While working on his first two books about businesses that scaled to great heights, Jim Collins found that the executives responsible for leading America's most successful ventures possessed a wide range of characteristics--everything from brashness to introversion; some were wildly creative while others were disciplined. That's why by the time he was working on his third book, which would become the bestselling business bible, Good to Great: Why Some Companies Make the Leap ... and Others Don't, he'd become convinced that leadership wasn't a determining factor in a company's long-term success. Fortunately for him, he had by that time founded his management company in Boulder, Colorado, and there had surrounded himself with a crew of younger researchers. He benefited greatly from their ability to challenge his thinking. Collins explains. --As told to Christine Lagorio-Chafkin

WHEN WE WERE doing research for my third book, Good to Great, I was dubious of anything having to do with "leadership." I had long believed that if you observe a successful company over time, you find its success is not the result of a single leader. As extraordinary as Steve Jobs was, Apple has succeeded beyond him--meaning it's ultimately about building a company that doesn't depend on a single leader, right? The founders of our country understood that. So I was deeply skeptical of studying leadership at all.

But my research team revolted. They told me that, in studying the inflections of companies that went from "good" to "great," it was clear the leader played a big role. I said, "Well, let's go to our comparison companies that didn't make it. Some have towering, charismatic leadership but didn't make that good-to-great leap. Leadership is an irrelevant variable."

Again, my team fought back, marshaling evidence and arguing that the critical element is the source of the leader's ambition. Is that leader truly ambitious for the cause, and the company, and the work--not for him- or herself? That will show up in their actions, regardless of personality. Look at Anne Mulcahy of Xerox and Katharine Graham of The Washington Post. They have vastly different personalities, but both showed ambition for a bigger cause when taking over their companies during tough times. In the end, I'm happy to admit my team was right: Great leadership, in its many personality packages, matters a lot.