Oxytocin and shared intentionality are positively associated with cooperation in children.

While humans cooperate with unrelated individuals to an extent that far outstrips any other species, we also display extreme variation in decisions about whether to cooperate or not. A diversity of cognitive, affective, social, and physiological mechanisms interact to shape these decisions. For example, group membership, shared intentionality talk (i.e. talk about shared goals), and natural initial oxytocin levels are linked to cooperation in adults in an optimal foraging paradigm that is loosely modelled on the iterated prisoner's dilemma. In this 'egg hunt', shared intentionality talk was key to achieving cooperation, and it drove cooperation more between participants who shared the same group membership (and who also had higher initial oxytocin levels). Such complex interactions raise the question of the age at which humans develop the necessary mechanisms to cooperate effectively in the egg hunt game. Here, we tested children in secondary school aged between 10 and 14 years. We found that, as for adults, shared intentionality talk was crucial for successful cooperation. Furthermore, initial oxytocin levels correlated with cooperation through shared intentionality talk. In contrast, group membership did not affect behaviour. Finally, pre- and post-experiment oxytocin levels showed various interactions with group membership and sex. Thus, children's performance was relatively similar to adults while showing some differences with respect to underlying mechanisms. Our study is a rare contribution to further our understanding of the role of oxytocin in early adolescent social behaviour.