[thomassmith65]

Evolution is surprising and complicated enough that that might not be the full picture.

People have relatives and relatives have many genes in common.

If there are genetic traits that predispose people to remaining childless, it could wind up that populations with those genes end up with larger branches on their family tree.

It's been a while since I read the Selfish Gene, but its central point is that natural selection is about genes, not individuals.