The age at which individuals are most physically aggressive is 22 months. However, some children fail to inhibit this normal aggression and by the time they are three or four are showing signs of oppositional defiant disorder. In older children persistent antisocial behaviour is classified as conduct disorder. Epidemiological follow-up surveys show that the risk of poor outcomes in antisocial children is very high. The causes are multiple but two sets of factors stand out. First, genetic predisposition. Even children adopted away from violent or criminal parents have three or four times the rate of antisocial behaviour and second, poor parenting. [With external links to the evidence base]