The latest Harper’s Index (not yet online) includes the claim that a male child becomes 33 percent more likely to be gay for each (full biological) older brother he has. Science News apparently reported the finding over the summer.
The fact that the connection holds only for biological older brothers suggests that the cause here is also biological, rather than some psychological effect. And, though it oddly goes unmentioned in the SN article, dovetails very nicely with one popular theory of the evolutionary function of homosexuality. In many different species, there appear to be organisms that totally forego reproduction in order to help parents and siblings raise their own children. The most extreme case is the social insects, where workers are all sterile and work to aid the reproducing queen. And this makes sense from a selfish-gene perspective, since what matters for evolutionary purposes is how many copies of your genes get into subsequent generations—genes shared by the queen and her offspring—rather than whether the individual organism spreads them through its own reproduction. The same “helper organism” behavior is found, to a less extreme extent, in a variety of other species as well.
Of course, it would be bad for your genes if your very first child were “programmed” to be a non-reproducing helper, because if you then die without having any more children, that’s the end of the line. There’s some benefit if child number two is a helper, but probably still not as much as if you had two reproducing kids. But for male offspring in particular, there’s probably diminishing marginal returns to having them reproduce, because one male can, potentially at least, impregnate a very large number of females. So especially for a limited pool of mates, the more male offspring you have, the less chance that they’re providing an added genetic benefit: There’s already plenty of sperm carrying your genes to go around. Therefore, with each added male child, it becomes more likely that your genes get a greater benefit from having him devote his energy and resources to helping his siblings’ children survive than to having his own offspring.
Now, there don’t seem to be a whole lot of gay bees, but then, their whole species structure is geared around a dramatically collectivist reproductive model. Mammals have a bunch of machinery set up to produce a powerful individual sex drive. So it’s not a huge leap to imagine that it was easier for evolution to produce non-procreative helper siblings by redirecting the sex drive than it would’ve been to selectively shut down the system altogether. Voila: Gay dudes.
This is, of course, a wholly speculative, armchair-generated Just So Story. But it fits a bunch of available evidence, and has a satsfying sort of internal logic to it. So much so that it seems inevitable that someone else has already proposed it. Anyone know if this notion’s been floated so far?