THE IMPLICATIONS OF A REVERSE FAVOURITE-LONGSHOT BIAS IN A PREDICTION MARKET

We examine 330,857 trades of prediction market contracts, the values of which are based on against-the-spread outcomes of NFL games, and find the presence of a significant reverse favourite-longshot bias. Surprisingly, the timing of this bias is identical to that observed in traditional casino-style NFL betting markets. That is, late-season away favourites so profoundly underperform expectations that the set of all favourites underperforms on average. Prior research shows that in prediction markets having asset prices ranging from $0 to $100, win rates are below (above) expectations when prices are low (high). However, we show that observed win rates for contracts on late-season away favourites are below expectations across all prices. The presence of a strong RFL bias in a prediction market provides evidence against the theories that this bias is caused by line shading or due to the effects of unpredicted weather variables on team performance.