Psychological theories of hyperactivity : a behaviour genetic approach
This study was an attempt to combine two research literatures on hyperactivity: the behaviour genetic research and the studies testing psychological theories of hyperactivity. We obtained behavioural ratings from the teachers of 1316 twin pairs, aged 7-12, from the general population. For a subsample of 268 twin pairs we obtained ratings also from their parents. Forty-six hyperactive twin pairs (pairs in which at least one twin was pervasively hyperactive) and 47 control twin pairs were then assessed on tests relating to three theories of hyperactivity, those of response inhibition deficit, working memory impairment and delay aversion. Confirming previous findings, genetic factors accounted for 50-70% of the variance in hyperactivity when considered as a continuous dimension. There was also significant evidence of genetic effects on extreme hyperactivity, although the present group heritability estimates were somewhat lower than previous estimates. The hyperactive group performed worse than the control group on the delay aversion measure and some of the working memory tasks. Controlling for IQ removed the significant group differences on the working memory measures, however. Although there were no significant group differences on the inhibition variables, the inhibition measure, stop task, produced evidence of a pattern of responding that was strongly characteristic of hyperactivity: hyperactive children were variable in their speed, generally slow and inaccurate. This pattern of responding may indicate a non-optimal effort/activation state. To investigate the possibility that the cognitive impairments or task engagement factors associated with hyperactivity mediate the genetic effects on the condition, bivariate group heritability analyses were carried out. There was significant evidence of shared genetic effects only on extreme hyperactivity and the variability of speed. The findings are interpreted as supporting the state regulation theory of hyperactivity. Although delay aversion is a characteristic of hyperactivity, it seems to have an environmental rather than a genetic origin.