A milder version of mania, hypomania often occurs in the relatives of manic-depressives and affects an estimated 5 percent to 10 percent of Americans. "If you're manic, you think you're Jesus," says Gartner. "If you're hypomanic, you think you're God's gift to technology investing. We're talking about different levels of grandiosity but the same symptoms."

Gartner theorizes that there are so many hypomanics--and so many entrepreneurs--in the U.S. because our country's national character rose on waves of immigration. "We're a self-selected population," he says. "Immigrants have unusual ambition, energy, drive, and risk tolerance, which lets them take a chance on moving for a better opportunity. These are biologically based temperament traits. If you seed an entire continent with them, you're going to get a nation of entrepreneurs."

Though driven and innovative, hypomanics are at much higher risk for depression than the general population, notes Gartner. Failure can spark these depressive episodes, of course, but so can anything that slows a hypomanic's momentum. "They're like border collies--they have to run," says Gartner. "If you keep them inside, they chew up the furniture. They go crazy; they just pace around. That's what hypomanics do. They need to be busy, active, overworking."

I know so many hypomaniacs. So many.