OBJECTIVETo examine three possible causes of the relationship between attention deficit and hyperactivity disorder and conduct disorder: additive genetic factors(A), common environmental factors(C) and individual-specific environmental factors(E).

METHODSOne hundred and forty pairs of twins from the Southwestern China Twin Registry were examined with the parent-rated Strength and Difficulties Questionnaire (SDQ). The cross-twin within-variable, within-twin cross-variable and cross-twin cross-variable correlations were calculated. Using structural equation modelling, bivariate models were fitted. The best fitting model was chosen based on likelihood and parsimony.

RESULTSThe observed phenotypic correlation between HYPER and COND was 0.44 (95% CI: 0.09, 0.27), with genetic factors accounting for about 70% of the observed correlation. Bivariate model fitting quantified the genetic correlation between HYPER and COND at 0.76 (95% CI: 0.31, 1) and the individual-specific environmental correlation at 0.28 (95% CI: 0.02, 0.51).

CONCLUSIONIn children, three different genetic factors may exist: one that solely affects the liability to hyperactivity behaviour, one that has only an effect on conduct behaviour and one that influences both hyperactivity and conduct behaviour. Our results suggests that most of the environmental factors that increase the risk of hyperactivity behaviour do not influence conduct behaviour and vice versa.