Prior research shows that fairness judgements regarding the income distribution have a substantive impact on redistribution preferences. Those who perceive incomes as unfair demand more redistribution. However, the association is undertheorized in previous studies. This article adds to the literature by offering a comprehensive theoretical explanation of why incomes are perceived as unfair and how this influences the demand for redistribution. Based on equity theory from social psychology, it is argued that individuals develop a preference for redistribution if they consider their own income and incomes in general to be disproportional to relevant exchanged inputs. They assess proportionality by using social comparisons with observable reference groups such as colleagues, family members or other labour market participants. Multilevel models with survey data from 39 diverse countries support this theory. Individuals who perceive their own income as disproportional in comparison to their efforts and those who perceive incomes in general as disproportional demand more redistribution. These findings have several implications for research on political economy and social policy. Most importantly, they explain the inconclusive results of empirical tests of rational choice theories such as the medianvoter hypothesis.