Twin studies have provided some of the strongest evidence im-plicating environmental factors in the etiology of the AD-DBDs. These studies indicate that a moderate-to-significant propor-tion of the susceptibility to AD-DBDs is accounted for by non-shared factors. Nonshared factors have their greatest effect in CD (Slutske et al., 1997); and are less important contributors to ADHD (Sherman et al., 1997). Examples of nonshared factors include low verbal intelligence, poor school performance, dif-ficult temperament inflated self-esteem and biological events, such as perinatal insults and head trauma. Among the most sali-ent risk factors for CD is the presence of early ODD and ADHD (Hinshaw, 1987).
Shared family, peer and neighborhood risk factors also play a role in the etiology of ODD and CD, and to a lesser degree of ADHD. These so-called adversity factors include large sib-ships, families that have experienced separation, single-parent households, child neglect, parental conflict and poverty. Paren-tal child-rearing practices, such as harsh physical discipline and poor supervision, have also been implicated in ODD and CD. However, the most salient familial risk factor for CD is paren-tal criminality which likely has both environmental and genetic components. Delinquent peer membership and repeated victim-ization by peers also add to the etiology of CD and aggression. Finally, residing in a neighborhood with high rates of crime, pov-erty and/or unemployment is associated with an earlier onset of CD. These factors seem to operate in an additive fashion, with the probability of CD increasing linearly with the aggregation of risk factors (Rutter et al., 1975).