Futarchy: an experiment we'd learn a great deal from, but please don't try it on me
Robin and Mencius both make some great points in this debate -- for example, Mencius observes that GDP and similar estimates of "national welfare" are poor criteria for decision-making, analogous to a corporation making decisions to maximize revenue instead of profits. GDP in the United States, for example, rose dramatically during World War II, but its standard of living was probably significantly lower for most people than in the Great Depression due to rationing, death, and other traumas of war.
But both Mencius and Robin have misconstrued or vaguely glossed over two of the biggest problems with zero-sum decision markets, especially when applied to government decisions. First an observation: all prediction markets are decision markets unless the resulting prediction is so useless that nobody ever makes a decision based on it or some government somehow bans all possible kinds of decisions based on those markets. The distinction Robin and Mencius make between the two serves to obfuscate those issues that are problems with real decision markets rather than with theoretical or play prediction markets. Two of the biggest problems are:
1. The problem of "morons' money", deemed by Mencius to be the money of some less-informed fraction of players who are disincentivized to play the zero-sum prediction game, actually means the money of anybody with information worse than the best player's information. Since it's a zero-sum game, and much less entertaining than sports betting, any money that is stupider than the average money has a disincentive to play -- in sharp contrast to normal positive-sum markets. But when the less-informed half of the players leave the market, we have a new half of the market that has a disincentive to play, and so on. There's only one person left for whom it would be rational to play, if there were any market left. Prediction markets can work in an experiment only because students of the professor or fans of the idea feel they have a duty to play, or want to signal to fans that they like the idea, or because there are a few people who, like the much larger population of sports bettors, find it genuinely entertaining and thus worth wasting some time and/or money on. Robin I believe has tacitly recognized this problem in his proposals to goose these markets with subsidies. But how big subsidies are needed? It's a high-risk market, so you need to guarantee a high rate of positive return to the average player to compensate for the risk if you want good information entering the market.
2. Moral hazard: As I stated, prediction markets are also decision markets. We're right now going through major economic problems that stem from neglect of moral hazard. Selling securities, insurance, or betting without the investors/bettors or their agents or regulators exercising due diligence and control creates pathological incentives for people to do things like, as we've seen in the mortgage market, give cheap loans to bad credit risks because there were a bunch of number-crunchers with woefully incomplete models buying or insuring these loans who didn't understanding that most kinds of markets create additional risk by creating moral hazard, that this moral hazard must be well controlled if the market is to work, and that controlling moral hazard is usually far from straightforward.
In these betting markets, which are decision markets, people inside and outside government will make their own decisions based on the market. Coercive decisions will have an especially pathological effect: they turn the zero-sum market into an overall negative-sum game. For example, a market predicting the death of someone can, as Tim May long ago observed, readily be used as an assassination market. That's why, for example, PAM, a since-abandoned effort by the U.S. Department of Defense to try out Robin's ideas for prediction markets, had a market for predicting the death of Yasser Arafat but not of George W. Bush. Since these are markets for government decisions, the decisions it drives will, like assassinations, be primarily coercive in nature, for example to wage war on country X or to tax one group of people in order to subsidize another. If the decisions resulting from such a market were voluntary, you could just go onto a voluntary market to satisfy your needs instead of bothering with deciding that government must make some decision and then betting on its outcome. Furthermore, Mencius, although waxing entirely too rude behind his pseudonymous mask, is probably quite right that these markets will reflect the demand for desired decisions far more than a "supply" of information about how to best make that decision: per (1.), there is little to no incentive to supply such information without huge subsidies to give the markets an overall positive rate of return comparable to other high-risk markets.
For example the market for "if the U.S. gives $20 billion to GM the U.S. GDP will rise by 1.1% or more [instead of the expected consensus by some group of economists of 1.0%]" will be dominated by GM, its unions, its dealers and suppliers, and so forth goosing it to a "probably true" prediction, not by people betting on the extremely uncertain, practically lost in the random noise, outcome of the proposition. If GM thinks it can sway the odds of a decision in its favor by 5% by, increasingly as the time for decision approaches, investing up to nearly $1 billion in the market, it will do so. GM has far more to gain by the gift of other people's money than arbitragers making risky bets with their own money have to gain from arbitraging the market back down to whatever they believe to be the true odds of the GDP rising 1.1%. If GM is banned directly from participating in the market, there are numerous other parties with similar stakes in the outcome and they can't all be banned. GM and its allies also have much to gain by lobbying the government to set up this kind of market for direct subsidies instead of futarchic markets for more generic decisions you might find more useful, such as a "remove the President if the GDP doesn't rise 1.1% or more" market. Futarchy thus becomes little more than an auction for government favors, like the time the Praetorian Guard auctioned off the emperorship to the highest bidder.
It would be interesting to try straightforward government-by-the-highest-bidder. We'd probably discover why historians have equated this kind of process with corruption, but it would be well worth the suffering of some hapless residents in some small county somewhere to try the experiment and learn from it. It would of course be interesting to try out futarchy in a similarly real but small scale, restricting the players as well as the victims of the market to our guinea pig jurisdiction since there are no big GM-subsidy stakes at play.
Of course, some would argue that we are living in this kind of experiment anyway, just under a blizzard of euphemisms instead of a straightforward honest auction. Others would argue that democratic voters bring no more information to governmental decisions than "moron money" would in these markets. I have no ready refutations for these arguments, but they don't prove futarchy is better than either what we have now or other possible alternative reforms.
In summary, fans of prediction markets in general, and futarchy in particular, need to actually specify how to deal with moral hazard, as insurance companies and their regulators have, rather than mysterious hand-waving about the supposed "several ways to deal with it", as well as to acknowledge the need to subsidize these markets and figure out how to fairly distribute these endowments.