Prediction Markets Meet Wall Street

Besides helping us determine the next President or Hollywood blockbuster, crowd wisdom may also help predict flu epidemics or whether a bailout will work. With all the fuss about Wall Street lately, one fact has gone unobserved: The stock market has performed brilliantly as a form of artificial intelligence.

Consider what happened on Monday, Sept. 29. A $700 billion bailout package was on the table in Congress, and many constituents were opposed to it, complaining that it would help fat cats on Wall Street but not mainstream America. The House of Representatives, where members are up for re-election in November, stopped the bill.

That same day, the Dow Jones industrial average plunged almost 778 points, its biggest one-day percentage decline in two decades. A vast group of investors bet instantly that the political logjam was very bad for the economy. And as the global stock sell-off continued in the succeeding days, Americans began realizing that failure to do something to help the credit markets was not helping their personal fortunes.

How did Wall Street know what would happen? It acted like a prediction market, a pool of intelligence that can foresee the future. Prediction markets are simply bets on ideas: What do you think something is worth, and more important, what will it be worth tomorrow? When groups of people bet on something, their combined intelligence is often remarkably prescient. British scientist Francis Galton was one of the first to notice this in 1906, when he observed 787 people at a country fair try to guess the weight of an ox after it was slaughtered. The average of all guesses was 1,197 pounds, only one pound less than the correct answer.

James Surowiecki expanded on Galton's observation in his 2004 book The Wisdom of Crowds. The most obvious example of collective intelligence is sports wagers, where the odds are really set by all people placing a bet, he wrote. Think of a horse race where the odds are 3 to 1 that one horse will win. If suddenly more people start betting on that horse, the odds rebalance in that horse's favor. The tendency of sports fans to collectively set the right odds is how bookies make money. If it works in sports, "wouldn't people betting on other kinds of events be equally as good, as a group, at predicting them?" Surowiecki asked.

Recently new types of markets have emerged to specifically predict the future. The Iowa Electronic Markets is one of the most famous prediction markets, set up to foretell U.S. Presidential elections. Thomas Rietz, a University of Iowa finance professor who helps direct the IEM, notes it has accurately foretold each election result since 1988 with only a 1.33% error rate in voter totals.

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