[gwern]

I don't know how that is supposed to work. What sort of social pressure could possibly produce near-zero fitness impact when it makes you not want to have sex with women, in an environment where the men who do want to have sex with women are like 95% of the population and will happily put enormous efforts into doing that and taking your place? In addition, as I said, given the apparent fitness impact and frequent childlessness of gay men, we should be able to see homosexuality rates plummeting over the past century, but as far as I know, there's no evidence. (Plus the wild animal thing, yeah.) There's something stranger going on. The 'gay germ' hypothesis, as unpopular as it may be, at least adequately reconciles all of the observations.

[toasterlovin]

Maybe you're misunderstanding the parent (or I'm misunderstanding you!). I believe the parent is saying that the genes which predispose somebody to being gay remain in the population, despite their seemingly zero reproductive fitness, because gay people face enormous pressure from their relatives to not be gay and reproduce. And they face this pressure because their family members have a genetic interest in the gay person's offspring and so have evolved to coerce gay family members into having children.

[gwern]

I understand what they're saying. My point is that it's highly unlikely: coercion just doesn't work that well when it comes to something extremely expensive and requiring a lifetime of work to maybe succeed, when the task is not just not pleasant but outright aversive. It's difficult to see how such coercion could be so extraordinarily successful as to reduce the average fitness penalty to something so tiny that homosexuality could still exist at current frequencies like 5%; it'd be like you'd have to have such pervasive and super-effective coercion that not a single gay man out of 100 fails to reach his quota, and this would have to obtain in all societies forever, effectively, or else eventually the coercion would slacken and the homosexuality genes would almost immediately vanish, permanently. This is pencil-balancing-on-its-point-for-millennia territory.

[toasterlovin]

Right, I agree that if the genes which predispose somebody to being gay had no benefits whatsoever, then they could not even exist; they would have a purely negative effect on fitness. So they must have some positive effect on genetic fitness to even exist at all. And they must be relatively significant and intractable from the side effect that some of their carriers turn out gay, given how common gay people are.

But that is separate from whether or not pressure from relatives to reproduce can be used as a mitigating strategy for the cases where these genes do result in someone being gay. And this strategy, then, would lower the amount of fitness those genes would need to provide to be viable.

[gwern]

"So they must have some positive effect on genetic fitness to even exist at all."

That's not true. There's a number of ways variants can exist without being fit. They can be regularly created by mutation, or they can increase due to genetic drift, especially in a bottleneck scenario. And there's even more ways that a phenotype can persist while being highly unfit - if it's a side-effect of a co-evolving pathogen, being one of them, as then the human natural selection is constantly fighting it but the pathogen easily evolves even faster.

[JoeAltmaier]

Genes can also come along with other stuff - like Neanderthal genes that (must have?) provided some benefit for living in Northern Europe, while also adding depression, alcoholism etc.

Sorry, off topic :)

[Snargorf]

You don't even need to look that hard.

Poor eyesight has no adaptive benefit. Yet many, many people need glasses. I guess it's just 'difficult' for our genes to grow a sharp eye focusing system, so they often get it wrong.

The same may be true of homosexuality. So even if there's nothing adaptive about being gay, it keeps happening because sexual tuning in the brain is a hard target to hit.

Homophobia may be an adaptation to this. It makes parents force their children to act straight even if they're gay.

[gwern]

> Poor eyesight has no adaptive benefit. Yet many, many people need glasses.

Many people... in particular industrializing countries. Our genes have a great sharp eye focusing system. You won't find much myopia in a random tribe.

[JoeAltmaier]

Many animal populations have non-reproductive members. It takes a village to raise a child. In such an extremely social animal as Homo Sapiens, many of our adaptations are social. E.g. it only takes a few men to guarantee another generation, yet 50% men are born in each. Another: we don't die the instant we give birth; we're there (and non-reproductive) for decades afterward, competing for resources. Its the social benefits that allow for this.

[makomk]

There's something called Fisher's principle that explains why the sex ratio of most species is 50:50. In short, if less than 50% of humans are men, then men must have more children on average than women and it becomes reproductively advantageous to have more make kids in order to take advantage of it.

[JoeAltmaier]

Still, in many animal populations the ratio 1:1 doesn't hold. Something else is going on then.

[internaut]

I think it's not quite 1:1 in humans, the males have a slight lead.

http://www.scientificamerican.com/article/is-a-pregnant-woma...