Genetic and environmental mechanisms determining intelligence, neuroticism, extraversion and psychoticism: an analysis of Irish siblings.

The theory that shared family environment has an effect on intelligence but no effect on personality was examined by a study of correlations between young adolescent Irish siblings for intelligence, neuroticism, extraversion and psychoticism. The correlations obtained for 386 sibling pairs were 0.48 (intelligence), 0.06 (neuroticism), 0.31 (extraversion) and 0.14 (psychoticism). The correlation for IQ confirms the operation of both genetic and shared family effects on intelligence. The low correlations for neuroticism and psychoticism confirm the thesis that shared family effects have no influence on these traits. They are also inconsistent with an additive genetic model and suggest that non-additive genetic mechanisms are present to make siblings so dissimilar. The higher sibling correlation for extraversion suggests that shared family environment does have some effect on this trait among young adolescents and is also consistent with an additive genetic model.