Two studies investigate young children's beliefs about aggression and withdrawal in others with reference to the possibility of stability and change. Study 1 (N = 41) provides evidence that preschool children (1) view aggression in more essentialist ways (i.e. they believe it to be more stable and less changeable) than withdrawal and (2) believe that friends hold a greater potential to create change in aggression and withdrawal than do other potential sources of influence, such as parents and teachers. Study 2 (N = 25) replicates the findings of Study 1 and also demonstrates that by preschool age, children hold systematic ideas about the effectiveness of strategies that friends can use to change the behaviour of their peers. These ideas include the belief that prosocial strategies, such as showing peers how to make friends, are more effective than requests to stop engaging in undesirable behaviour. Study 2 also demonstrates that preschool-aged participants engaged in essentialist reasoning to a greater extent than did a comparison group of 20 7-8 year olds. (Contains 2 tables and 3 notes.)