It is true that in AG the players share a first preference for a)bp. But, as indeed Sen says, they do both need assurance, and the prospect of the enforcement of a contract might be how the assurance was secured. This points up an important difference in the possible role of sanctions, a difference relevant to political theory. The addition of sanctions may change the utilities for a basically egoistic agent, or they may provide assurance for an agent who is basically cooperative. The latter, a very important possibility, is connected to a point I shall consider later: that the disposition to cooperation is typically cost sensitive.

XREF: This contrasts two distinct roles of sanctions in game theory and political theory — deterrence for egoists versus assurance for cooperators — which connects to Hobbesian vs. Rousseauian accounts of the social contract. DEFINE: Distinguishes between the utility-changing role of sanctions for egoistic agents and the assurance-providing role for cooperative agents, a key conceptual distinction in the author's framework.

Diego Gambetta, Trust - Making and Breaking…, loc. 104