FORTUNE -- When you picture a leader, the image that usually comes to mind is someone like Jack Welch or Bill Clinton -- gregarious, energized by crowds. Organizations tend to celebrate and promote such extroverted personalities, as opposed to introverts, who draw energy from ideas or one-on-one interactions. Such quiet types are often not as visible within companies, but by some calculations, introverts make up half of the population. That's an awful lot of talent to exclude from executive ranks. It's the numerical equivalent of excluding women -- and similarly shortsighted, says Susan Cain, author of the new book Quiet: The Power of Introverts in a World That Can't Stop Talking. "There's a bias in our culture against introversion," she says. To use Betty Friedan's language from The Feminine Mystique, "it's a problem in our culture that has no name" -- pervasive, yet seldom discussed, at least until recently.