doesn't have to be genetic at all
diet, can be an influence that would seem to follow race lines. The amount of fat an infant intakes can affect his or her brain development. Just heard recently on NPR that Omega 3's can also play a factor.
Asians eat more fish on average. Those omega 3s could be transferred across the breast milk.
Poorer communities where malnourishment affects the motther and child are certainly going to have an impact on the infants brain development as well.
Why look only at skin color when other factors could easily explain the difference in the averages.