Intelligence and Heredity
Adult height is a trait that is known to have large environmental and hereditary determinants. Taller parents usually have taller children but changes in nutrition, sanitation, health also have large effects. While it has proved possible to identify a number of important discrete environmental effects that make large differences, the hereditary component has proven much harder to pin down. With the exception of a few rare genes that make big differences, hereditary effects seem to be spread over a large percentage of the whole genome. In the largest scale studies, thousands of gene differences each seem to have small but measurable effects.
Intelligence, as measured by IQ tests, may well be similar. Nutritional deficiencies, in particular, are big culprits in low IQ. Introduction of iodine in regions of low iodine soils have been known to raise average IQs by as much as 16 points - more than one standard deviation. Exposure to lead and pesticides can produce large IQ deficits. Large scale Genome Wide Association Studies (GWAS) of IQ are far less developed than similar studies for height, but early indications are similar to the case for height: many genes each with small effect.