Of course it's a combination of the two. In the ancestral environment, kids "just happened". There were people who specifically tried to have more children, bu

Of course it's a combination of the two. In the ancestral environment, kids "just happened". There were people who specifically tried to have more children, but they would only have a small advantage over the vast majority of accidentally made children. Fast forward to modernity. Now, children are only being born to people who specifically choose to do so. A small change in socioeconomics accidentally privileges people with a biological quirk.