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Reply to: https://imgur.com/gallery/nKvZpDK/comment/1655199307

Heritability always answers the question "What makes Joe different from Jane?". It doesn't answer "in what proportions do genes and environment constitute Joe's intelligence", for example. If we take a pile of Joes and Janes and see how different they are from each other, heritability only talks about those *differences* and their makeup.

As one classic example, we both presumably have five digits in our hands and feet. This is entirely genetic and how basically every human is genetically coded. The most common reason any human would have a different number of digits is usually accidents that lead to some of them being chopped off. So the differences in number of fingers between humans are 100% environmental, barring an extraordinarily small number of freak accidents.

This is not the case with cats - a small but not freak-accident-low number of cats are naturally born with six digits on their feet, and they're also prone to accidents just as humans are. So some part of the variation in toe numbers in cats is genetic, some of it is environmental.

In the case of human intelligence, SES raises heritability because it eliminates some environmental factors we know can fuck up children's intellectual development - lead exposure, not having enough iodine in the diet growing up leading to cretinism, not having enough food to begin with (intelligent people's brains run more efficiently, but building a brain - anyone's brain - is an expensive affair), and so on.

In high-SES environments, these kinds of fuckups are understandably really rare, and so the intelligence differences between Yale students Jack and Jill can be predominantly genetic (to the tune of 80%). Again, some of their intelligence level is environmental - they had the iodine and didn't grow up cretins - but is the same for people of their background, so it doesn't contribute to differences between people of that kind of background.

You could do a similar thought experiment between say, Kyle and Karen who grew up in human society and poor Hannah who was raised by wolves and is Karen's twin sister. The difference in acquired vocabulary between Kyle and Karen can be heavily genetic, but Hannah's lack in that department is squarely down to her family communicating by howling and barking - it's environmental, she doesn't share Kyle's and Karen's baseline of "grew up in human society".