This is interesting, I wonder if there's anything to it: International variation in IQ – the role of parasites (paper) by Christopher Hassall of U. Carleton.
It strikes me as the sort of thing that could be as big an issue as lead in the environment. Raise the sanity waterline: improve health!
Also note that there is at least one plausible candidate for a nurture variable among African Americans in low breast-feeding rates.
IIRC, breast-feeding rates were plummeting in Europe in the late 20th century, but there was no substantial decline in IQ as far as I know.