We agree that the intergenerational transmission of psychopathy could be driven by genetic factors as well as by environmental factors. The degree of influence that environmental factors have on intergenerational transmission of personality features and the possible confounding by genetic features are of central importance in understanding the aetiology of psychopathology.

The Cambridge Study in Delinquent Development (CSDD) has not collected any genetic material. However, it has collected detailed information on environmental measures and can provide some insight into this issue. Current work involves comparing the intergenerational transmission of psychopathy for offspring with resident fathers (up to age 16) with that for offspring with non-resident fathers. We would expect that, to the extent that the intergenerational relationship is driven by genetics, it would be just as strong for those with non-resident fathers as for those with resident fathers. To the extent that environment matters, the relationship should be stronger for those with resident fathers.

We have found that, for male offspring with a resident father, transmission of Hare Psychopathy Checklist: Screening Version (PCL:SV) Factor 1 and Factor 2 scores is strong and statistically significant (P<0.001). However, for males with a non-resident father, the transmission is weaker. It was significant only for the more behavioural Factor 2 scores (P = 0.021) and not for the Factor 1 scores, which measure psychopathic personality features.

These results suggest that environmental factors might be important in the transmission of psychopathic personality features to male offspring. For female offspring with a resident father, the transmission of Factor 1 and Factor 2 scores was not significant. For female offspring with a non-resident father, the transmission of Factor 1 scores was not significant but, surprisingly, the transmission of Factor 2 scores was significant (P = 0.003). Our results suggest that, for both male and female offspring, genetic factors may be important in the transmission of the more behavioural Factor 2 scores. However, it may be that environmental factors are more important for male offspring.

Our results agree with previous analyses of the CSDD dataset examining the intergenerational transmission of convictions. Farrington et al ^{Reference Farrington, Barnes and Lambert1} found that the relationship between the convictions of same-gender intergenerational pairs was stronger than for opposite-gender pairs; father–son was stronger than father–daughter correlation, and mother–daughter was stronger than mother–son correlation.