairly stable. That is, there is roughly a single child born for each person in the country, which means that there is one successful pregnancy for every ten thousand sexual experiences.

Humans are unusual creatures in this regard, though hardly unique. Certain apes, dolphins and wolves are among the animals that use sexual interactions for things like pleasure, bonding, and establishing social structure. But intercourse for most other organisms is all about making babies.

Even though humans rarely have sex in order to get pregnant, it's primarily our genetic mandate to mate and bear young that is reflected in our sexual desire. Evolution ensures things that are good for the propagation of our genes bring us pleasure. For most people, and apparently many animals as well, the orgasm is the benchmark of pleasure. The fact that it produces the most enjoyable sensations and the strongest desires that we are likely to ever experience suggests that sex is effectively the most important thing we do in life, from an evolutionary point of view. Although we have found ways to separate intercourse from procreation, sexual ecstasy is nature's reward to us for continuing the species.

Sex between a male and a female, however, is not the only way for organisms to reproduce. For millions of years, when life was new on Earth, plants and animals got on just fine without intercourse. Most of them were simple single-celled organisms. At times when there were plentiful resources to keep some organism alive, rather than letting anything go to waste, or go to a competitor, the organism would simply duplicate its DNA and split into two identical copies of the original. Modern bacteria, amoebas, and many types of algae and molds continue the practice.

A look at the pros and cons of various types of reproduction can help explain why we mate the way we do instead of opting for asexual reproduction, or some other scheme altogether. For physicists who ponder reproductive strategies, the subject is similar to many other sorts of problems in physics where systems, in this case populations, naturally find the optimal solution to complex and competing demands.