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Is IQ hereditary?

Is IQ hereditary?
Is IQ hereditary? Chimp studies add support to the 50% heritability conclusion in humans

Is IQ hereditary? Although many people have asked the question, few people have wanted to hear the real answer to this question.

In 1987, Shockley, a Nobel Prize winner in Physics repeated the claim from the controversial book “The Bell Curve” that IQ was in fact genetic. So is IQ hereditary really? Let’s turn to the academic literature to find out.

One interesting place to start is studies of identical vs fraternal twins. Identical twins have almost identical DNA and cannot be told apart in this way. Fraternal twins, on the other hand, although born at the same time, are just like any other pair of siblings in that they share 50% of genes.

Is IQ hereditary then? In a study as early as the 1970s, Plonmin and Petrill found that 50% of IQ is likely to be heritable. This is so because the IQ score for identical twins was very highly correlated compared to that of fraternal twins, despite either type of twin (across populations) being born at the same type and the assumption that both types of twins would like be reared by the same care-taker and benefit from equal environmental settings.

Adoption studies are also interesting in this regard. Adoption studies have shown that the correlation of coefficient between the IQ of identical twins was 0.86 (which is very high), which drops to 0.76 (still very high correlation) if the siblings are reared apart.

This compares to an IQ score correlation of 0.47 for siblings reared together (medium strength correlation), dropping to 0.22 for siblings reared apart (weak correlation). The correlation between the IQ of adoptive parents and their children is 0.19, which is even lower than siblings reared in different households.

Is IQ hereditary? It would appear so. In a 1997 study, Petrill revisited the earlier studies with refined statistical procedures. This study found that the heritability of IQ was 50%.

In a 1998 study, Bouchard found that the correlation of IQs of unrelated children reared together was 0.28, which dropped to a near nil value in adulthood.

Is IQ hereditary in animals?

A new study by William Hopkins of Georgia State University has looked at whether IQ is genetic through chimp studies. The results closely match those found is humans. That is, more intelligent chimps are found to have more intelligent offspring, and a statistical analysis of the results suggests that the estimate for heritability is 50%.

Coming back to the original question: Is IQ hereditary? I would appear so, for at least 50%. And importantly, the heritability component prevails  in adulthood.

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