[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.