A total of 72 boys and 72 girls, of six age groups from 6 to 11 years, was observed in the school classroom over eight 5-minute periods, under conditions of low structure, non-directive teaching; aggressive and other categories of behaviour were recorded. There were no overall gender differences in aggression, but boys showed significantly more physical aggression than girls (overall effect size d = 0.36), whereas girls showed significantly more verbal aggression than boys (overall effect size d = 0.66). The physical aggression difference was variable both in direction and magnitude across the age categories and included a large reversal at 9 years. Boy's higher levels of physical aggression were not associated with higher levels of activity or more time spent in social interactions. The gender difference in verbal aggression was consistent in direction and magnitude across age categories, but in the opposite direction to the majority of previous findings. There were no gender differences in other categories of verbal behaviour.

An observational study was carried out to investigate whether sex differences in aggression, found in pre‐school children in free‐play situations, would also be found at older ages in the classroom. Equal numbers of boys and girls, aged 6 and 11 years, were observed individually in their school classrooms for eight 5 min periods, carried out over several weeks. At 11 years of age, boys showed more physical aggression than girls, a finding which resulted from a few boys showing higher scores than the remaining boys and the girls; girls showed more verbal aggression. These sex differences were not found at 6 years of age. At both ages, there was a higher proportion of same‐sex than opposite‐sex encounters. These results are discussed in terms of the generality of sex differences in aggression, the possibility of a sex difference in the form of aggressive acts, and the sex of the recipient of aggression.