  It has been hypothesized that the evolution of modern human cognition was
catalyzed by the development of jointly intentional modes of behaviour. From an
early age (1-2 years), human infants outperform apes at tasks that involve
collaborative activity. Specifically, human infants excel at joint action
motivated by reasoning of the form "we will do X" (shared intentions), as
opposed to reasoning of the form "I will do X [because he is doing X]"
(individual intentions). The mechanism behind the evolution of shared
intentionality is unknown. Here we formally model the evolution of jointly
intentional action and show under what conditions it is likely to have emerged
in humans. Modelling the interaction of hunter-gatherers as a coordination
game, we find that when the benefits from adopting new technologies or norms
are low but positive, the sharing of intentions does not evolve, despite being
a mutualistic behaviour that directly benefits all participants. When the
benefits from adopting new technologies or norms are high, such as may be the
case during a period of rapid environmental change, shared intentionality
evolves and rapidly becomes dominant in the population. Our results shed new
light on the evolution of collaborative behaviours.
