The Heritability of g
Individual differences in mental test scores have a substantial genetic component indexed by the coefficient of heritability (in the broad sense), that is, the proportion of the population variance in test scores attributable to all sources of genetic variability. The broad heritability of IQ is about .40 to .50 when measured in children, about .60 to .70 in adolescents and young adults, and approaches .80 in later maturity.
Environmental variance can be partitioned into two sources: (1) environmental influences that are shared among children reared in the same family but that differ between families, and (2) nonshared environmental influences that are specific to each child in the same family and therefore differ within families. The shared environmental variance diminishes from about 35 percent of the total IQ variance in early childhood to near zero percent in late adolescence. The nonshared environmental variance remains nearly constant at around 20 to 30 percent from childhood to maturity. That is, virtually all of the nongenetic variance in adult IQs is attributable to within- family causes, while virtually none is attributable to the kinds of environmental variables that differ between families. The specific sources of much of the within-family environmental variance are still not entirely identified, but a large part of the specific environmental variance appears to be due to the additive effects of a large number of more or less random and largely physical events--developmental "noise"--with small, but variable positive and negative influences on the neurophysiological substrate of mental growth.
More of the genetic variance in test scores is associated with g than with any other common factor. Hence the relative g loadings