Unfortunately, although our cohort was longitudinal and behavioral data was longitudinally collected from childhood, DNA from blood cells were collected only once the participants in the study reached adulthood. Future longitudinal studies should address these questions. Second, the results are based on a small sample of chronic aggressive subjects. The two longitudinal studies we used to recruit our subjects from had followed more than 1 000 males from childhood to adolescence. Young adult Caucasian males with a history of chronic physical aggression during childhood are relatively rare [11] and even more difficult to recruit for biological sampling. Thus, the limited numbers are an inherent property of the trait that we are examining. However replications of the present study with other longitudinal samples are obviously needed and should address this issue in part although we don’t expect a large number of CPA subjects in these future studies as well. In any case, the subgroup that was available for the DNA methylation analysis did not differ in its basic behavioral and socioeconomic characteristics from the original group (Table 3). Third, although we
