Of all the aptitudes and attributes Mitt Romney likes to cite as having prepared him to be an effective president, the one credential he returns to most emphatically is his lifetime career in business. “I’ve lived in the real economy,” he said at one point during the primaries. “For 25 years, I spent my life in business. I only spent four years as a governor, and I joke that I didn’t inhale. I’m still a business guy, all right? I know how to lead us out of this stagnant Obama economy and into a job-creating recovery.” More recently Romney suggested that, in addition to the constitutional requirements that a president be a natural-born citizen and at least 35 years old, there should be a requirement that a president have several years working in business.

Romney makes his assertions about the importance of business experience so eagerly and so often as to suggest that the relationship between business success and presidential success is simply a given. The only problem with the claim is that there’s no evidence to suggest it’s true.

The greatest presidents? Notable business failures almost to a man, if they had any business experience at all. Abraham Lincoln racked up so many unpaid notes in his brief career as a storekeeper in New Salem, Illinois, that he referred to the obligations as his “national debt.” For 15 years after his haberdashery in Kansas City went under, Harry Truman was still working to pay off his creditors, and was strapped for money until well into his career in the United States Senate. George Washington was a sharp-elbowed, tightfisted planter and entrepreneur (he owned a distillery!), but he spent so much time winning American independence and then inventing the job of president that his financial affairs were a mess by the end of his first term. As for Franklin Delano Roosevelt, the spoiled country squire who led the nation through the Depression and World War II: it could be argued that, until he found his passion in politics, he never really worked a day in his life (even if he put in some desultory time as a Wall Street lawyer).

This is not to say that no successful businessmen have ever become president. A few have, among them Warren G. Harding (an Ohio newspaper publisher and editor), Herbert Hoo­ver (a multi-millionaire mining engineer, investor, and consultant), and Jimmy Car­ter (a Georgia peanut farmer and warehouse owner).

But no one would argue that the one-term presidencies of Harding, Hoover, or Carter were anything close to successes.

A few other businessmen have made serious stabs at the White House—including Wendell Willkie, a lawyer and utilities executive; Ross Perot, a data-processing entrepreneur; and Romney’s own father, George, the successful president of American Motors and then governor of Michigan. But only Willkie managed to get as far as winning the Republican nomination (in 1940, before losing to Roosevelt). Mitt Romney would be only the second president with an M.B.A. The first was George W. Bush, and we know how that worked out.

Why is it that, when it comes to the presidency, businessmen—those presumed avatars of competition and competence—just can’t seem to cut the mustard? For one thing, the basic goals of business (productivity and profit) are not completely congruent with those of national office (the general welfare, the pursuit of which may be neither efficient nor profitable in any narrow sense). Sure, it may be possible and even advisable to run certain government activities in a businesslike way, but the very definition of “bottom line” is different in the private and public sectors—and needs to be. Romney is right enough to suggest that efficiency, effectiveness, and good customer service are the hallmarks of successful businesses, and who doesn’t trust FedEx more than the U.S. Postal Service?