Homo Mysterious: Evolutionary Puzzles of Human Nature (16 page)

Perhaps the strongest argument for a biological underpinning to same-sex preference is a simple generalized one, deriving from how remarkably widespread it is in the biological world. In one of the earliest such reports, published in 1979, anthropologist Linda Wolfe presented detailed data on the sexual behavior of a troop of 60 adult female Japanese macaque monkeys, within a troop composed of 140 animals overall.
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Wolfe described a substantial frequency of female–female mountings, results that essentially were not believed—at least, not initially. But as further observations accumulated from a wide array of species, the conclusion became inescapable: Homosexuality is a common fixture in the lives of many animals, perhaps most.

More than 30 years after Wolfe’s pioneering observations, same-sex pairing and attempted copulation have been described for a vast array of animal species, including all the major vertebrate groups as well as mollusks, insects, even nematode worms, and among free-living individuals as well as under captive conditions. Moreover, it is likely that the actual frequency of homosexual behaviors among animals is higher than reported, simply because when sexual behavior is observed among “monomorphic” species (those in which male and female look similar), it is typically assumed even now that the interaction is heterosexual.

A popularized book titled
Biological Exuberance: Animal Homosexuality and Natural Diversity
described a wide range of same-sex behaviors among 470 different species.
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This number should be taken with a grain of salt, however, since in his eagerness to overcome what he perceived as a pro-heterosexual scientific bias and to “document” the full range of animal homosexuality, the author, Bruce Bagemihl, seems to have deviated in the opposite direction, uncritically accepting nearly any account of same-sex sociability as indicating homoerotic behavior. Moreover, his explanation for the existence of animal homosexual behavior is downright silly: that nature is simply abundant, surprising, and—his word—exuberant.

In one respect, Bagemihl is altogether correct: Nature really
is
abundant, surprising, and wonderfully exuberant, but this is simply a description of how it appears to us. In no way does the word “exuberance” explain anything at all, just as Aristotle didn’t really explain why objects accelerate when they have been dropped, by claiming that they become increasingly “jubilant” as they approach the ground. There is something delightfully liberating about exuberance generally, and perhaps sexual exuberance in particular, but it is bone-headedly unscientific to maintain that gay and lesbian sexuality, for example, simply result from nature’s profligate fondness for variety as such.

It is doubtless more pleasant to revel in diversity—whether of human or nonhuman sexuality—than to morosely deny its existence. Moreover, it must be challenging for religious fundamentalists to maintain that homosexuality is contrary to God’s law and therefore a mortal sin once it is seen to be widespread in the natural world. Alternatively, perhaps there is simply a widespread prudish disinclination to look honestly at animal sexuality in general and at homosexuality in particular, discernible in the shocked surprise evoked when the facts are made clear. In 2005, for example, the blockbuster movie
March of the Penguins
was widely embraced by the viewing public in general and by Christian fundamentalists in particular as testimony to the potency of monogamy in nature. Shortly thereafter, considerable media attention, seasoned with outright disbelief, followed publication of the fact that male
chinstrap penguins form long-standing pair bonds … with other male penguins!
¹⁶

To be sure, the situation among animals could be illuminating for
Homo sapiens
as well, especially when it comes to possibly revealing certain generalizable evolutionary pressures. At least, this seems to be a fear on the part of many biologists, most of whom are constitutionally disinclined to extrapolate from their animal-based research to the human condition, as well as for antigay activists, who worry that animal studies will somehow “naturalize” what they see as immoral behavior. Thus, those male chinstrap penguins at the Central Park Zoo successfully fostered a chick and also gave rise to a best-selling children’s book,
And Tango Makes Three
. For several years running, this book has exceeded all others as the one most requested to be banned from libraries, a situation that does not seem to have been ameliorated by the fact that after constituting a male–male unit for 6 years, one of these male penguins ran off with a female named Linda.

Animal research, applied indiscriminately to human beings, can be misleading. There have, for example, been numerous research findings dealing with same-sex courtship in that long-time favorite of research geneticists, the fruit fly, genus
Drosophila
. Several different mutations have been identified that induce males to court other males instead of females.
¹⁷
Sexual behavior in these animals is very strongly controlled by pheromones and how the fruit fly’s brain responds to them. Behavior geneticists have known for decades of a particular mutation—appropriately known as
fruitless
—that induces males to court other males. Others have also been identified. But nothing as clear-cut applies to human beings: It is simply not the case that gay men prefer sex with other men because they are unable to distinguish them from women.

So let’s turn briefly to some of the evidence for homosexual genetics among human beings in particular before we consider various hypotheses as to how any such “gay genes” might be maintained.

In the early 1990s, Dean Hamer, a geneticist at the U.S. National Institutes of Health, led a study that reported the existence of a
specific allele located on the X chromosome—Xq28—that predicted gay versus straight sexual orientation in men.
¹⁸
It generated a media firestorm.
^iv
Subsequent research has been confusing, showing that at minimum, the situation is considerably more complicated than had been hoped by some (notably, most gay rights advocates) or feared by others (who insist that sexual orientation is entirely a “lifestyle choice”). Thus, some studies have failed to confirm any such role for Xq28,
¹⁹
while others have been supportive.
²⁰

It is also increasingly clear that whatever its impact on male homosexuality, this particular allele does not relate to lesbianism.
²¹
Moreover, other research strongly suggests that there are regions on the “autosomal” (nonsex) chromosomes that influence sexual orientation in people.
²²
A reasonable summary statement at present 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 it is increasingly clear that the genetic mechanism(s) and almost certainly the relevant genes themselves differ between the sexes.

