Kin Altruism and Homosexuality
Caveat: I believe that human sexuality is a rich spectrum, not binarily hetero or homo. When I use the term homosexual in this writeup, I mean individuals exhibiting strictly same-sex sexual behavior. I know it's shorthand, but it makes things smoother in the text. Also, I presume the studies showing a moderate heritability of homosexuality are correct, i.e. that there is some genetic basis for its presence in humans.
Homosexuality presents quite a problem to evolutionary thinking. How could homosexuality have ever survived evolutionary forces to be as common as it today across the world? After all, time was it took a man and a woman to spread genes by making babies. Since same-sex pairings can't make babies, aren't "the gays" a genetic dead end?
(This line of thinking is what led cackling plastic sculpture Dr. Laura Schlessinger to announce that homosexuality is a "biological error", with all of its terrifying Third Reich inferences. But even without the vitriol, the question seems fairly tough at first.)
The short answer is no, because of course it's not that simple.
As Gritchka notes in the above writeup of the general theory of kin altruism, evolution supports preferential behavior towards individuals who share genetic makeup. In his 1978 Pulitzer Prize-winning book On Human Nature, Edward Wilson extended this idea (it was his in the first place, Dawkins just made it more accessible) to posit that while exclusive homosexuality does not directly contribute to the gene pool, the kin altruism of homosexuals has a greater effect than that of heterosexual kin because the homosexuals are unburdened with children of their own, and this compensates. Put another way, your kids are more likely to get more presents on their birthday if you have a gay sibling than a straight one. (That is, of course, if you're on good terms with them and don't call them biological errors.)
Other theorists have elaborated indirect evidence to support the theory:
- Male homosexuals are poor heterosexual mating prospects, and so the fitness costs for not having their own progeny is low.
- They are on average more empathic and nurturing, and therefore more likely to act on their kin altruism.
- Homosexuals tend to be born later rather than earlier in birth order, increasing the likelihood that there would be nieces and nephews around to spoil.
David M. Buss in his 2004 book The Evolution of Desire, cites three problems with the theory.
- It doesn't account for exclusive homosexual behavior in the first place: why wouldn't it encourage asexuality instead?
- To be valid, there must be some evidence to indicate that greater empathy translates to specific kin benefit.
- The kin benefits would have to be quite high to compensate for the loss of direct reproduction.
I'm not an evolutionary scientist, and this last one seems like it would take more statistics that I can muster, but the first two seem easy to address from a layman's point of view.
Why homosexuality rather than asexuality? Because the sexual drive is evolutionarily much older and much more deeply embedded than mammalian communal drives. For this reason it seems that it would be much easier for evolution to tweak sexual drive than to eradicate it. Furthermore, the theory doesn't seek to explain the origin of homosexuality (after all, it appears in thousands of species much older that that arrogant upstart Homo Sapiens Sapiens), just the reasons why it became optimized at a consistent 1%-4% rate across the species.
Why must greater empathy translate to specific benefit? I think this is a straw man argument, because increased empathy is not the lynchpin of the theory. It would still hold true if homosexual kin had the exact same levels of empathy as their heterosexual counterparts, because they are unburdened with children of their own. As long as kin altruism exists, this translates into some gain for the children, and evolution would seek to optimize it.
For his death knell to the theory, Buss cites a cross-sectional study by David Bobrow and Michael Bailey that indicates that modern gay and lesbian kin investment is equal to heterosexual kin investment. But does saying it doesn't work that way now mean it couldn't have been that way in the past? The evolutionary forces that shaped homosexuality in our species may simply no longer pertain. (Just as the evolutionary forces that optimized our spinal column for brachiation don't apply now either, but that's the spine we got. See Gould's spandrel theory.) Modern mobility and the stigmatization of homosexuality have clearly lessened the likelihood, opportunity, and amount of investment in the grand scheme of things.
Seen this way, the recent (circa 1600 CE) Western stigmatization of homosexuality is strangely self-denying. A society truly interested in family values should embrace their gay uncles and lesbian aunts. It should encourage their kin altruism as much as possible. And yet. And yet. And yet.