Background. Previous studies have shown that the presence of conduct disorder may contribute to the persistence of attention deficit-hyperactivity disorder (ADHD) symptomatology into adolescence; however, the aetiological relationship between the two phenotypes remains undetermined. Furthermore, studies utilizing multiple informants have indicated that teacher ratings of these phenotypes are more valid than maternal reports.

Methods. The genetic structure underlying the persistence of ADHD and oppositional-defiant disorder/conduct disorder (ODD/CD) symptomatologies as rated by mothers and teachers at two occasions of measurement was investigated on a sample of 494 male and 603 female same sex adolescent twin pairs participating in the Virginia Twin Study of Adolescent Behavioral Development (VTSABD).

Results. Using structural modelling techniques, one common genetic factor was shown to govern the covariation between the phenotypes across informants and occasion of measurement with additional genetic factors specific to ODD/CD symptomatology and persistence of symptomatology at reassessment. Genetic structures underlying the phenotypes were, to some extent, informant dependent.

Conclusions. The findings indicate that it is unlikely that the co-morbidity between ADHD and ODD/CD is due to environmental influences that are independent of ADHD. Rather it is likely to be due to a shared genetic liability either operating directly, or indirectly through gene–environment correlations or interactions. The covariation between phenotypes across informants and time is governed by a common set of genes, but it seems that ODD/CD is also influenced by additional genetic factors. Developmentally, different forms of genetic liability control ADHD in males and inattention in females.