Much attention has been paid to the increase in delinquent behaviour from pre-adolescence to the middle of adolescence, followed by its decrease from late adolescence to early adulthood (Elliott, 1994; Farrington, 1986). Using official statistics, the director of the Brussels Observatory in the early nineteenth century, Quetelet (1833), described this phenomenon and concluded that ‘This fatal propensity seems to develop in proportion to the intensity of physical strength and passions in man’. More recently, Ellis and Coontz (1990) concluded that this crime bell curve could be explained by the increase of testosterone levels during puberty.

Surprisingly little attention has been paid to the link between the ‘strength of passions’ during early childhood and later delinquency. There is evidence that the most disruptive toddlers are at highest risk of becoming the most deviant adolescents and adults (Caspi, Moffitt, Newman and Silva, 1996; Stattin and Klackenberg-Larsson, 1993; White, Moffitt, Earls, Robins and Silva, 1990). Furthermore, there is evidence that the peak frequency of physical aggression during an individual's life is generally attained at around 24 months after birth (see Figure 6.1, and Tremblay et al., 1996; Tremblay, Mâsse, Pagani and Vitaro, 1996) and not during mid- or late adolescence. Observational studies of the frequency of physical aggression between toddlers count the number of physical aggressions within a fifteen or thirty minute period (e.g., Hay and Ross, 1982), while studies during adolescence generally count the frequency of aggressions over a twelve-month period.