Maybe none of the above. Blinder and Watson propose that the answer has less to do with policies—taxing, spending, redistributing—and more to do with dumb luck. "The Democratic edge stems mainly from more benign oil shocks, superior [productivity growth], a more favorable international environment, and perhaps more optimistic consumer expectations about the near-term future," they wrote.

As Jordan Weissmann and Brad Plumer both observed in analyses of previous drafts of this paper, some factors that Blinder and Watson file under "luck" might not be entirely outside of the president's control. Oil shocks, for example, are kryptonite for growth. But both Iraq wars, started by Republicans, coincided with oil spikes. It seems fair to point out that oil prices are a barometer of global supply and demand, not just the state of war in Iraq. But it's a telling example of how broader economic forces aren't entirely separate from U.S. policy.

For many voters and pundits, the fact that the President of the United States presides over the economy makes him entirely responsible for the economy. This is not quite the Pottery Barn Theory of presidential power—"you break it, you buy it." Often, it resembles the Used Car Theory of presidential power—"it's broken, but you bought it." Like a lemon, the economy is constantly breaking down, demanding emergency fixes, or sputtering along with agonizing sluggishness. But if the Blinder/Watson paper tells us anything, it's that we should employ an informed humility about the White House's ability to control every ingredient that bakes into GDP growth and employment.

That's not to say policy is irrelevant to the economics of people's lives. On the contrary, we have mountains of evidence that Social Security dramatically reduces poverty, that cutting taxes raises take-home pay in the short run, and that passing a law to broaden health care coverage reduces the number of uninsured households. The fact that income inequality has grown in Republican administrations and fallen under Democrats illustrates a major point. The government has vast control over how much of your paycheck goes home with you and what your taxes are used for. But it exercises little control over the global vicissitudes that determine our overall growth, prices, and wages.

Maybe we'd be better off thinking about international economics less like Washington's little private laboratory and more like the weather—a massive force we cannot hope to control, even as we debate how to respond to its worst excesses.