Beyond the suggestive but inconclusive search for specific sex-orientation alleles, other genetic evidence has emerged. On average, approximately 3% of men and about 2% of women in the United States are gay, and yet, if we look at the sisters of homosexuals (gays and lesbians taken together), it turns out that more than 6% are lesbian. This is about three times what would be expected from a random sample. Similarly, 8% of the brothers of homosexuals turn out to be gay, which is significantly higher than the 3% that would be expected from chance alone.
²³
To summarize: If one sibling is homosexual, this means that his or her sibling is significantly more than randomly likely to be homosexual as well. Although such a finding is
consistent
with genetics, by itself it doesn’t prove a genetic connection, since a comparable result could arise if environmental factors alone were operating. After all, siblings don’t only share half their genes; they are also likely to
share an environment. On the other hand, had the siblings of homosexuals turned out to be
no more likely
to be homosexual than would be expected by chance, then we would have to conclude that genetic factors (along with environmental ones) are unlikely to be involved. So a
prima facie
case exists, even without the independent gene sleuthing of Hamer and others.

Twin studies have yet more to contribute. Monozygotic (MZ) or “identical” twins share 100% of their genes, since they develop from a single egg that splits in two immediately after fertilization. Of course, MZ twins have also shared the same prenatal environment. Dizygotic (DZ) or “fraternal/sororal” twins shared the same prenatal environment as well, and since they have the same mother and father, they share 50% of their genes, which makes them no more similar genetically than full siblings born separately. Halfsibs (the same mother or father, but not both) share 25% of their genes. By comparing different kinds of twins, it is possible to learn quite a lot about the general degree of genetic influence for any trait, although precise alleles cannot be identified.

Numerous comparisons can be made, not only between MZ twins, DZ twins, half DZ sibs, and unrelated individuals, but also by introducing another distinguishing variable: whether such pairs were reared together or apart. For example, comparing MZ twins reared apart with MZ twins reared together helps tease out the impact of genotype: More precisely, it helps control for the role of shared environment, since such individuals will have the same genes while experiencing different environments. A key dependent measure here is “concordance rate,” essentially the probability that both members of a pair possess a particular trait given that one of them has it—or, alternatively, the probability that both lack the trait given that one of them doesn’t have it. To conduct such analyses is to confront a rapidly multiplying subuniverse of logical and statistical complexity, but the bottom line strongly supports genetic influence … although stopping short of genetic determinism.
²⁴

For example, the concordance of homosexuality among MZ twins is about 50%, showing that the role of genes is real, since the concordance among DZ twins is considerably lower.
²⁵
But the fact that the MZ concordance rate isn’t 100% shows a definite role for their environment, including gene–environment interactions.
(Since MZ twins share 100% of their genes, if genes totally determined sexual orientation, their concordance for homosexuality and heterosexuality should also be 100%.) Other comparisons are also important: MZ twins reared together share the same genes as well as the same environment (although the environments for two different individuals can never be precisely the same!); MZ twins reared apart share the same genes but very different environments; DZ twins reared together share 50% of their genes and somewhat the same environment; DZ twins reared apart share 50% of their genes and very different environments; adopted siblings have altogether different genes and somewhat the same environment; random pairs from the population at large have different genes and different environments.

It is impossible to summarize in detail the welter of twin-based findings without devoting an entire book to the effort. Among some of the interesting conclusions that can be drawn, however, is that the concordance of homosexuality for adoptive siblings, for example, is lower than for biological siblings, which in turn is lower than that for DZ twins, which is lower than that for MZ twins. This is precisely what would be expected if genes exert a nonzero effect. In addition, concordance rates among female MZ lesbians is higher than that among gay male MZ twins, and similarly, the concordance rate among female DZ lesbians is higher than among their male DZ counterparts—suggesting that the genetic influence on homosexuality may be higher among women than among men, while further supporting the idea that the genetic influence upon homosexuality differs somewhat, in some way, between women and men.
²⁶
Other studies confirm the fact that the tendency to be lesbian or gay has a substantial heritability—which means that a significant proportion of the variation in sexual preference correlates with variation in underlying genotype.
²⁷

Prior to recent times and the prospect of
in vitro
fertilization or do-it-yourself “turkey baster babies,” the likelihood was that people who were exclusively homosexual did not reproduce at all; they, too, had to have descended from heterosexual ancestors, which by itself suggests a complex heredity for the trait.

Finally, perhaps the most cogent argument for a substantial genetic component to sexual orientation comes from a single qualitative, nonexperimental, nontechnical observation, but one that is
so widespread as to be unarguable and highly revealing, namely, the stunning fact that around the world, huge numbers of people struggle against their sexual orientation, often desperately seeking to become heterosexual so as to avoid social ostracism and/or conform to their religious expectation of what constitutes appropriate sexual behavior. If sexual orientation were simply a personal “lifestyle” choice, it is inconceivable that so many people would voluntarily subject themselves to so much misery.

It is also notable that intrapsychic battles of this sort are overwhelmingly lost, despite the typically earnest—even desperate—desire of the participants to change, and (at least in the United States) notwithstanding a small army of ideologically committed therapists. Even those relatively few cases of “successful” conversion therapy typically last less than a year, and, moreover, they involve a disproportionate number of subjects who were initially bisexual rather than exclusively homosexual in the first place.
²⁸

Unlike certain traits that are under strict genetic control (eye color, blood type, etc.) and others that are entirely determined by the environment (e.g., what language someone speaks), most behavioral traits—including but not limited to sexual orientation—are influenced by both genes and environment. Accordingly, isn’t it likely that individuals with an inclination toward homosexuality would have been more prone to behave homosexually in a more gay-tolerant social environment, and vice versa? If so, then in societies that did not welcome sexual diversity, homosexual-prone people might well have historically been more inclined to marry and even have children, all the better to fit in. This would have contributed to the maintenance of same-sex preference, but as we have already seen, it couldn’t have done the job by itself.