Common sense is uncommon in individuals and, at first blush, seems even more so in groups. No one expects crowds to produce useful thought. We fear the tyranny of the majority and mob rule, avoid peer pressure where we can, and immediately see the aptness of Charles Mackay's 19th-century book title Extraordinary Popular Delusions and the Madness of Crowds.

But the idea of collective intelligence shouldn't seem so far-fetched. After all, democracy is built on the principle that large groups know how to govern themselves. Commodities markets, which set prices on the basis of group knowledge, play a growing role in everyday business decisions. Even lone geniuses build on the work of others: for every Einstein there are Poincares and Lorentzes and Hilberts lurking in the background.