How Does Inside Information Affect Sports Betting Odds?

We describe how the presence of insiders with superior information about potential outcomes of sporting events affects odds set by bookmakers, using a generalized version of the model in Shin (1991). The model has been widely cited as an explanation for the pattern of favorite-longshot bias observed in fixed-odds betting markets. We show that disagreement among those bettors without inside information causes favorite-longshot bias. The presence of insiders reduces odds but does not necessarily exacerbate favorite-longshot bias. For realistically calibrated beliefs, the fraction of insiders has a minimal effect on the ratio of favorite to longshot odds and the betting market collapses if this fraction rises above low levels.