Adolescence is a period of life often characterised by behaviours that, prima facie, are irrational, such as seemingly excessive risk-taking and impulsivity. However, these behaviours can be interpreted as adaptive and rational if one considers that a key developmental goal of this period of life is to mature into an independent adult in the context of a social world that is unstable and changing. It is proposed that, for adolescents, the ‘social risk’ of being rejected by peers outweighs other potential negative outcomes of decisions, such as threats to one’s health or the prospect of getting caught. Furthermore, peer influence in adolescence can lead to prosocial as well as antisocial behaviours. Neurocognitive mechanisms of peer influence include social reward of being accepted by one’s peer group, arousal and increased mentalising, which is associated with development of the social brain network. The findings from cognitive neuroscience and developmental psychology studies have implications for education and public health and fit with evidence that the opinions of peers are particularly important to adolescents in areas such as school anti-bullying campaigns.