Or, smart people had smart parents. Smart parents tend to do that kind of thing, but the main effect to the kids’ IQ comes from having the right genes, not from the actual behavior. (A lot of the things that correlate with kids having higher IQs are like this, including things like “amount of books in the house”, IIRC—control for heredity, and the effect on IQ vanishes.)
Or, smart people had smart parents. Smart parents tend to do that kind of thing, but the main effect to the kids’ IQ comes from having the right genes, not from the actual behavior. (A lot of the things that correlate with kids having higher IQs are like this, including things like “amount of books in the house”, IIRC—control for heredity, and the effect on IQ vanishes.)