Association between extreme autistic traits and intellectual disability: insights from a general population twin study.

BACKGROUND Autism is associated with intellectual disability. The strength and origin of this association is unclear. AIMS To investigate the association between extreme autistic traits and intellectual disability in children from a community-based sample and to examine whether the association can be explained by genetic factors. METHOD Children scoring in the extreme 5% on measures of autistic traits, IQ and academic achievement were selected from 7965 7/8-year-old and 3687 9-year-old twin pairs. Phenotypic associations between extreme autistic traits and intellectual disability were compared with associations among the full-range scores. Genetic correlations were estimated using bivariate DeFries-Fulker extremes analyses. RESULTS Extreme autistic traits were modestly related to intellectual disability; this association was driven by communication problems characteristic of autism. Although this association was largely explained by genetic factors, the genetic correlation between autistic traits and intellectual disability was only modest. CONCLUSIONS Extreme autistic traits are substantially genetically independent of intellectual disability.