Cooperation is one of the most advantageous strategies to have evolved in small- and large-scale human societies, often considered essential to their success or survival. We investigated how cooperation and the mechanisms influencing it change across the lifespan, by assessing cooperative choices from adolescence to old age (12–79 years, N = 382) forcing participants to decide either intuitively or deliberatively through the use of randomised time constraints. As determinants of these choices, we considered participants’ level of altruism, their reciprocity expectations, their optimism, their desire to be socially accepted, and their attitude toward risk. We found that intuitive decision-making favours cooperation, but only from age 20 when a shift occurs: whereas in young adults, intuition favours cooperation, in adolescents it is reflection that favours cooperation. Participants’ decisions were shown to be rooted in their expectations about other people’s cooperative behaviour and influenced by individuals’ level of optimism about their own future, revealing that the journey to the cooperative humans we become is shaped by reciprocity expectations and individual predispositions.

Cooperation is one of the most advantageous strategies to have evolved in small and large-scale human societies, often considered essential to their success or survival. We investigated how cooperation and the mechanisms influencing it change across the lifespan. Using incentivised economic games, we assessed cooperative choices from adolescence to older age (12-79 years) forcing participants to decide either intuitively or deliberatively through the use of randomised time constraints. As determinants of these choices, we considered participants’ level of altruism, their social expectations, their optimism, their desire to be socially accepted, and their attitude toward risk. We found that intuitive decision-making favours cooperation, but only from age 20 when a shift occurs: while in young adults, intuition favours cooperation, in adolescents it is reflection that favours cooperation. Adults and older adults were found to be most cooperative, with intuition still favouring cooperation in older adults, but not in working-age adults where well-established social cooperative heuristics govern both intuitive and deliberative decision making. Participants’ decisions were shown to be rooted in their expectations about other people’s cooperative behaviour and influenced by individuals’ level of optimism about their own future. We show that intuition promotes cooperation only after adolescence, and that the journey to the cooperative humans we become is shaped by reciprocity expectations and individual predispositions.