What pressures keep homosexuality in populations?
Twin studies indicate that genetics does seem to play a role: “Bailey and Pillard (1991) found that [identical] twins are concordant for homosexuality 52% of the time, and [fraternal] twins only 22% of time time, a statistically significant difference that points to a large role for genes.” That link addresses a lot of your question I think. Much of what I would say has been said. The genetic basis for homosexuality is well established but that only means that some genes include, among their effects, a greater or lesser propensity for homosexuality. To really evaluate the worth of those genetic factors you would have to know what they were, you would have to know what all their effects are, and you would have to be able to weigh those effects in your theorized evolutionary environment (not just individual survival of one organism). Fully untangling this sort of thing is (forever?) beyond the reach of genetics, s
It can be “genetic” but not as simple as most people view genetics; that is, like Mendel’s smooth and wrinkled peas. Even eye color is currently understood to be 2 genes. Other reactions are even more complex. There was a brief discussion of this in Nature via Nurture by Matt Ridley, basically analyzing Ray Blanchard’s studies into male birth order and birth weight. He found that males who were born after older brothers were lighter at birth, potentially due to an increasingly strong maternal immune response in the womb. And those younger brothers had an increasingly strong “chance” to be homosexual. In a sense too, the “gay gene” (or genes, or gene complex…) maybe be on the mother. As an amusing sidenote, finger length may correlate with birth order
The Boston Globe ran an interesting article about a research project on what makes people gay. It did not address what would be the evolutionary pressures for homosexuality. If it is partly genetic, one would expect it to quickly disappear due to natural selection unless it conferred some hidden advantage to the genetic lines which carried it. My pet theory is that its a form of population control. If certain environmental cues suggest overpopulation is imminent then the trait is activated at some point during the child’s or fetus’s development. Just as some salamander species produce cannibal mutants in overcrowding situations, we produce useful hunter-gatherers. Of course, if this theory were correct it would predict sparsely populated regions would have noticeably lower gay populations. I’m not aware of any studies to support that.
1) I would check your facts on the “10% throughout history” statement. Bisexuality and homosexuality were prevalent pre-Christianity. 2) Even assuming homosexuality is purely genetic, I think you’re grossly underestimating the amount of time it would take to make any significant change in the proportion of gay people to straight people. 3) It is interesting to note that studies have found gay animals.
