Extreme do-gooders – what makes them tick?

"We call it a moment of obligation," says Lara Galinsky, of the Echoing Green Foundation, whose 471 fellows have raised more than $1 billion for their causes since 1987. "It's usually not a dramatic moment; it's a gathering of moments, but it's very clear. It's when something gathers such force that you can't ignore it."

It doesn't happen that often. There is only 1 social entrepreneur for every 10 million of the rest of us, according to calculations of Ashoka, an organization that funds social entrepreneurs around the world. Ashoka founder Bill Drayton bases his calculations on nearly 30 years' worth of seeking out the elusive combination of vision and passion that social entrepreneurs put into practice.

"The core defining element is that they simply cannot come to rest ... until their dream has become a new pattern across all of society," says Mr. Drayton. "This is very different from everyone else: the scholar or the artist expresses an idea, and they're happy. The manager ... make[s] the company work. The social worker, the professional help people ... make their lives better. None of that would remotely satisfy the social entrepreneur. Their job is to change the system."

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