What about the Duke brothers in “Trading Places”, lol? Seems a lot of rich kids like Obama and Hillary and Warren, so brain size difference is not so much of an issue as compared to how it is actually used.
I read another study on families---I think by Kay Hymowicz, but I could be mistaken on the author---that said that poorer mothers, even "good ones," will interact with a kid differently than more educated mothers (though not necessarily more affluent). For ex., a poorer mother who even reads to her kid will read the word "dog" and the kid will repeat "dog." And that is the end of it, where as more educated mothers will say, "And what sound does the dog make? And what sound does the cat make?" and start a whole learning conversation.