Ancient beginnings and correlation
The period of Ancient Greece 4th century B.C. and later of the Roman Empire marks the beginning not only of democracy, but as well as its connection to economic growth. All throughout history, up until the present they have stayed intertwined. While there is no doubt of their existing relationship, whether it being in favor of economic development or democracy, there is no evidence to claim that it is in fact a causal relationship.
In other words, a country that undergoes democratization does not have to necessarily experience economic growth, most often measured in income per capita, or vice versa. For every such case there exists a counter example. What this means is that there are multiple factors, such as political stability and political institutions, social insurance, government capacity, religion and many other which influence the outcome. In two similar countries, almost identical democratic regimes can yield completely different results. However, the concepts highly complement each other, and in cases through history where they were separated there has been great difficulty.