Of all the developmental parameters of antisocial behaviour, perhaps the most extensively studied has been the age at which it begins (LeBlanc and Loeber, 1998). An exhaustive review by Farrington and eight expert colleagues (Farrington, Loeber, Elliott, et al., 1990) precludes the need for us to review this large literature here. It can be summarised by the welldocumented observation that early onset of antisocial behaviour problems is the single best predictor of serious adult criminal outcomes. Despite the importance of age of onset for theory and prevention planning, we found no published systematic comparisons of sex differences in this important variable. Unpublished sex comparisons of onset are cited by Elliott (1994) who mentioned that fewer girls than boys initiated self-reported serious offending at every age, and by Weiner and Wolfgang (1989) who mentioned that more girls than boys were first registered by police before age 14. Because these reports are unpublished, and seem to conflict, this chapter compares the sexes on age of onset.

Sex comparisons of onset age are interesting because it is possible to make opposing predictions about what the results might be. On the one hand, if, as we have seen, girls compared to boys have less extreme antisocial behaviour, or less aetiological press toward antisocial behaviour, we might expect girls would show onset at older ages on average.