At what ages do boys and girls begin to show average differences in aggressive behaviour?

In the study of aggressive behaviour, research indicates that sex-linked differences in aggression typically become evident starting around two to three years of age, with boys generally exhibiting higher levels of aggression than girls.1

While some studies suggest that differences in antisocial behaviours—such as fighting, stealing, or vandalism—may not appear until after the age of three, other observations note that boys are more likely to be irritable and aggressive beginning at the toddler stage. By age seven, a small proportion of boys may display an extreme and consistent tendency toward aggression, a pattern that, if left unaddressed, can persist into adulthood.21

It is important to note that while biological factors and early hormonal exposure play a role in these developmental differences, the social and cultural environment is also a critical influence on how and whether aggressive responses are expressed. Psychologists emphasize that consistent, nonpunitive parenting is an effective approach to helping children manage and reduce aggressive tendencies.312