For children with long-term behavioral problems, signs of conduct disorder often arise by early school age, but few children meet the full criteria for the disorder before 10 years of age. These early signs involve aggressive tendencies, impulsivity, and failure to comply with requests, which are features of attention deficit–hyperactivity disorder (ADHD) and oppositional-defiant disorder (see the Glossary). Prospective data show a trajectory of behavioral problems, with progression from ADHD in early school years to oppositional–defiant disorder in subsequent years, followed by conduct disorder as children approach adolescence.11 Although this developmental pattern is common, it is not typical in children who have early behavioral problems — that is, conduct disorder does not develop in most children with ADHD or oppositional–defiant disorder,12,13 and successful treatment of these two conditions may reduce the risk of progression. Like ADHD and oppositional–defiant disorder, callous–unemotional traits are expressed early. Such traits have been identified in children as young as 2 years of age,14 and among young children with conduct problems, they predict a particularly early onset of a severe,15 persistent variant of conduct problems.7
