 Decision theory should address all possible decision problems and not just realistic decision problems. However, if Newcomb’s problem seems untroubling because unrealistic, realistic versions of the problem are plentiful. The essential feature of Newcomb’s problem is an inferior act’s correlation with a good state that it does not causally promote. In realistic, medical Newcomb problems, a medical condition and a behavioral symptom have a common cause and are correlated although neither causes the other. If the behavior is attractive, dominance recommends it although expected utility maximization prohibits it. Also, Allan Gibbard and William Harper (1978: Sec. 12) and David Lewis (1979) observe that a Prisoner’s Dilemma with psychological twins poses a Newcomb problem for each player. For each player, the other player’s act is a state affecting the outcome. Acting cooperatively is a sign, but not a cause, of the other player’s acting cooperatively. Dominance recommends acting uncooperatively, whereas expected utility computed with conditional probabilities recommends acting cooperatively. In some realistic instances of the Prisoner’s Dilemma, the players’ anticipated similarity of thought creates a conflict between the principle of dominance and the principle of expected-utility maximization.