[pfannkuchen]

It feels sad because it goes against human instinct. In hunter gatherer groups people generally didn’t leave, and when they did leave (because the band got too big) I think they generally fractured voluntarily along bilateral sentiment lines (because why wouldn’t they, what are such sentiments for after all).

Which isn’t to say that we should prevent it in our own lives today, because we do live in a society. But I think it’s worth reflecting on how the structure of today’s society prevents the formation of larger kin groups.

To the child it feels less sad because the parent is basically a sort of captor, who is implicitly forced to mold the child to the behaviors required by society and the child generally holds some level of grudge, often subconscious, against the parent for the actions that are taken to do this to them (such as forcing them to live inside a box and suppress a large variety of natural instincts from early childhood on, among other things).

[unD]

I might be wrong, but IIRC in other animal species as well as in humans, individuals generally do leave when pairing—either females joining the male's tribe (many hunter-gatherer societies) or vice versa (e.g., baboons).

[pfannkuchen]

That’s fair. I have read that also, but I have some skepticism that what we’ve seen in extant hunter gatherers there represents a universal instinctive practice in hunter gatherer humans.

As the mating pool grows and includes more genetic variation, the propagation penalty for non-obvious genetic defects decreases because feature breakages are mitigated by reinforcement from the pool.

In a small and relatively homogenous mating pool (let’s say a band cluster with some non-zero interband genetic flow velocity), a genetic feature breakage that does not get filtered out by sexual selection or consistent early death will be extremely harmful to the group and could kill it or cause it to be outcompeted by neighbor clusters. However, in a large scale society, a breakage in an individual is likely not also in the mate, so the breakage can persist as a recessive without causing consistent damage.

So, if an extant hunter gatherer group has any substantial mixture with or heritage from any group that was ever living in a large scale society with a large mating pool, we would expect them to face a problem of recessive diseases from this source and an appropriate response would be intermarriage.

Which isn’t to say there aren’t other potential sources of recessive diseases, but I do believe the observations of extant hunter gatherer groups is likely corrupted by this vis a vis true ancestral human behavior.

There are also other reasons why interband mating could be advantageous as a social cooperation strategy within the band network, but I wouldn’t necessarily expect that to be so universal that it would be baked into instinct, and it seems like it isn’t based on how people feel in similar situations today.

Beyond this, even if that was universal (which it could be), that would be like half your children moving in with your cousin’s family in the next neighborhood over, in exchange for a similar number of your cousin’s children coming to you. Not really much like what happens today.