As Bill Clinton is fond of reminding us all, it is a well-established fact that, on average, the U.S. economy grows faster when a Democrat is president. Is it because liberals have better ideas or because they just have better fortune?

According to a new paper by Princeton Professors Mark Watson and Alan Blinder (himself a Clinton administration veteran), it's mostly about luck.

Since 1947, real gross domestic product has grown 4.35 percent per year under Democratic presidents and just 2.54 percent under Republicans. Even Jimmy Carter, whose presidency lives on in the public's imagination as an economic failure, averaged about the same growth rate as Ronald Reagan.

But it's hard to trace the relative success of Democratic presidents to their actions in office. After running the data and ruling out potential explanations such as fiscal policy, the Federal Reserve, or Congress, Watson and Blinder zero in three factors that, according to their math, manage to explain 46 to 64 percent of the differences.* Those factors are: