False Confidence, Real Consequences: Measuring Post-Truth via Epistemic Error and Populist Support

The paper investigates the foundations of populist support by proposing the Post-Truth Index (PTI) as a novel measure of epistemic overconfidence. An overconfident individual has it exactly wrong on some political facts. This paper utilizes two primary datasets, the Populist Leaders and the Economy and the World Values Survey in Wave 7, covering more than 34,498 respondents in 32 countries. Logistic regression with marginal effects shows that the independent variable of epistemic overconfidence stands out as the strongest and most positive explanatory variable for populist support besides economic, institutional, and media trust-related variables. This result is robust to nonlinearities in media use and trust, indicating that excessive use of digital media increases populist support. Because the PTI substantially improves model fit, it is essential to the understanding of contemporary populism.