Individual Behavior and Beliefs in Experimental Parimutuel Betting Markets

We study experimental parimutuel betting markets with asymmetrically informed bettors. We propose a theoretical model, the Adaptive Model, which serves as our source of null hypotheses about individual behavior and the capacity of the markets to aggregate information. In one treatment, groups of eight participants bet against each other in twenty repetitions of a sequential betting market. The second treatment is identical, except that bets are observed by other participants who assess the winning probabilities of each outcome. In the third treatment, the same individuals place bets and assess the winning probabilities of the outcomes. A favorite-longshot bias is observed in the first and second treatments, but it is sharply reduced in the third treatment. Information aggregation is better in the third than in the other two treatments, because contrarian betting is almost completely eliminated by the belief elicitation procedure. Placing bets improves the accuracy of belief statements. A statistical generalization of the Adaptive Model explains the data very effectively