Do a Google search for "manic" and "businessman"--and you get nearly 18,000 hits. A psychologist at Johns Hopkins Medical School thinks he knows why. It's not just that these folks are go-getters. According to clinical assistant professor John Gartner, many U.S. entrepreneurs actually meet the diagnostic criteria for hypomania, a mild form of mania characterized by restlessness, creativity, grand ambitions, euphoria, risk-taking and impulsivity.

Not that this is a bad thing. In Gartner's new book, "The Hypomanic Edge: The Link Between (a Little) Craziness and (a Lot of) Success in America," he argues that the dynamic business environment of this country over the last 200 years has been due largely to people with hypomania. Gartner cites historical figures like Alexander Hamilton and filmmaker Louis B. Mayer, as well as modern genome decoder Craig Venter. "They may be our greatest natural resource," he writes, for these are precisely the people who have the zeal and arrogance to conceive and implement bold schemes. They come by it naturally, Gartner says. After all, they bear the temperament-shaping genes of the great risk takers who settled the New World.

But as Gartner makes clear, hypomania cuts both ways. Those big ideas can be brilliant schemes or ill-conceived flops, but the hypomanic entrepreneur is likely to endorse both with equal zeal. "When you dig into their biographies," he says, "it's amazing the stupid, impulsive mistakes they made." Henry Ford wanted to build a $250 car that every working man could afford. But in a lesser-known episode, he also formed the grandiose notion of sailing to Europe and personally bringing a speedy end to World War I. The endeavor brought him nothing but ridicule. "When it succeeds, we say they're a genius," says Gartner. "When they fall flat on their face, we say, 'What were they thinking?' "Let the investor beware.