This paper attempts to identify general, cross-cultural cognitive factors that trigger the default commissive interpretation of assertions about one’s future action. It is argued that the solution cannot be found at the level of the semantics of the English will, or any other future tense marker, but should be sought in the structure of rational intentions, as combined with the pragmatics of felicitous predictions and with parameters linked to the evolutionary advantage of cooperative behaviour. Some supporting evidence from language development studies is briefly presented.