"The expense of spirit in a waste of shame" is how William Shakespeare described lust, but he was speaking as a poet, not a pragmatist. True, copulation is not cheap--it always exacts a toll, spiritual or otherwise, from its participants. But "a waste"? Hardly. Sex perpetuates the species, and lust--shameful though some human primates choose to make it--is the overnight express to sex.
For men and women alike the objective of lust is orgasm. It's hard to imagine a more powerful inducement to sexual activity. Indeed, orgasm is the kind of experience that could have been invented by gametes (reproductive cells). Imagine being stuck in somebody's gonads, where your goal in life is to form a union with someone else's gamete. The objective? To produce an organism that makes more gametes. What possible incentive could you offer your host to bring about that union? Try a somatic blitzkrieg of ecstasy, courtesy of the limbic system, the pleasure (as well as the pain) center of the brain. That's orgasm.
Today, when orgasm has been divorced almost entirely from reproduction, the "how" of sexual climax has been largely demystified. Masters and Johnson established the physical parameters of orgasm in their landmark 1966 book, Human Sexual Response. These include, in men, contraction of the rectal sphincter at .8-second intervals, a reduction of voluntary muscle control, and involuntary muscle spasms throughout the body (one distinctive example being the so-called carpopedal spasm, in which the big toe is held straight out while the other toes bend back and the foot arches--a contortion most people couldn't pull off consciously if they were paid to). And of course, there is ejaculation, which is usually, but not always, a telltale sign. In women the telltale signs are similar: along with its chaotic effects on other muscles, orgasm causes contractions of the uterus, vagina, and rectal sphincter, again at .8-second intervals. For both sexes, the pudendal cataclysm typically lasts less than a minute.
Despite such insights on the mechanics of orgasm, the origins of the phenomenon are as mysterious as ever. When did orgasm evolve? Who (or what) had the first orgasm? What selective pressures shaped the redoubtable reflex? Without fossil evidence, scientists interested in those questions must examine in excruciating detail the sexual practices of contemporary human cultures as well as the sex lives of chimpanzees, gorillas, orangutans, and other uninhibited relatives of Homo sapiens.
