Prevalence rates vary across studies due to both definition and measurement differences. Self-reports from a Christchurch sample of young people indicated that 5.1% of 15-year-olds behaviours were consistent with a conduct disorder diagnosis (Ferguson et al., 1993). However, parent ratings of the same adolescents resulted in only 1.8% being diagnosed with conduct disorder.

In their article on the cause of disruptive behaviour disorders, Lahey and colleagues (1999) stated that relatively little can be concluded about the prevalence of such disorders due to variations in assessments, diagnostic definitions, informants, and adjustments made for developmental impairments.

Nevertheless Lahey et al. (1999) concluded that conduct disorder is more common in boys than girls, and there is a decrease in aggressive behaviours with an increase in non-aggressive conduct disorder features over time. Research also suggests that as young people get older the gender differences decline.

In the Dunedin study, long-term follow-up of a group of children born in the same year by McGee and colleagues (1992) found that the male-to-female ratio for severe conduct disorder problems declined from 2.6:1 at age 11 years to 0.7:1 at age 15 years. These results suggest that there are gender differences in the development of conduct disorder with a delayed onset pathway for girls similar to the childhood onset pattern for boys (Silverthorn and Frick, 1999).