The fact that each of us has significantly greater confidence in the claims of co-partisans – those belonging to groups with which we identify – explains, in large part, why so many people believe a significant amount of the misinformation they encounter. It's natural to assume that such misinformed partisan beliefs typically involve a rational failure of some kind, and philosophers and psychologists have defended various accounts of the nature of the rational failure purportedly involved. I argue that none of (...) the standard diagnoses of the irrationality of misinformed partisan beliefs is convincing, but I also argue that we ought to reject attempts to characterize these beliefs as rational or consistent with epistemic virtue. Accordingly, I defend an alternative diagnosis of the relevant epistemic error. Specifically, I maintain that such beliefs typically result when an individual evaluating testimony assigns more weight to co-partisanship than he ought to under the circumstances, and consequently believes the testimony of co-partisans when better alternatives are available. (shrink)