The wisdom of crowds

is an excellent book by James Surowiecki (isbn 0-316-86173-1). As usual I'm going to quote from a few pages:

Diversity and independence are important because the best collective decisions are the product of disagreement and contest, not consensus or compromise.

The best way for a group to be smart is for each person in it to think and act as independently as possible.

To be smart at the top, the system has to be smart all the way through.

Adding in a few people who know less, but have different skills, actually improves the group's performance.

We have bad intuitions about averaging.

Keep the mistakes that people make from becoming correlated.

Information isn't in the hands of one person. It's dispersed across many people.

The problem starts when people's decisions are not made all at once, but rather in sequence.

People are more overconfident when facing difficult problems than when facing easy ones.

Encouraging people to make incorrect guesses actually made the group as a whole smarter.