We propose that a "social exchange heuristic" is as important as the cheater detection mechanism for attaining mutual cooperation in social exchange. The social exchange heuristic prompts people to perceive a mixed-motive situation, such as the Prisoner's Dilemma (PD), as an Assurance Game (AG) situation in which cooperation is a personally better choice than defection insofar as the partner is cooperating as well. We demonstrate the operation of the social exchange heuristic through a comparison of the ordinary one-shot, simultaneous PD with the one-shot, sequential PD. Participants in the current experiments, involving a total of 261 volunteers, committed a logical error in the direction of favoring mutual cooperation as the situation involved more serious consequences. This result strongly suggests the operation of a domain specific "bias" that encourages pursuit of mutual cooperation in social exchange.