People who grow up in a small town, or an old and stable neighborhood, often know their neighbors. More than than that, they know pretty much everything that’s happened for the past couple of generations, whether they want to or not. For many Americans, probably most, this isn’t the case. Mobility breeds anonymity. Suburban kids haven’t necessarily been hanging out with the same peers since kindergarten, and even if they have, they probably don’t know much about their friends’ sibs and parents.
If you do have that thick local knowledge, significant trait heritability is fairly obvious. You notice that the valedictorians cluster in a few families, and you also know that those families don’t need to put their kids under high pressure to get those results. They’re just smart. Some are smart but too rebellious to play the game – and that runs in families too. For that matter, you know that those family similarities, although real and noticeable, are far from absolute. You see a lot of variation within a family.
If you don’t have it, it’s easier to believe that cognitive or personality traits are generated by environmental influences – how your family toilet trained you, whether they sent you to a prep school, etc. Easier to believe, but false.
So it isn’t all that difficult to teach quantitative genetics to someone with that background. They already know it, more or less. Possession of this kind of knowledge must have been the norm in the human past. I’m sure that Bushmen have it.
The loss of this knowledge must have significant consequences, not just susceptibility to nurturist dogma. In the typical ancestral situation, you knew a lot about the relatives of all potential mates. Today, you might meet someone in college and know nothing about her family history. In particular, you might not be aware that schizophrenia runs in her family. You can’t weigh what you don’t know. In modern circumstances, I suspect that the reproductive success of people with a fair-sized dose of alleles that predispose to schiz has gone up – with the net consequence that selection is less effective at eliminating such alleles. The modern welfare state has probably had more impact, though. In the days of old, kids were likely to die if a parent flaked out. Today that does not happen.