Intelligence is highly familial, yet the extent and nature of the genetic contribution to intelligence differences has been controversial11. Twin and adoption studies suggest that additive genetic effects contribute over half of the population variance in intelligence in adulthood3,6. However, no single genes or gene variants have been identified that are robustly associated with intelligence-related phenotypes3. Moreover, it has been suggested that the apparent high heritability for intelligence is the result of a correlation (confounding) between genetic and environmental factors and that breaking up this correlation would result in the trait being much less heritable12,13.
