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Abstract Background Studies of aggression often confound physical aggression with verbal aggression, indirect aggression, relational aggression, opposition, competition and other externalizing behaviours. Developmental models of aggression have suggested that onset of physical aggression occurs after the onset of hyperactive, stubborn and oppositional behaviour. Yet little is known about the onset and development of physical aggression before school entry. Method The age of onset and frequency of physical aggression was studied in a population sample of 17‐month‐old children. Results The 511 maternal reports on 11 physically aggressive behaviours indicated that the rate of cumulative onset of physically aggressive behaviours increased substantially from 12 to 17 months after birth. By 17 months of age, onset of physical aggression was reported for close to 80% of the children. However, the sex of the child and the presence of a sibling appeared to influence age of onset. Conclusions These results, and those of other studies of preschool children, suggest that the notion of increased probability of ‘onset’ of physical aggression as children approach adolescence needs to be reconsidered. Most children have had their ‘onset’ of physical aggression by the end of their second year after birth, and most have learned to inhibit physical aggression by school entry. Thus, there may be two main developmental trajectories of physical aggression: childhood limited, and life‐course persistent. This may be an indication that there is a sensitive period for learning to inhibit physically aggressive behaviour. To understand and prevent cases of chronic physical aggression, which appear most at risk of violent criminal behaviour, we need to study the factors which help children learn to inhibit physical aggression during the preschool years. Copyright © 1999 Whurr Publishers Ltd.