In every other case you get worse predictions. Since those who are predicting have to now construct their bets such that they know they can always get run over by an insider. So in the general case it reduces the ability of the predictors to push the market in the right direction, because they always have to risk manage the fact that someone out there might run them over with insider information.
Suppose some large private company has to decide whether they're going to build a new facility in city A or city B. This is useful information for all kinds of reasons. If you're a vendor then you need to start making preparations to set up shop in the city where your big customer is moving etc.
The company's analysis shows it would derive a $10M advantage from building in city A. The prediction market is correctly leaning that way. If there are only enough counterparties that someone who now bets on city B and wins would make $5M, everything works the way it should and the company goes with city A. But if there are enough counterparties that a winning bet on city B would net you $25M then the company can place the bet, eat the $10M loss by choosing city B and come out $15M ahead.
But the $10M number isn't public. It's essentially the thing you wanted the market to predict and it could be arbitrarily larger or smaller than that. So how are you supposed to know if the prediction market will be predicting the result or determining it?
On the other hand, at the point where the prediction market winnings are material enough that they might alter the underlying decision itself, you've clearly got an antisocial structure. Prediction markets that don't want to be seen as mere prop betting venues should refuse to run markets on those questions.
It is insider trading, the thing everyone here is talking about
If you're approaching a market with hard facts, detailed comparisons and solid evidence; while I'm trading in the same market based on vibes and intuition, surely it's expected that your returns would be better, and mine worse?
"When a measure becomes a target, it ceases to be a good measure"
Insiders can change the facts.
Perfect example from today. Allbirds just announced that they're going all in on AI infra, skyrocketing the stock. Had I bought a million dollars worth of Allbirds yesterday, everyone would think I'm an idiot. But now, they would think I have insider information and would no longer want to participate because it would make no sense to buy Allbirds yesterday unless I knew the announcement was coming.
Sports betting is so profitable for prediction markets because they're mostly unsophisticated retail flow making lots and lots of trades, giving the platforms commission. If an insider just pushes market prices in their direction the platforms are going to lose on volume.
The average person does not do this. People trade individual stocks all the time, despite every other market participant (banks, hedge funds, etc.) having better information and technology.
It's why institutions like Citadel pay for retail order flow. They know that retail traders don't have an edge and, if anything, often end up being negative signal.
That depends on what the effects are.
Suppose that predicting things well requires both information and analysis. Early access to information is therefore a competitive advantage: Even if you're not as good at analysis, having the information before anyone else and then getting the analysis right 65% of the time is more often than not going to let you beat the people who get the analysis right 85% of the time once they have the information. Which is to say, it will make it less profitable for the people who are better at analysis to participate in the market, and then fewer of them will.
So the question is, what do you want? An answer which is right 65% of the time slightly sooner, or an answer which is right 85% of the time slightly later? It's valid to want the second one.
You can disagree with the libertarian argument, but I don't see how you can say that Polymarket et al. are something other than a prediction market. Can you explain where you see the difference?
So if a market is trying to maintain a veneer of fairness it’s just using a prediction market as cover and is something else.
If you're being that puritan about the definition, then having a "real" prediction market is completely impossible. Because actors like the DOJ do not wait for a statement by the Kalshi CEO to bring charges. And rational actors will know and anticipate that, and hence preemptively comply. So you never get the unfettered version of a prediction market.
I don't think it makes sense to be that puritan about a definition that the thing it's trying to define becomes an impossibility. Polymarket, Kalshi et al are clearly prediction markets in the messy reality that we live in, and we're figuring out as we go what the legal reality of a real-world prediction market is and should be.
I'd like regulations to cut into that too, so the market isn't just a weird "Did trump tweet something deranged today?"