Michel Serafinelli and Guido Tabellini have a new paper on that question, here is the abstract:
Creativity is often highly concentrated in time and space, and across different domains. What explains the formation and decay of clusters of creativity? In this paper we match data on thousands of notable individuals born in Europe between the XIth and the XIXth century with historical data on city institutions and population. After documenting several stylized facts, we show that the formation of creative clusters is not preceded by increases in city size. Instead, the emergence of city institutions protecting economic and political freedoms facilitates the attraction and production of creative talent.