Matt Manley for The Chronicle Review

Critics claim that evolutionary biology is, at best, guesswork. The reality is otherwise. Evolutionists have nailed down how an enormous number of previously unexplained phenomena—in anatomy, physiology, embryology, behavior—have evolved. There are still mysteries, however, and one of the most prominent is the origins of homosexuality.

The mystery is simple enough. Its solution, however, has thus far eluded our best scientific minds.

First the mystery.

The sine qua non for any trait to have evolved is for it to correlate positively with reproductive success, or, more precisely, with success in projecting genes relevant to that trait into the future. So, if homosexuality is in any sense a product of evolution—and it clearly is, for reasons to be explained—then genetic factors associated with same-sex preference must enjoy some sort of reproductive advantage. The problem should be obvious: If homosexuals reproduce less than heterosexuals—and they do—then why has natural selection not operated against it?

The paradox of homosexuality is especially pronounced for individuals whose homosexual preference is exclusive; that is, who have no inclination toward heterosexuality. But the mystery persists even for those who are bisexual, since it is mathematically provable that even a tiny difference in reproductive outcome can drive substantial evolutionary change.

J.B.S. Haldane, one of the giants of evolutionary theory, imagined two alternative genes, one initially found in 99.9 percent of a population and the other in just 0.1 percent. He then calculated that if the rare gene had merely a 1-percent advantage (it produced 101 descendants each generation to the abundant gene’s 100), in just 4,000 generations—a mere instant in evolutionary terms—the situation would be reversed, with the formerly rare gene occurring in 99.9 percent of the population’s genetic pool. Such is the power of compound interest, acting via natural selection.

For our purposes, the implication is significant: Anything that diminishes, even slightly, the reproductive performance of any gene should (in evolutionary terms) be vigorously selected against. And homosexuality certainly seems like one of those things. Gay men, for example, have children at about 20 percent of the rate of heterosexual men. I haven’t seen reliable data for lesbians, but it seems likely that a similar pattern exists. And it seems more than likely that someone who is bisexual would have a lower reproductive output than someone whose romantic time and effort were devoted exclusively to the opposite sex.

Nor can we solve the mystery by arguing that homosexuality is a “learned” behavior. That ship has sailed, and the consensus among scientists is that same-sex preference is rooted in our biology. Some of the evidence comes from the widespread distribution of homosexuality among animals in the wild. Moreover, witness its high and persistent cross-cultural existence inHomo sapiens.

In the early 1990s, a geneticist at the National Institutes of Health led a study that reported the existence of a specific allele, Xq28, located on the X chromosome, that predicted gay-versus-straight sexual orientation in men. Subsequent research has been confusing, showing that the situation is at least considerably more complicated than had been hoped by some (notably, most gay-rights advocates) and feared by others (who insist that sexual orientation is entirely a “lifestyle choice”).

Some studies have failed to confirm any role for Xq28 in gay behavior, while others have been supportive of the original research. It is also increasingly clear that whatever its impact on male homosexuality, this particular gene does not relate to lesbianism. Moreover, other research strongly suggests that there are regions on autosomal (nonsex) chromosomes, too, that influence sexual orientation in people.

So a reasonable summary is that, when it comes to male homosexuality, there is almost certainly a direct influence, although probably not strict control, by one or more alleles. Ditto for female homosexuality, although the genetic mechanism(s), and almost certainly the relevant genes themselves, differ between the sexes.

Beyond the suggestive but inconclusive search for DNA specific to sexual orientation, other genetic evidence has emerged. A welter of data on siblings and twins show that the role of genes in homosexual orientation is complicated and far from fully understood—but real. Among noteworthy findings: The concordance of homosexuality for adopted (hence genetically unrelated) siblings is lower than that for biological siblings, which in turn is lower than that for fraternal (nonidentical) twins, which is lower than that for identical twins.

Gay-lesbian differences in those outcomes further support the idea that the genetic influence upon homosexuality differs somewhat, somehow, between women and men. Other studies confirm that the tendency to be lesbian or gay has a substantial chance of being inherited.

Consider, too, that across cultures, the proportion of the population that is homosexual is roughly the same. We are left with an undeniable evolutionary puzzle: What maintains the underlying genetic propensity for homosexuality, whatever its specific manifestations? Unlike most mystery stories, in which the case is typically solved at the finish, this one has no ending: We simply do not know.

Here are some promising possibilities.

Kin selection. Scientists speculate that altruism may be maintained if the genes producing it help a genetic relative and hence give an advantage to those altruistic genes. The same could be true of homosexuality. Insofar as homosexuals have been freed from investing time and energy in their own reproduction, perhaps they are able to help their relatives rear offspring, to the ultimate evolutionary benefit of any homosexuality-promoting genes present in those children.

