Conduct disorder is the most commonly referred psychological problem in child psychiatric and mental health services (Sholevar, 1995). It is characterised by externalizing behaviours such as aggression and antisocial behavior, and is often associated with impulsivity and hyperactivity. Research has shown that externalizing behaviour has a sizable genetic component and is thought to be associated with the dopaminergic system, in which the dopamine D4 receptor gene (DRD4) plays a role. The current study explores whether detainees or conduct disorder individuals can be differentiated from controls based on the variation in genetic polymorphisms of DRD4. Allele, genotype, and haplotype frequencies for the exon 3 VNTR and the C_1611535 (rs58688157), rs9366, rs11246226 SNPs in the DRD4 gene were compared in 179 incarcerated adolescent delinquents and 60 matched controls. Results indicated that genetic variation in DRD4 is associated with criminality (i.e., incarceration) and conduct disorder. Understanding the influence of DRD4 on externalizing behaviour problems associated with conduct disorder is important for understanding etiological mechanisms of conduct problems.