A twin study of hyperactivity--II. The aetiological role of genes, family relationships and perinatal adversity.

In a large representative sample of 13-year-old twins, monozygotic pairs were more alike than same-sex dizygotic pairs on objective measures of attentiveness and on parent and teacher ratings of hyperactivity. Comparison of recognized and unrecognized monozygotic pairs indicated that parents and teachers rated twins more similarly when the twins were perceived as "identical" rather than "non-identical". After allowing for this stereotyping, genetic effects accounted for approximately half of the explainable variance of hyperactivity and inattentiveness. Data from mixed-sex pairs did not support a 2-threshold genetic explanation for the male excess of hyperactivity. The link between adverse family factors and hyperactivity was weak. Perinatal adversity was not related to later hyperactivity.