Last week The Wall Street Journal published an op-ed article by Carly Fiorina titled “Hillary Clinton Flunks Economics,” ridiculing Mrs. Clinton’s assertions that the U.S. economy does better under Democrats. “America,” declared Ms. Fiorina, “needs someone in the White House who actually knows how the economy works.”

Well, we can agree on that much.

Partisan positioning on the economy is actually quite strange. Republicans talk about economic growth all the time. They attack Democrats for “job-killing” government regulations, they promise great things if elected, they predicate their tax plans on the assumption that growth will soar and raise revenues. Democrats are far more cautious. Yet Mrs. Clinton is completely right about the record: historically, the economy has indeed done better under Democrats.

This contrast raises two big questions. First, why has the economy performed better under Democrats? Second, given that record, why are Republicans so much more inclined than Democrats to boast about their ability to deliver growth?

Before I get to those questions, let’s talk about the facts.

The arithmetic on partisan differences is actually stunning. Last year the economists Alan Blinder and Mark Watson circulated a paper comparing economic performance under Democratic and Republican presidents since 1947. Under Democrats, the economy grew, on average, 4.35 percent per year; under Republicans, only 2.54 percent. Over the whole period, the economy was in recession for 49 quarters; Democrats held the White House during only eight of those quarters.