Since we know IQ is at least 50% hereditable, wouldn’t we expect this correlation?
I think the main factor is pre- and post-natal care. And the post-natal goes all the way to the teenaged years. It includes not just a proper diet but the best training.
i.e. take an illegitimate baby from a dirt poor mom that at least gave it good pre-natal care, adopt it out to a well off and caring family, and you could end up with a VERY intelligent individual in adulthood.
Of course two very smart people can have a dumb kid and the reverse.
So what? The children of intelligent people are on average going to be more intelligent than the children of stupid people.
Why would anybody even try to argue this point?
Two tall people can have a short child, and two short people can have a really tall child, but on average heredity does its thing.
How does anybody think we developed dog and horse breeds?
Many (most) characteristics are hereditable.
One could, in theory, breed humans for desired characteristics (intelligence, beauty, height, etc.) just like any other species of animal. Ideas for doing so have always foundered on two rocks: the people in question won't cooperate, and the long breeding cycle of humans makes it impractical relative to other animals. (Not to mention ethical considerations. :))
That is what people are, you know, animals. We are not JUST animals, but that is what at root we are.