The basic theory behind prediction markets is that a lot of people with money on the line can better predict an outcome than any one expert. “Financial markets are generally pretty efficient, and the evidence suggests that the same is true of prediction markets,” Eric Zitzewitz, an economics professor at Dartmouth, tells me. The shares trade between $0 and $1, and once the event is resolved, shares tied to the correct outcome pay out a dollar. If you bought Trump shares on Monday, when they were 58 cents, you can expect to make 42 cents on the dollar. Later that fall, the popular betting market PredictIt gave Hillary Clinton an 82% chance of beating Donald Trump.

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