This study investigates the roles of intelligence and school-related motivation in sex differences in school achievement and whether there are sex differences in the genetic and environmental influences on these variables. In a sample of 4464 9-year-old twins, intelligence, ablility self-perceptions, intrinsic values and achievement scores were assessed. Girls outperformed boys in English and had better corresponding ability self-perceptions, whereas in Math boys showed better attainment and ability self-perceptions. For both sexes and all three domains, intelligence was the strongest predictor of achievement and ability self-perceptions added incrementally to the prediction. Evidence of genetic influences was found for all measures but shared environmental influences were not important. These findings challenge some widely held assumptions about the development of children's motivation. Copyright © 2008 John Wiley & Sons, Ltd.