Examining the comorbidity of ADHD-related behaviours and conduct problems using a twin study design

Abstract

Background: Although attention-deficit hyperactivity disorder (ADHD) and conduct disorder (CD) frequently co-occur, the underlying mechanisms for this comorbidity are not well understood. Aims: To examine whether ADHD and conduct problems share common risk factors and whether ADHD+CD is a more heritable variant of ADHD. Method: Questionnaires were sent to 2846 families. Parent-rated data were obtained for 2082 twin pairs and analysed using bivariate genetic analysis and a liability threshold model approach. Results: The overlap of ADHD and conduct problems was explained by common genetic and non-shared environmental factors influencing both categories. Nevertheless, the two categories appeared to be partly distinct in that additional environmental factors influenced conduct problems. It appeared that ADHD+CD was a genetically more severe variant of ADHD. Conclusions: Conduct problems and ADHD share a common genetic aetiology; ADHD+CD appears to be a more severe subtype in terms of genetic loading as well as clinical severity.