Why Does Homosexuality Exist?

The Supreme Court decides this week whether to hear an appeal of the California Supreme Court's ruling that Proposition 8, the 2008 voter initiative that limited marriage to a man and a woman, is unconstitutional.

In addition to the constitutional questions surrounding gay marriage, there are some interesting biological questions to be asked about homosexuality as well. For instance, given the assumption that homosexuality is genetically based and the further evolutionary assumption that traits that confer no reproductive advantage eventually die out in a population, why does homosexuality exist in the first place?

Evolutionary biologist David Barash tackles this question at The Chronicle of Higher Education. He writes:

If evolution is true then homosexuals, who of course reproduce at far lower rates than do heterosexuals, should not exist, but of course they do.

This is a vexing enigma for evolutionary theorists and Barash surveys the various speculations scientists have advanced to explain the existence of a phenomenon which, evolutionarily speaking, should not exist. The speculations he adduces sound unconvincingly feeble, but they're evidently the best that the theorists have been able to come up with.

Barash admits that the solution to the mystery of the existence of a trait that confers no reproductive benefit has eluded our best scientific minds:

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?

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.

The author continues at some length to try to explain the inexplicable fact of homosexuality. He's adamant that homosexuality is a product of evolution, but despite his promise to explain why he offers no substantive evidence to support the claim. He also insists that it's a genetically based phenomenon but offers scant evidence to support that view.

Maybe someone ought to call the departed ship back to port and give it another look.