GENETIC AND ENVIRONMENTAL-INFLUENCES ON THE COVARIATION BETWEEN HYPERACTIVITY AND CONDUCT DISTURBANCE IN JUVENILE TWINS

Structural equation models were applied to the maternal ratings of 265 MZ and 163 DZ male-male, 347 MZ and 160 DZ female-female, and 262 mal e-female twin pairs, aged 8-16 years, who participated in the Virginia Twin Study of Adolescent Behavioral Development (VTSABD). Substantial additive genetic influences and contrast effects were found for hyper activity, and additive genetic and shared environmental effects or pos itive comparison effects (particularly for the girls) for oppositional /conduct disturbance. Bivariate model fitting showed that the covariat ion between hyperactivity and oppositional/conduct problems in both yo unger and older boys and girls is almost entirely attributable to gene tic factors. However, whereas in the younger males and females the sam e set of genes explain all the variation in hyperactivity and conduct disturbance, in the older cohort at least some of the genetic effects are behavior- and gender-specific.