Humans have a long history of coping with particular recurring risks. We expect natural selection to have resulted in specific physiological and psychological adaptations that respond well to these risks. Why, then, does it seem so difficult to communicate risk? We suggest that the human mind has been structured by natural selection to use a mental calculus for reckoning uncertainty and making decisions in the face of risk that can be substantially different from probability theory, propositional calculus (logic), or economic rationality (utility maximization). We argue that this is because of the unique armamentarium of strategies humans have evolved to cope with the risks faced during our long history living as hunter-gatherers. In particular, we believe the risk of social contract violation (not contributing a fair share to cooperative endeavors) was an important selective factor because reciprocity, reciprocal altruism, and cooperation are primary adaptations to the most important risks our ancestors faced.