Heritability of IQ

Research on heritability of IQ infers from the similarity of IQ in closely related persons the proportion of variance of IQ among individuals in a study population that is associated with genetic variation within that population. This provides a maximum estimate of genetic versus environmental influence for phenotypic variation in IQ in that population. ""Heritability"", in this sense, ""refers to the genetic contribution to variance within a population and in a specific environment"". There has been significant controversy in the academic community about the heritability of IQ since research on the issue began in the late nineteenth century. Intelligence in the normal range is a polygenic trait. However, certain single gene genetic disorders can severely affect intelligence, with phenylketonuria as an example.Estimates in the academic research of the heritability of IQ have varied from below 0.5 to a high of 0.8 (where 1.0 indicates that monozygotic twins have no variance in IQ and 0 indicates that their IQs are completely uncorrelated). Some studies have found that heritability is lower in families of low socioeconomic status. IQ heritability increases during early childhood, but it is unclear whether it stabilizes thereafter. A 1996 statement by the American Psychological Association gave about 0.45 for children and about .75 during and after adolescence. A 2004 meta-analysis of reports in Current Directions in Psychological Science gave an overall estimate of around 0.85 for 18-year-olds and older. The general figure for heritability of IQ is about 0.5 across multiple studies in varying populations. Recent studies suggest that family environment (i.e., upbringing) has negligible long-lasting effects upon adult IQ.