 To make expected utility track efficacy rather than auspiciousness, causal decision theory interprets the probability of a state if one performs an act as a type of causal probability rather than as a standard conditional probability. In the Prisoner’s Dilemma with twins, consider the probability of one player’s acting cooperatively given that the other player does. This conditional probability is high. Next, consider the causal probability of one player’s acting cooperatively if the other player does. Because the players are isolated, this probability equals the probability of the first player’s acting cooperatively. It is low if that player follows dominance. Using conditional probabilities, the expected utility of acting cooperatively exceeds the expected utility of acting uncooperatively. However, using causal probabilities, the expected utility of acting uncooperatively exceeds the expected utility of acting cooperatively. Switching from conditional to causal probabilities makes expected-utility maximization yield acting uncooperatively.