Bringing together the classic defense of liberty and democracy, the political economy of hierarchy, endogenous growth theory, and the new institutional economics on growth, we propose a new institutional theory that identifies democracy's unique advantage in prompting economic growth. We contend that the channel of liberty‐to‐innovation is the most critical channel in which democracy holds a unique advantage over autocracy in promoting growth, especially during the stage of growth via innovation. Our theory thus predicts that democracy holds a positive but indirect effect upon growth via the channel of liberty‐to‐innovation, conditioned by the level of economic development. We then present quantitative evidence for our theory. To our best knowledge, we are the first to propose such an indirect and conditional effect of democracy upon economic development and provide systematic evidence. Our study promises to integrate and reconcile many seemingly unrelated and often contradictory theories and evidence regarding regime and growth, including providing a possible explanation for the inconclusive results from regressing overall regime score against the rate of economic growth or change in level of GDP per capita.