We report the first empirical data showing a significant amount of double conjunction fallacies in physicians’ probability judgments concerning prognosis and diagnosis. Our results support the hypothesis that physicians’ probability judgments are guided by assessments of evidential impact between diagnostic conditions and clinical signs. Moreover, we show that, contrary to some influential views, double conjunction fallacies represent an experimentally replicable reasoning bias. We discuss how the phenomenon eludes major current accounts of uncertain reasoning in medicine and beyond and how it relates to clinical practice.
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