A twin-family study of general IQ

In this paper we assess the presence of assortative mating, gene¿environment interaction and the heritability of intelligence in childhood usinga twin family design with twins, their siblings and parents from 112 families. We evaluate two competing hypotheses about the cause of assortativemating in intelligence: social homogamy and phenotypic assortment, and their implications for the heritability estimate of intelligence. The RavenProgressive Matrices test was used to assess general intelligence (IQ) and a persons IQ was estimated using a Rasch model. There was asubstantial correlation between spouses for IQ (r=.33) and resemblance in identical twins was higher than in first-degree relatives (parents andoffspring, fraternal twins and siblings). A model assuming phenotypic assortment fitted the data better than a model assuming social homogamy.The main influence on IQ variation was genetic. Controlled for scale unreliability, additive genetic effects accounted for 67% of the populationvariance. There was no evidence for cultural transmission between generations. The results suggested that an additional 9% of observed IQ testvariation was due to gene¿environment interaction, with environment being more important in children with a genetic predisposition for lowintelligence.© 2007 Elsevier Inc. All rights reserved.Keywords: Intelligence; Genetics; Assortative mating; Heritabilit