Of preferences and priors: Motivated reasoning in partisans’ evaluations of scientific evidence

Despite decades of research, it has been difficult to resolve debates about the existence andnature of partisan bias—the tendency to evaluate information more positively when it supports, rather than challenges, one’s political views. Whether partisans display partisan biases, and whether any such biases reflect motivated reasoning, remains contested. We conducted four studies (total N = 4,010) in which participants who made unblinded evaluations of politically- relevant science were compared to participants who made blinded evaluations of the same study. The blinded evaluations—judgments of a study’s quality given before knowing whether its results were politically-congenial—served as impartial benchmarks against which unblinded participants’ potentially biased evaluations were compared. We also modeled the influence of partisans’ preferences and prior beliefs to test accounts of partisan judgment more stringently than past research. Across our studies, we found evidence of politically motivated reasoning, as unblinded partisans’ preferences and prior beliefs independently biased their evaluations. We contend that conceptual confusion between descriptive and normative (e.g., Bayesian) models of political cognition has impeded the resolution of longstanding theoretical debates, and we discuss how our results may help advance more integrative theorizing. We also consider how the blinding paradigm can help researchers address further theoretical disputes (e.g., whether liberals and conservatives are similarly biased), and we discuss the implications of our results for addressing partisan biases within and beyond social science.