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."

Only those people who can fulfill both promises and threats are trustworthy allies.

You can tell when someone is truly committed to a course of action. He is decisive and serious. He continues on his chosen path even when it demands effort, discomfort, or sacrifice.