Wisdom of Crowds

This is the title of a book written by James Surowiecki and first published in 2004.

The author said many examples where the decision of a group is often better than could have been made by any single member of the group (including an expert). These examples are taken from several fields, primarily economics and psychology.

The opening story was about a crowd in a county fair who were asked to guess the "slaughtered and dressed" weight of an ox. The median weight of the individual guesses was closest to the ox's true weight than the estimates of most individual members, including the estimates made by cattle experts.

This theory refers to decisions of independently-deciding individuals, and is not the same as crowd psychology (where most people blindly follow the crowd).