title: 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.
  
  
  OBJECTIVE
  To examine whether ADHD and conduct problems share common risk factors and whether ADHD+CD is a more heritable variant of ADHD.
  
  
  METHODS
  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.
authors.affiliation: Child and Adolescent Psychiatry Section, Department of Psychological Medicine, University of Wales College of Medicine, Cardiff, UK.
authors.forename: A
authors.initials: A
authors.lastname: Thapar
authors.forename: R
authors.initials: R
authors.lastname: Harrington
authors.forename: P
authors.initials: P
authors.lastname: McGuffin
mesh_terms: D000293:Adolescent; D000704:Analysis of Variance; D001289:Attention Deficit Disorder with Hyperactivity / Q000453:epidemiology / Q000235:genetics*; D002648:Child; D002675:Child, Preschool; D015897:Comorbidity; D019955:Conduct Disorder / Q000453:epidemiology / Q000235:genetics*; D004200:Diseases in Twins / Q000235:genetics*; D004739:England / Q000453:epidemiology; D004777:Environment; D005260:Female; D006801:Humans; D008297:Male; D008957:Models, Genetic; D012307:Risk Factors; D011795:Surveys and Questionnaires