I have long been critical of many evolutionary psychologists for their over-the-top stories, but today I am forced — albeit briefly — to join their ranks. I have thought of a hypothesis that shares all the salient traits of the best ideas of evolutionary psychology: it is brilliant, makes evolutionary sense, and is untestable.

It is the conventional wisdom in human sexuality that semen tastes bad. Anyone with minimal sexual experience knows that although many women will perform fellatio on their partners, most bridle at the thought of swallowing the ejaculate. Its flavor is frequently characterized as revoltingly bitter or salty. The “swallow or spit” dilemma faces any woman who performs such an act, and whose partner regards swallowing as a gesture of love.

The universal distastefulness of semen is attested by the many internet sites that give advice about how to improve the taste of one’s ejaculate, for example, here, here, and here.

To get a better scientific handle on this idea, I took a poll, asking a woman friend, Dr. Fawzia Rasheed, to canvass her female acquaintances about their willingness to swallow after the act of fellatio. Twenty-four women were asked this question:

Sperm…would you spit or swallow? In other words, can you abide by or do you hate the taste?

There were sixteen responses, many including pungent asides that I cannot repeat on a family-oriented website. One answer was a non-response (“I should be so lucky”). The other fifteen included eleven “spits” and four “swallows”. But among the latter, two women commented that they did not like the taste: one, in fact, swallowed to get rid of the flavor as quickly as possible. Two others said “swallow” but did not comment on whether they enjoyed it. Therefore, 13/15, or 87% of informative respondents could not abide the taste of semen.

This near-unanimous response to a random poll demands an evolutionary explanation. Why does semen taste so foul? One answer, of course, is that the chemicals necessary to make an ejaculate effective have the side effect of tasting bad. Semen is only about 5% sperm, with the remainder of the fluid consisting of a complex mixture of compounds from the prostate gland and seminal vesicle. These compounds include sugars such as lactose [CORRECTION: fructose; see below ] (to provide energy for the swimming sperm), enzymes, amino acids, zinc, hormones, and various amines to counteract the acidic environment of the vagina (these are said to give sperm its characteristic smell and flavor). Some of these amines have the names putrescine and cadaverine, which give an idea of how they smell. The function of some of the compounds is unknown; they may help overcome female immune defenses or even function in male-male sperm competition when females are multiply inseminated.

But this proximate answer will not satisfy the diligent evolutionary psychologist. After all, natural selection could presumably add some sugars or good-tasting stuff to semen if it were advantageous to do so. Why does it not do so?

A moment’s reflection gives the answer.

Natural selection maintains the repugnant taste of semen so that a man’s sperm will wind up in the appropriate place: the vagina and not the stomach. So long as sperm tastes bad, women will not be tempted to swallow it, but will turn their male partner towards conventional intercourse, which of course is the only act that will produce children. In other words, any male with good-tasting sperm would have fewer offspring than his competitors. A man whose sperm tasted like honey would probably not have any children at all.

I can think of only two ways to test this hypothesis, both of them impractical or impossible:

1. If women gave birth through their stomachs, semen would taste great

2. Those males with genes giving them better-tasting semen will leave fewer offspring than other males.

This theory is offered as a modest proposal, only partly (excuse me) tongue in cheek. It may even be true.

Notes added post facto: Although light-hearted, the post is somewhat serious; it’s the kind of interesting speculation that evolutionists indulge in over a few beers. And everything in the post is true, including the survey of women.

And note to T.R. Gregory: I don’t think this idea is refuted by finding, say, primate species that don’t have oral sex but do have similar compounds in the semen. The whole idea rests on those compounds TASTING BAD to females, and we’d need to know something about the taste reactions of females in these other primates. The evolution, after all, might have been in the female taste receptors rather than in the semen.

Finally, apologies to readers who find the subject distasteful.