Homo Mysterious: Evolutionary Puzzles of Human Nature (19 page)

Alternatively, if such a genetic factor exists on the Y chromosome, it could only be expressed in males—but then, what about female homosexuality? (Since women are XX and lack a Y chromosome, a genetic factor for lesbianism couldn’t reside on the Y chromosome.)

For all the excitement about identifying a “gay gene”—and all the reality that to a significant extent, homosexuality clearly is mediated at least partly by genetics—it must be emphasized that genes don’t tell the whole story. For one thing, it is possible to explain any trait, and especially to
explain away
any finding that appears to disconfirm a genetic hypothesis, by arguing that the trait in question shows “incomplete penetrance and variable expressivity” … which is to say, sometimes the trait shows itself (whether or not the underlying genes are there), and even when it does, its actual manifestation will vary greatly.

Let’s conclude this section by emphasizing that no trait derives from genes alone or from the environment alone. To be more affirming: Every trait derives from the interaction of the two. That said,
we’ll continue to examine the role of genetics in particular, simply because DNA is the substrate upon which natural selection acts.

Time, now, for a reasonable historical assumption about how we got to be the way we are: During our long Pleistocene adolescence as a species, men were probably evolutionarily fit in proportion as they were good with projectile weapons, at anticipating the habits of game and of potential enemies and competitors, and at attracting and keeping mates, of course … with much of the latter two occurring in proportion as men succeeded at the former four. With the development of agriculture and early civilization, however, it is likely that the optimally adapted person—of either sex—tilted more than previously toward social skills, verbal ability, etc.
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The likelihood is that these traits had always been favored to some degree, but perhaps especially so once people occupied large settled communities. And so, the argument goes, natural and sexual selection came to favor social and communicative skills—at which homosexuals tend to exceed heterosexuals.
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What does this have to do with genetics? Just this. Maybe in evolutionary time, exclusive homosexuality has been a fitness catastrophe, like sickle cell disease, but just as sickle cell disease persists because in single dose its underlying allele conveys a benefit with respect to malaria, perhaps one or more homosexuality-promoting alleles were retained because they also conveyed a particular payoff. That fitness-supporting homosexual benefit could have derived from the verbal facility and/or social and communicative skills just described. The outcome might then be a stable frequency of exclusive same-sex preference, just as heterosis produces a steady frequency of people with sickle cell disease.

C. R. Dewar, who first developed this line of reasoning, argues that if selection has favored an intermediate degree of “gayness,” perhaps because of verbal facility and related assets, this would lead to exclusive homosexuality cropping up persistently at one tail of the distribution, even though homosexuality itself would not have been selected for directly. He turns to head size of human embryos for an example. Thus, selection favors the production of
babies whose heads are pretty much as large as possible, so as to accommodate maximum brain development. Sometimes, however, this selective pressure results in babies whose heads are simply too big to be accommodated by the mother’s birth canal: The resulting cephalopelvic disproportion is a significant cause of mortality during childbirth, especially in societies without access to modern obstetrical procedures.

Nevertheless, Dewar argues, such genes aren’t edited out of the population because they are advantageous when present at intermediate levels:

It might be safer at birth to have a small head with a small brain and to be born with the ease of a puppy, but it is just too massive a disadvantage throughout the remainder of life. In reproductive terms it’s better to take the risk associated with a large head. Similarly it might be too big a disadvantage in a post-hunter–gatherer society to be aggressive with poor communication and social skills (as a result of being highly responsive to available androgens) even if the alternative means there is a 5% chance of being exclusively homosexual. This also parallels the observation that homosexual children are born to heterosexual parents. Parents with large heads who have survived childbirth may themselves conceive children who do not. Indeed it is invariably the case that parents of children who die during childbirth survived their own birth.
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Bear in mind that in traditional societies, homosexuality often served a social role, beyond possible assistance in rearing the offspring of genetic relatives.
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Perhaps the social payoff associated with same-sex preference—quite aside from any kin-selected payoff—was sufficient to keep same-sex alleles around, even though some proportion of regularly produced exclusive homosexuality (analogous to having a too-large head) was the price to be paid. Not quite heterosis as with sickle cell disease, but close.

A final gene-based hypothesis goes by the unfortunate name of “sexually antagonistic selection.” It sounds complicated but is actually quite straightforward. What if one or more alleles that predispose toward homosexuality (and with it, lower reproductive output)
in one sex actually work in the opposite manner in the other sex? I prefer the phrase “sexually complementary selection,” since the hypothesis does not so much imply antagonism between the genetic effect in males
versus
females as complementarity in outcome: A fitness decrement experienced when the relevant genes exist within, say, males is more than compensated—for the genes in question—by a fitness enhancement within females. And vice versa.

There is some supportive evidence. In one study, it was found that the fecundity of mothers of heterosexuals averages 2.07, whereas mothers of homosexuals, from the same population, average 2.73 offspring.
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It turns out that numerous other relatives from the mother’s line also showed higher fecundity.
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In general, sexually complementary selection is suggested any time it can be shown that the female relatives of male homosexuals have a higher fitness than do the female relatives of heterosexual males. The result would be that in the process of selecting for greater female reproductive success, enhanced numbers of male homosexuals have also been produced, as a side effect.

