The U.S. economy has performed better since World War II when the president is a Democrat rather than a Republican, two prominent Princeton University economists conclude. The reasons, they say, involve more luck than successful policies.

Over the past 64 years and 16 presidential terms, the U.S. grew at an average rate of 4.35% when a Democrat was in the White House and at a 2.54% when a Republican was, a gap the economists call “astoundingly large.”