The causal mechanism isn't the same thing as "why hasn't evolution selected against this"[0] and neither is the same thing as subjective incentives.

[0] one suggestion is the "gay uncle hypothesis" which posits that people who themselves do not have children may nonetheless increase the prevalence of their family's genes in future generations by providing resources (e.g., food, supervision, defense, shelter) to the offspring of their closest relatives; this hypothesis seems to be consistent with the evidence without being sufficient on its own.

Near as I can tell, it’s also evolutionary advantageous to have significant ‘randomness’ as far as traits across a population. Especially for humans due to the extreme variation in environmental conditions and stresses we end up producing for ourselves.

Being a hero by jumping on a grenade is a pretty terrible survival trait for an individual for instance, but essential for the group to have at least one in any decent sized population. All the non-grenade-coverers will strongly support such folks, as long as it doesn’t hurt their own survival chances somehow.

So if we look at individual tendencies as coming more from die rolls than anything else, with a wide distribution, a lot of outlier behavior makes a lot of sense.

A hardcore survivalist most of the time is going to be selected against, for instance, for many reasons when things are going well. They’re dumping all their stats points in the wrong categories!

But any population that doesn’t have at least a few is going to completely disappear on those rare long tail events (an actual nuclear war?).

And if those survivalists are actually capable, they get the benefit of ‘seeding’ the next generation without any competition! Long odds, but potential huge payoff biologically.

That sounds plausible, I wonder how strong the effect size is?

Part of the problem is how you’d measure/quantify it, I imagine. Especially since many of the really interesting edge cases will want to hide.

Also many of the traits might also only show up under extreme stress, and we’ve learned to frown on doing destructive a/b tests on entire cultures.

That's the 'how', not the 'why'. I think biological incentive here means, the reason human evolution retained certain traits. An interesting theory for that is that gays and lesbians can cooperate with their siblings to raise nieces and nephews, instead of competing with them for resources.

And individually would benefit from positive relationships with those nieces and nephews (financially, physically when old, socially), without having to directly bear the costs of having those kids either.

JBS Haldane: I would be willing to lay down my life for two brothers or eight cousins.