Unfortunately, available evidence does not show that homosexuals spend an especially large amount of time helping their relatives, or even interacting with them. Not so fast, however: Those results are based on surveys; they reveal opinions and attitudes rather than actual behavior. Moreover, they involve modern industrialized societies, which presumably are not especially representative of humanity’s ancestral situations.

Some recent research has focused on male homosexuals among a more traditional population on Samoa. Known asfa’afafine, these men do not reproduce, are fully accepted into Samoan society in general and into their kin-based families in particular, and lavish attention upon their nieces and nephews—with whom they share, on average, 25 percent of their genes.

Social prestige. Since there is some anthropological evidence that in preindustrial societies homosexual men are more than randomly likely to become priests or shamans, perhaps the additional social prestige conveyed to their heterosexual relatives might give a reproductive boost to those relatives, and thereby to any shared genes carrying a predisposition toward homosexuality. An appealing idea, but once again, sadly lacking in empirical support.

Group selection. Although the great majority of biologists maintain that natural selection occurs at the level of individuals and their genes rather than groups, it is at least possible that human beings are an exception; that groups containing homosexuals might have done better than groups composed entirely of straights. It has recently been argued, most cogently by the anthropologist Sarah B. Hrdy, that for much of human evolutionary history, child-rearing was not the province of parents (especially mothers) alone. Rather, our ancestors engaged in a great deal of “allomothering,” whereby nonparents—other genetic relatives in particular—pitched in. It makes sense that such a system would have been derived byHomo sapiens, of all primate species the one whose infants are born the most helpless and require the largest investment of effort. If sufficient numbers of those assistants had been gay, their groups may have benefited disproportionately.

Alternatively, if some human ancestors with a same-sex preference reproduced less (or even not at all), that, in itself, could have freed up resources for their straight relatives, without necessarily requiring that the former were especially collaborative. Other group-level models have also been proposed, focusing on social interaction rather than resource exploitation: Homosexuality might correlate with greater sociality and social cooperation; similarly, it might deter violent competition for females.

Balanced polymorphisms. Perhaps a genetic predisposition for homosexuality, even if a fitness liability, somehow conveys a compensating benefit when combined with one or more other genes, as with the famous case of sickle-cell disease, in which the gene causing the disease also helped prevent malaria in regions where it was epidemic. Although no precise candidate genes have been identified for homosexuality, the possibility cannot be excluded.

Sexually antagonistic selection. What if one or more genes that predispose toward homosexuality (and with it, reduced reproductive output) in one sex actually work in the opposite manner in the other sex? I prefer the phrase “sexuallycomplementary selection": A fitness detriment when genes exist in one sex—say, gay males—could be more than compensated for by a fitness enhancement when they exist in another sex.

One study has found that female relatives of gay men have more children than do those of straight men. This suggests that genes for homosexuality, although disadvantageous for gay men and their male relatives, could have a reproductive benefit among straight women.

To my knowledge, however, there is as yet no evidence for a reciprocal influence, whereby the male relatives of female homosexuals have a higher reproductive fitness than do male relatives of heterosexual women. And perhaps there never will be, given the accumulating evidence that female homosexuality and male homosexuality may be genetically underwritten in different ways.

A nonadaptive byproduct. Homosexual behavior might be neither adaptive nor maladaptive, but simply nonadaptive. That is, it might not have been selected for but persists instead as a byproduct of traits that presumably have been directly favored, such as yearning to form a pair bond, seeking emotional or physical gratification, etc. As to why such an inclination would exist at all—why human connections are perceived as pleasurable—the answer may well be that historically (and prehistorically), it has often been in the context of a continuing pair-bond that individuals were most likely to reproduce successfully.

There are lots of other hypotheses for the evolution of homosexuality, although they are not the “infinite cornucopia” that Leszek Kolakowski postulated could be argued for any given position. At this point, we know enough to know that we have a real mystery: Homosexuality does have biological roots, and the question is how the biological mechanism developed over evolutionary time.

Another question (also yet unanswered) is why should we bother to find out.

There is a chilling moment at the end of Ray Bradbury’sThe Martian Chronicles, when a human family, having escaped to Mars to avoid impending nuclear war, looks eagerly into the “canals” of their new planetary home, expecting to see Martians. They do: their own reflections.

It wasn’t terribly long ago that reputable astronomers entertained the notion that there really were canals on Mars. From our current vantage, that is clearly fantasy. And yet, in important ways, we are still strangers to ourselves, often surprised when we glimpse our own images. Like Bradbury’s fictional family, we, too, could come to see humanity, reflected in all its wonderful diversity, and know ourselves at last for precisely what we are, if we simply looked hard enough.

Unlike the United States military, with its defunct “don’t ask, don’t tell” policy, many reputable investigators are therefore asking ... notwho is homosexual, butwhy are there homosexuals. We can be confident that eventually, nature will tell.