Free Homo Mysterious: Evolutionary Puzzles of Human Nature by David P. Barash

among evolutionary biologists, simply because its adaptive significance—its biological payoff—is obscure.
    Events on the male side pretty much speak for themselves. Even though orgasm (a subjectively experienced phenomenon) is technically different from ejaculation (expulsion of semen from the body), it occasions no great surprise that for men the two are tightly connected, and that evolution has doubtless contrived to use the former as a carrot, inducing men to engage in the latter. But what about women?
Why Orgasm Is a Womanly Mystery
     
    Of course, not all women experience orgasm, and that is part of the mystery, although not as one might think: The enigma of female orgasm is
not
why some women don’t climax but why some
do
. The data are quite clear that unlike its male counterpart, female orgasm isn’t necessary for reproduction; among the many complaints of nonorgasmic women, inability to conceive is not one.
     
    For generations, old and young wives’ tales—husbands’, too—along with scores of Victorian romance novels claimed that there was some sort of connection between a woman “really giving” herself and finally becoming pregnant. And to be sure, it is easy to speculate how female orgasm might facilitate fertilization, especially if the waves of muscular contraction provide greater access of sperm to egg. The problem, however, is that most of these contractions go in the wrong direction! It has alternatively been claimed that uterine contractions during orgasm literally generate a suction effect, which draws semen up toward the fallopian tubes. There may yet be some truth to this rather inelegantly named “uterine upsuck” hypothesis, which was generated by heroic laboratory research in which a radio telemetry device was insertedinto a uterus, thereby revealing a vacuum cleaner–like negative pressure following orgasm. 1 This “finding,” however, was based on an unacceptably small sample size: one woman!
    Moreover, even if uterine upsuck turns out to be a valid phenomenon, it is far from what scientists call a “robust” one, or else it would have been noted previously. More to the point, it would have generated a cause-and-effect relationship between female orgasm and subsequent pregnancy, which simply does not exist.
    It has also been suggested—although again, the data are inconclusive—that orgasm reduces the amount of “flow-back” (the leakage of semen out of a woman’s reproductive tract), thereby increasing the likelihood that fertilization will be achieved by the partner who helped induce that orgasm. 2 But this, too, is controversial, based on a very small sample of remarkably cooperative couples.
    Although there are, in theory, many ways by which female orgasm could facilitate fertilization (including a range of possible biochemical effects along with physical assistance to sperm or egg), there is currently no evidence that orgasmic women produce more babies, or better ones, than their less fortunate “sisters.” And of course, in vitro fertilization further italicizes that when it comes to baby making, female orgasm is simply not a physiologic or anatomic prerequisite.
    On the other hand, female orgasms are unquestionably real and are, if anything, more dramatic than their male counterparts, especially given a woman’s capacity for multiple orgasms. Given that there are no free lunches in biology, the question presents itself: Why orgasm?
Some Easy-to-Exclude Hypotheses
     
    The redoubtable Desmond Morris, whose fertile imagination gave us the “buttocks mimic” hypothesis for the evolution of breasts, unburdened himself of yet another howler, proposing that orgasm is natural selection’s way of keeping a woman horizontal after sex, which in turn supposedly makes fertilization more likely. This “knock-down” hypothesis has problems. For one, despitesubstantial efforts, it has never been demonstrated that postcoital positions influence fertilization. And if they did,