Belief in free will is a pervasive phenomenon that has important consequences for prosocial actions and punitive judgments, but little research has investigated why free will beliefs are so widespread. Across 5 studies using experimental, survey, and archival data and multiple measures of free will belief, we tested the hypothesis that a key factor promoting belief in free will is a fundamental desire to hold others morally responsible for their wrongful behaviors. In Study 1, participants reported greater belief in free will after considering an immoral action than a morally neutral one. Study 2 provided evidence that this effect was due to heightened punitive motivations. In a field experiment (Study 3), an ostensibly real classroom cheating incident led to increased free will beliefs, again due to heightened punitive motivations. In Study 4, reading about others' immoral behaviors reduced the perceived merit of anti-free-will research, thus demonstrating the effect with an indirect measure of free will belief. Finally, Study 5 examined this relationship outside the laboratory and found that the real-world prevalence of immoral behavior (as measured by crime and homicide rates) predicted free will belief on a country level. Taken together, these results provide a potential explanation for the strength and prevalence of belief in free will: It is functional for holding others morally responsible and facilitates justifiably punishing harmful members of society. © 2014 American Psychological Association.