Sibling resemblance in mental ability: a review.

Empirical evidence on sibling resemblance in intelligence published since 1915 in the United States and Europe, including more than 27,000 sibling pairs, is reviewed. The results of these investigations are presented in tables showing the date, investigator, test, sample size, and correlations found for each specific study. Collectively, the results are highly consistent with the polygenic hypothesis and the conclusion that genetic factors are the major source of individual differences in intelligence. The most likely estimate of the sibling correlation for IQ in the population is +0.49.