Here's one way to control for genetics - compare orphans who were adopted early (Pre 1yo) and then see what the correlation between their IQ and both the parent

Here's one way to control for genetics - compare orphans who were adopted early (Pre 1yo) and then see what the correlation between their IQ and both the parents and adoptive parents IQ. (Or use income as a proxy for intelligence; it might be hard to get the dead parents to take an IQ test)