The main aim of this study was to combine two research approaches to hyperactivity: the behaviour genetic approach and the testing of psychological theories of hyperactivity. For a sample of 268 twin pairs aged 7-11 years we obtained ratings on the Conners' scales from both teachers (CTRS-28) and parents (CPRS-48). Forty-six hyperactive twin pairs (pairs in which at least one twin was pervasively hyperactive) and 47 control twin pairs were assessed on a psychological test battery. Confirming findings from previous twin studies, a substantial proportion of the variance in hyperactivity considered as a dimension was due to genetic effects. There was significant evidence of genetic effects also on extreme hyperactivity, although the present group heritability estimates were somewhat lower than those reported in most previous studies. We investigated the possibility that the psychological mechanisms we reported to be associated with hyperactivity (Kuntsi, Oosterlaan, & Stevenson, 2001) share common genetic factors with hyperactive behaviour. The data produced significant evidence of such shared genetic effects only on hyperactivity and the variability of reaction times. Given that the high variability in speed of responding would indicate a state-regulation problem, this is the psychological mechanism that could possibly be the "link" between genetic effects and hyperactive behaviour.