By David P. Barash
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.
Across cultures, the proportion of the population who
are homosexual is roughly the same. What maintains the genetic
propensity for the trait?
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 in
Homo 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 as
fa'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 by
Homo 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 "sexually
complementary
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's
The 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 ... not
who is homosexual, but
why are there homosexuals. We can be confident that eventually, nature will tell.
David P. Barash, an evolutionary biologist,
is a professor of psychology at the University of Washington. His most
recent book is Homo Mysterious: Evolutionary Puzzles of Human Nature
(Oxford University Press, 2012).