The Price Is the Forecast: Why 63 Cents Means 63 Percent, Mostly

A prediction market share pays $1 if an event happens and $0 if it doesn’t. From that single design choice, everything follows: the price of that share, in cents, is the market’s probability forecast. Here’s the mechanism, the frictions that bend it, and the measured record.

The mechanism: expected value forces the mapping

If a YES share trades at 63 cents, buying it is a bet that pays $1 with some probability p. The purchase breaks even exactly when p = 63%. Believe the true probability is higher — you buy, pushing the price up; believe it’s lower — you sell or buy NO, pushing it down. Every trader’s money votes their probability estimate, and the price settles where the marginal dollar disagrees with itself. It’s the same arithmetic as sports betting odds, stripped of the bookmaker’s margin: price equals implied probability, discovered continuously in public.

The frictions: why 63 isn’t always exactly 63

Real markets bend the clean mapping in measurable ways. Fees and spreads shave the edges — a market quoting 63/65 has a two-cent zone where no trade corrects mispricing. Longshot bias, inherited from betting markets generally: cheap tails trade slightly rich (5-cent events happen less than 5% of the time), because lottery tickets are fun and shorting them ties up capital for pennies. Time value: a share paying $1 in a year competes with interest rates, dragging long-dated prices below true probability. And resolution risk — the possibility the market’s fine print settles differently than traders assumed — puts a haircut on everything; the famously argued resolutions are a genre of their own. None of these breaks the mapping; they add a known error bar around it.

The record: calibration is genuinely good

The empirical question — when markets say 70%, does it happen 70% of the time? — has been studied across election cycles, sports, and platform datasets, and the answer is: yes, remarkably well, especially in liquid markets near the middle of the range. Aggregate calibration curves hug the diagonal through the 20–80% band, with the documented deviations exactly where the frictions predict — at the extremes. Head-to-head against polls and pundit forecasts, liquid markets have historically matched or beaten both at most horizons, not because traders are oracles but because the price aggregates every forecaster who’s willing to stake money, including the ones reading the polls.

Reading a price like a practitioner

The working rules: treat mid-range prices in liquid markets as honest probabilities with a couple points of slack; discount cheap tails; check the resolution criteria before trusting any number; and watch volume — a 63% price on ten dollars of trading is an opinion, on ten million it’s a forecast. The market’s great trick isn’t clairvoyance. It’s turning “put your money where your mouth is” into a continuously updated number anyone can read — which makes it the rare pundit that publishes its own scorecard.