Background. Rates of child and adolescent conduct disorder (CD) have increased steadily over the past several decades. What is not known is whether the underlying genetic and environmental influences on individual differences in CD have also changed.

Methods. Retrospective reports of antisocial behaviour prior to age 18 were obtained from a population-based sample of 2769 adult males from male–male twin pairs born between 1940 and 1974. Using a summary score of number of CD symptoms, structural equation modelling was used to investigate whether mean level and variation in CD increased with more recent cohorts, and whether any increase in variance could be explained by familial or non-familial factors.

Results. Both mean level CD symptoms and variation were increased in more recent cohorts. Model fitting indicated that the primary increase in variance was due to familial factors, most notably, an increase in the shared environmental influences on CD, from 0·01 (95% CI = 0·00; 0·27) to 0·30 (95% CI = 0·01; 0·44). Heritability estimates remained largely unchanged, although an increase in genetic factors could not be ruled out.

Conclusions. Secular changes in sociodemographic factors responsible for increasing rates of CD may also account for the greater magnitude of shared environmental influences on variation in CD found among more recent cohorts.