Inheritance of intelligence

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". In other words, heritability is a mathematical estimate that indicates how much of a trait’s variation can be attributed to genes. 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, meaning it's influenced by more than one gene.

The general figure for the heritability of IQ, according to an authoritative American Psychological Association report, is 0.45 for children, and rises to around 0.75 for late teens and adults. In simpler terms, IQ goes from being weakly correlated with genetics, for children, to being strongly correlated with genetics for late teens and adults. The heritability of IQ increases with age and reaches an asymptote at 18–20 years of age and continues at that level well into adulthood. Recent studies suggest that family and parenting characteristics are not significant contributors to variation in IQ scores; however, poor prenatal environment, malnutrition and disease can have deleterious effects.

"Heritability" is defined as the proportion of variance in a trait which is attributable to genetic variation within a defined population in a specific environment. Heritability takes a value ranging from 0 to 1; a heritability of 1 indicates that all variation in the trait in question is genetic in origin and a heritability of 0 indicates that none of the variation is genetic. The determination of many traits can be considered primarily genetic under similar environmental backgrounds. For example, a 2006 study found that adult height has a heritability estimated at 0.80 when looking only at the height variation within families where the environment should be very similar. Other traits have lower heritabilities, which indicate a relatively larger environmental influence. For example, a twin study on the heritability of depression in men calculated it as 0.29, while it was 0.42 for women in the same study. Contrary to popular belief, two parents of higher IQ will not necessarily produce offspring of equal or higher intelligence. In fact, according to the concept of regression toward the mean, parents whose IQ is at either extreme are more likely to produce offspring with IQ closer to the mean (or average).

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