Nearly a century of observational studies and more recent longitudinal surveys reveal that, in infancy, girls and boys use force at similar rates. Over the next few years boys become significantly more aggressive. Alternative hypotheses accounting for the widening gender gap are evaluated. These include hypotheses about normative patterns of male escalation and female desistance; boys' preference for active play that promotes aggression; girls' tendency to hide aggression; girls' use of alternative forms of aggression; boys' increased risk for the cognitive and emotional problems that are linked to aggression; boys' sensitivity to situational triggers of aggression; and boys' vulnerability to adverse rearing environments. The evidence bearing on each hypothesis is mixed. In general, the overall difference between the sexes appears to be produced by a minority of boys who deploy aggression at high rates. Three general principles govern the emergence of sex differences in aggression: female precocity, male vulnerability, and the salience of sex as a social category that shapes children's lives.