Many experimental and observational studies use the way that subjects respond to infor-
mation as evidence that partisan bias or directional motives influence political beliefs. For a natural and tractable formalization of belief formation with both accuracy and directional motives (i.e., motivated reasoning), this is not possible. Any subject influenced by directional motives has a "Fully Bayesian Equivalent" with identical beliefs upon observing any signal. As a result, comparing how individuals or groups with different partisanship or priors respond to information has no diagnostic value in detecting motivated reasoning, even in a multivariate or dynamic setting. Conversely, providing a “Bayesian rationalization” consistent with a pattern of updating is not meaningful evidence for a lack of directional motives. These results have theoretical implications for the convergence of beliefs among those with directional motives and practical implications for empirical studies that aim to detect directional motives.
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Cathy Hafer (firstname.lastname@example.org) and Congyi Zhou (email@example.com).