In one of his first papers, Steven Levit showed that there are indications that top sumo wrestlers collude. Indeed, a wrestler who loses height games (out of a total of 15) is demoted to an inferior league. By looking at matches involving one wrestler with 7 wins and 7 loses against another wrestler who has already height wins, Levit showed that the first wrestler beat the other one in 80% of the cases, suggesting that the wrestler already insured not to be demoted "helped" the wrestler who could be demoted. This article appears in the book Freakeconomics that Levit cowrote with Stephen Dubner.
A recent New York Times article brings additional confort to these earlier findings. Instead of being based on statistical evidence, like in Levit's paper, the current case is based on hard evidence, in this case the exchange of SMS messages between wrestlers. Interestingly, collusion (letting the marginal wrestle, who has 7-7 win-loses, win) takes the form either of monetary transfers or future exchange of loses and wins.
