“Steve Bainbridge”:http://www.professorbainbridge.com/2005/07/inside_informat.html makes an interesting argument about prediction markets.
bq. On the one hand, just as Henry Manne justified insider trading in stock markets by arguing that it improved the accuracy of stock market prices, bets by knowledgeable insiders will significantly enhance the predictive power of markets like the TradeSports contracts. Indeed, given the limits on the power of insiders to affect prices in the stock market, the effect is likely to be much stronger in prediction markets, where the ratio of activity by informed traders to that of uninformed ones is likely to be much higher than in stock markets. … On the other hand, in commercial prediction markets like TradeSports contracts, the proprietor of the market presumably has an incentive to eliminate informed insider trading. If there’s a fairly high probability that you’d be betting against somebody with inside information, who thus can’t lose, would you bet? Me neither.
Bainbridge’s argument here reminds me a little of this old “post”:https://crookedtimber.org/2003/08/05/hayekian-markets-reconsidered/ of Dan’s, which argues among other things that Hayek’s notion of the market as a knowledge-creating entity sits rather uneasily with more standard economic arguments such as efficient-market theory. But Bainbridge’s argument is somewhat different and points to a different tradeoff. If you want to use markets to make the best predictions possible on the basis of available information, you’ll want to allow insider trading, which is, by definition, trading by those with valuable hidden information. But this means that you’re likely to lose liquidity by driving out ordinary punters who don’t want to be fleeced by those in the know. And without ordinary punters, insider traders have no incentive to transact (the only reason that they would want to transact is to fleece suckers who know less than they do). The only way in which this contradiction can really be resolved is if there’s a supply of suckers out there, who are willing to make bets against people who are better informed than they are. As Bainbridge points out, this is a condition that can be satisfied. But by and large, it’s only satisfied when people have extraneous reasons to make a bet (they enjoy a flutter). Bets that aren’t “fun,” or otherwise attractive in some way aren’t likely to attract suckers. Thus, they’ll have low liquidity, and not be very useful as a source of information (this seems to be borne out by the empirics; as the authors of “this paper”:http://faculty-gsb.stanford.edu/zitzewitz/Research/Five%20Questions.pdf note, “as the wonkishness of the contract rises, however, volume and liquidity falls rapidly.”) Thus, even apart from the objections that Dan and John Q. have raised in past posts, prediction markets aren’t likely to be very useful for a very wide variety of important policy issues.