This paper estimates the longer-term effects of childhood conduct disorder on human capital accumulation and violent and criminal behaviour later in life using data of Australian twins. We measure conduct disorder with a rich set of indicators based on diagnostic criteria from psychiatry. Using ordinary least squares (OLS) and twin fixed effects (FE) estimation approaches, we find that early (pre-18) conduct disorder problems significantly affect both human capital accumulation and violent and criminal behaviour over the life course. In addition, we find that conduct disorder is more deleterious if these behaviours occur earlier in life.