I have blogged recently on the question of whether homosexuality has biological causes, and whether dissent from the view that it does should be allowed. I believe the answer to both questions is yes. You can read the posts here and here, along with comments from intrepid reader Z-Tek. Intrepid reader Vic Ulmer sent me this question by Gmail:
Isn't homosexuality inconsistent with and counter to the fundamental biological concept of reproductive success? Thus, why would there be a "genetic predisposition" to homosexuality?
This is a very good question, and to answer it I must put on my professor hat. First, one must distinguish the question whether some trait H has a biological cause from the question of what that cause is and how it works. Biological causation may be indicated by many things: for example if the trait is universal (all people grow old and die, regardless of where or when they are born or what culture they are exposed to); or if it shows signs of heritability (hair color and eye color). Homosexuality seems to indicated by both criteria: it exists in all significant populations, regardless of culture, and as I understand it, shows at least some signs of heritability.
Of traits that are biological, some are selected for, some against, and some are not exposed to selection pressure. Good eyesight, congenital blindness, brown rather than green eyes are, respectively, examples. As you indicate , homosexuality would seem to be subject to negative selection pressure, as homosexuals typically invest most or all of their sexual behavior in ways that cannot lead to successful reproduction. That is consistent with the fact that homosexuals constitute a relatively small portion of every general population (probably less than 2 or 3%).
However, a trait that is maladaptive in a general population can be adaptive for a small portion of the population if it finds some niche within that population where it is favored. Unusual boldness probably killed off most people who ever inherited it, but sometimes it leads to unprecedented reproductive success, as in the case of certain sports stars or the hero who saves the people against Goliath. Likewise, a generally maladaptive trait may be preserved if it is rare but unavoidable consequence of genes that are otherwise adaptive. There are two genes that code for resistance to malaria and that's adaptive; but if you have both, you get sickle cell anemia.
I know of at least two explanations for the genetic persistence of genetic homosexuality. One is that homosexual males made very good warriors: pairs of lovers sacrificed their own reproductive opportunities, but promoted the reproductive success of their heterosexual brothers and sisters. Those siblings carried the same genes that, in the proper mixture, would produce more homosexual warriors. That is a niche theory. Another explanation is that some of the genes that code for adaptive traits such as sexual attraction to the opposite sex and desire for friendship with the same sex will inevitably, on occasion, combine in such a way as to produce homosexuality. That seems more plausible, but we can hardly know yet.
All I can say now is that it is a good bet that there is a biological basis for homosexuality. That is a long winded answer to your question, but as I said, it is a good question.
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