Most of the environmental impact on IQ comes from nutrient deficiencies and diseases (especially in childhood). Most of the variation that isn't explained by b

Most of the environmental impact on IQ comes from nutrient deficiencies and diseases (especially in childhood). Most of the variation that isn't explained by biological heridity is simply unexplained at the moment. It is wrong to attribute all of it to the environment in the normal sense of that word. My guess is just noise during the construction of the brain. (And of course races are not clearly demarcated. Duh! There are still populations, though, and there tend to be differences, big and small, between them.)