once people pass a certain intelligence level

This seems crucial to me; you're really talking about a few percent of the population, right?

Also, I'll note that when (even very smart) people are motivated to believe in the existence of a phenomenon they're apt to attribute causal structure in.correlated data.

For example: It's common wisdom among math teachers that precalculus is important preparation for calculus. Surely taking precalculus has some positive impact on calculus performance but I would guess that this impact is swamped by preexisting variance in mathematical ability/preparation.