It shouldn’t be controversial to assert that there are only two sexes.
We have passed through one or two generations of post-Pill sexual liberation only for sex and gender to once again become topics impossible to discuss in a dispassionate way — even in a supposedly scientific venue, such as a listserv of the American Psychological Association (APA).
Recently, I was excluded from an email discussion listserv of the APA. I was not told exactly why, though I believe it was for a few mildly skeptical comments I had made about nonbinary sex. And I enjoyed the irony that my expeller told me, while in the act of kicking me out, that he recognizes “there are a wide range of views about many issues.”
Maybe so. But the biological facts here are clear. All mammals reproduce sexually; reproduction requires an egg and a sperm, the male supplies the sperm and the female the egg — no room for a third party. Male and female are it. A few non-mammalian species show surprising flexibility. Some zooplankton, such as rotifers, have a complex sharing arrangement that is still being sorted out (at least by us; the rotifers seem perfectly clear on the matter). Adult swordtails (Xiphophorus helleri), a popular live-bearing aquarium fish, sometimes change sex from female to male. Many other fish species change sex as adults — from female to male and the reverse. In clownfish (Amphiprion), the change is triggered by, for example, the composition of the fish school (if all female, one female may change to a male) or simply age.
Mammals are less flexible, but essentially all mammalian species that have been carefully studied show departures from normal heterosexual behavior — “normal” in the sense that it is both the majority pattern and essential to survival of the species. Human beings obviously also show a variety of abnormal patterns, such as homosexuality. (“Homosexuality is as natural as heterosexuality,” as Myra Breckinridge author Gore Vidal put it. “Notice I use the word ‘natural,’ not normal.”) Include also the wish to be, or the feeling of being, the opposite sex (transgender). These variants form a small fraction of the population. Between 2 and 10 percent of the U.S. male population may be homosexual and perhaps 1 to 2 percent of females. The proportion of bisexuals and transsexuals is probably much lower. All these figures are moving targets and vary with the winds of cultural change.
The causes of the different variants are uncertain; no doubt, cultural as well as biological (genetic, developmental) factors are involved in a still-unknown mix. The adaptive function of individuals who don’t reproduce is also unclear. Obviously if a substantial fraction of the population quits reproducing, the future of the species looks dim. On the other hand, division of interests and division of labor may mean that some nonreproductive minorities (celibate religious sects are an obvious example) contribute to rather than diminish the “fitness” of a culture.
How variants should be valued is a matter for ethics and politics, not science. But biology matters; and here, politics can warp science. That there are two sexes is biologically indisputable. That there are several minority variants is also indisputable. But sex is not a continuum.
There are real differences between men and women. Some claims achieve a consensus: Almost everybody will agree to them. Others, not so much. For example, men are in general stronger than women: They run faster, lift heaver weights, and are better football players. Despite the miraculous Simone Biles, men can even be better gymnasts. Men also tend to die sooner, commit more violent crimes, and take more risks, and they have more accidents. Most people agree on these things. What do they mean for women’s participation in the military? Participation of trans-women (i.e., men) in female sports?
Psychologists also tell us that men tend to be more interested in things, while women are more interested in people. Can this difference partly explain male–female disparities in professions such as nursing and engineering? Males, on most dimensions, are more variable than females. On some cognitive measures, for example, the population means are the same, but the male distribution is more spread out: more male idiots and also more male geniuses. Alluding to this as an explanation for the disproportionate representation of men in math-related departments at Harvard got then–Harvard president Larry Summers into trouble a few years ago. It is a fact nevertheless.
Passionate advocates for “gender equity” are likely to question if not ignore biological male–female differences. Sex differences very quickly become political rather than scientific issues. Equality before the law is a consensus view. But whether people should be equally treated in all other respects are matters where, in recent years, proven and relevant biological differences are sidelined. Does it make sense to treat men and women as identical, and equally valuable, in hazardous and physically stressful situations, for example? Should it really be “women and children, selected at random” — rather than “first,” for example?
We remain uncertain about less-well-established socio-cultural (gender) differences between males and females. Not all gender questions can be decided by science, but political and even ethical decisions that ignore established biology are unlikely to be wise.
A Pavlovian reaction to mild dissent is unlikely to advance our understanding. “Enthusiasts” in the APA and other supposedly scientific organizations should take note.