On Monday night, after the debate, Barack Obama was leading Romney on Intrade by around 60 percent to 40 percent. But at around 10:00 a.m. on Tuesday morning, Romney surged to 48 percent. Was this evidence that the conventional wisdom was wrong? Had Romney actually won the debate handily? Or, alternatively, was the nosedive in the stock markets putting a dent in Obama’s re-election chances?

Neither. As economist Justin Wolfers pointed out on Twitter, the huge swing toward Romney appears to have been driven by a single trader who spent about $17,800 buying up Romney shares and pushing the Republican candidate’s chances on Intrade up to 48 percent. But the surge only lasted a few minutes before other traders whittled the price back down to what they saw as a more accurate valuation. Romney’s odds of winning are currently back at around 41 percent.

…As Wolfers pointed out, this mysterious trader ended up overpaying by about $1,250 for shares that quickly collapsed in value. Was this just someone who made a bad trade? Or was somebody trying to influence Intrade odds in order to sway perceptions of the race? And if so, was it worth $1,250 to jolt the markets for less than 10 minutes?

This supports Robin Hanson’s and Ryan Oprea’s finding that manipulation can improve (!) prediction markets – the reason is that manipulation offers informed investors a free lunch. In a stock market, for example, when you buy (thinking the price will rise) someone else is selling (presumably thinking the price will fall) so if you do not have inside information you should not expect an above normal profit from your trade. But a manipulator sells and buys based on reasons other than expectations and so offers other investors a greater than normal return. The more manipulation, therefore, the greater the expected profit from betting according to rational expectations.