Commitment:

Randolph M. Nesse:

"In a world without commitments social exchanges arise mainly from helping relatives and trading favors. In such a world individuals can reliably be expected to act straightforwardly in their own interests. The advent of commitment changes everything. As soon as one individual finds a way to convince another that he or she will act other than in simple self-interest, social life is transformed. Now, individuals must consider the possibility that others may fulfill promises and threats."