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

We turn now from focused genetic hypotheses to social ones—but in doing so, we are not moving away from evolutionary considerations. In fact, we aren’t really stepping away from genetic matters, either. This is because if it turns out, for example, that homosexuality conveys a social benefit upon its practitioners—say, by providing a kind of social glue that enhances adult bonds—any long-term positive impact must depend on the “social benefit” translating into a genetic benefit and thus an evolutionary consequence … or else such a benefit would likely be short-lived. Behind the social payoffs hypothesized below there lurks presumed genetic and evolutionary payoffs as well.

Let’s start with social hypothesis number one: practice. In might not make perfect, but let’s face it, “doing what comes naturally”—that is, sex—is often surprisingly difficult without practice! After all, unlike eating, sleeping, keeping yourself warm, or scratching when you itch, courtship and copulation involve interaction with another individual, which introduces complexity into any occasion, especially one so inherently fraught as sex. In many cases, this is not simply a matter of becoming a “great lover,” or even a good one, but simply meeting the basic requirements for biological success.

Homosexual behavior could especially lend itself as an adaptive opportunity for sexual practice if, compared to heterosexual encounters, homosexual activity is less liable to serious social repercussions if done clumsily. One might predict that insofar as it provides practice and learning, homoerotic behavior should be particularly frequent among juveniles, and less so among adults. This is the case. Many free-living animal species show same-sex mounting, nearly always among juveniles and nearly always diminishing rapidly with adulthood. On the other hand, the existence of exclusive homosexuality—and in relatively high numbers, as among
Homo sapiens
—goes against the expectations of the practice hypothesis. It’s one thing for evolution to favor a behavior pattern that generates useful practice, quite another if the “practice” replaces the presumed goal. In addition, although some degree of homoeroticism occasionally accompanies human childhood and adolescence, there is no evidence that people who experiment sexually in this manner eventually function more successfully as heterosexuals. The “practice hypothesis” may thus apply more to animals than to people.

On to social hypothesis number two: sexual selection. The basic argument here is that evolutionary success isn’t merely determined by ability to survive (“ecological” or “survival” selection), but also by an individual’s success in attracting and keeping a mate. It is possible that a genetic tendency for homosexuality could be retained, even if it would otherwise seem to result in diminished breeding success, so long as such individuals were preferentially chosen by members of the opposite sex. This seems unlikely but is not impossible if, for example, some proportion of women have been favorably disposed toward homosexually inclined men who may also be especially caring and helpful, thus offering the promise
of enhanced parental care.
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Related to this is the observation, confirmed by many women, that they often find gay men appealing because they are less threatening than their heterosexual counterparts, whose overt female-directed sexuality can verge on the predatory. It remains to be demonstrated, however, that under such circumstances, gay men actually end up fathering a significant number of children. It is also unclear whether a reciprocal situation occurs, whereby at least some heterosexual men are especially attracted to women with tomboy traits, and if so, whether there is any connection between “boyishness” among adult women and same-sex preference.

The third social hypothesis revolves around the notion of homosexuality as social glue. Most research studies along these lines involve animal examples. Perhaps this is because homosexuality is more likely to have a social bonding effect among nonhuman creatures, or maybe biologists for some reason have simply been more intrigued than sociologists by this possibility. In any event, the thrust—pardon the expression—of many of these observations is captured in the title of one such research account: “Establishing Trust: Socio-sexual Behaviour and the Development of Male-Male Bonds Among Indian Ocean Bottlenose Dolphins.”
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Approximately one half of all observed homosexual copulations among these miniature whales are male–male. (Because male and female dolphins both possess genital slits, males can achieve intromission with other males.) Bonobos—“pygmy chimps”—are justly renowned for being hypersexual; what, then, are we to conclude about bottlenose dolphins, whose sexual “event rates”—for males—average more than 2 per hour, which is nearly 40 times higher than the frequencies reported for female bonobos in the wild?

Not coincidentally, bottlenose dolphins are highly social, and their breeding system is such that the reproductive success of males appears to hinge not merely on their heterosexual encounters but also on the extent of male–male cooperation in bringing such sexual liaisons about:

Of particular interest in bottlenose dolphin research is the relationship, if any, between male homosexual behaviour and alliance formation, a crucial part of male mating strategies. Males form first-order alliances (pairs and trios) that cooperate to sequester and maintain exclusive access to a single female for up to six weeks … an event known as a
consortship. Some first-order alliances appear to remain highly stable for 15–20 years. They typically pair with one or two other alliances to form second-order alliances. Second-order alliances cooperate by helping each alliance keep their respective females during consortships. Although popular accounts occasionally infer that males coerce copulations on the female, such behaviour has never been observed. Males may also form a super-alliance of up to 14 individuals. Pairings and trios within the super-alliance are labile, with no more than three males consorting with a female at any time. However, if the pair or trio is challenged by an outside alliance, the entire super-alliance may help the pair or trio defend the female … a pattern otherwise seen only in humans.

The bottom line is that bottlenose dolphins form unusually intense male–male bonds, albeit in the service of heterosexual breeding success, and they also engage in an unusually high frequency of male–male sexual interactions, especially as juveniles. It seems reasonable—maybe even likely—that the latter takes place in furtherance of the former. But no one knows for sure. It is tempting to extrapolate to humans, who also form intense male–male bonds, although usually not with an eye toward cooperative defense of a breeding female. It is interesting, though, that human male–male bonds are probably most intense among combat soldiers, and that positive sexual relationships among warriors has a long and honored history.
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(This might also suggest a new perspective on the controversy surrounding “gays in the military” within the United States.)