Genetics and the Plasticity of Intelligence
Heritability, denoted h^, is defined as the proportion of variance of any trait that can be accounted for by variation in the degree to which genes are shared (Gustafsson & Undheim, 1996; Neisser, 1998). Although the precise value of h^ for IQ varies from study to study, central tendencies of heritability estimates are around 0.50, with as much error as 0.2 in either direction (see also Bouchard & McGue, 1981). This means that roughly 50% of the variance in measured IQ is associated with genetic variation. This also happens to be the approximate genetic contribution to psychological traits in general (Harris, 1995), including personality traits (Bouchard, 1994), narrower cognitive factors such as verbal and spatial ability (Plomin & DeFries, 1998), and psychopathology (Scarr, 1992). Heritability values are about the same magnitude in accounting for variation in academic achievement (Plomin & DeFries, 1998).