What explains contemporary developed-world populism? A largely-overlooked hypothesis, advanced herein, is economic unfairness. This idea holds that humans do not simply care about the magnitudes of final outcomes such as losses or inequalities. They care deeply about whether each individual’s economic outcomes occur for fair reasons. Thus citizens turn to populism when they do not get the economic opportunities and outcomes they think they fairly deserve. A series of cross-sectional regressions show that low social mobility – an important type of economic unfairness – consistently correlates with the geography of populism, both within and across developed countries. Conversely, income and wealth inequality do not; and neither do the prominent cultural hypotheses of immigrant stocks, social media use, nor the share of seniors in the population. Collectively, this evidence underlines the importance of economic fairness, and suggests that academics and policymakers should pay greater attention to normative, moral questions about the economy.