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Belief polarization occurs when two people with opposing prior beliefs both strengthen their beliefs after observing the same data. Many authors have cited belief polarization as evidence of irrational behavior. We show, how-ever, that some instances of polarization are consistent with a normative account of belief revision. Our analysis uses Bayesian networks to char-acterize different kinds of relationships between hypotheses and data, and distinguishes between cases in which normative reasoners with opposing be-liefs should both strengthen their beliefs, cases in which both should weaken their beliefs, and cases in which one should strengthen and the other should weaken his or her belief. We apply our analysis to several previous studies of belief polarization, and present a new experiment that suggests that people tend to update their beliefs in the directions predicted by our normative account. How much should we rely on prior beliefs when evaluating new evidence? Prior beliefs allow us to make sense of ambiguous evidence, but if weighted too heavily, they can cause us to ignore unexpected outcomes. In this paper, we address the question of whether people rationally combine their prior beliefs with evidence. Researchers from several disciplines have explored this issu