Where does democracy come from? What are the social, demographic, and economic factors that shape the onset of democracy in a country and its subsequent stability and subsequent development?

This is a question that has vexed social scientists for decades. A new study puts the question to perhaps its most systematic test yet.

Researchers from ETH Switzerland and Georgetown University used a statistical procedure called "extreme bounds analysis" to test the salience of 59 factors identified in more than three million previous statistical regressions (h/t: Charlotta Mellander). The study finds a "humbling result": Out of all those studies, all those variables, and all those millions of statistical analyses, just five factors predict the emergence of democracy, while four predict its survival.

Most surprising of all is the role played by economic growth, measured as gross domestic product (GDP) per capita. The study finds that GDP per capita is negatively associated with the transition to democracy. Contrary to "modernization theory", the study finds that richer countries are not more likely to become democracies. Richer countries are more likely to remain democracies once they become one.