"Each of us knows very little, really, about the companies we invest in—certainly not how they're going to perform in the future. Yet our collective buying and selling often sends stunningly accurate signals about a company's prospects, which are reflected in the stock price. "When our imperfect judgments are aggregated in the right way," Jim concludes, "our collective intelligence is often excellent." Well, okay, not in the middle of a bubble—that's the madness of crowds—but most other times."

I wonder what line differentiates wisdom and madness in collective knowledge.

The Wisdom of Crowds