Whether there IS a bias on nutrition, lead, or anything in the environment, it is also known that intelligence is highly inheritable. Adopted black kids that grow up in white families have lower IQs than adopted white kids who grow up in black families.

There's some bias there, though, like black kids are usually adopted later in life.

I find it interesting how everyone loves theories about selection among the Jews (having to learn Hebrew for religious purposes and studying religious texts) has led them to have higher intelligence. But when it comes to race it's such a politicized discussion, that everyone wants to prove there is no difference.