Pari-mutuel odds for longshots in UK horseraces exceed those offered by bookmakers. This effect is less for winning compared to losing horses and is explained by informed gamblers betting disproportionately with the pari-mutuel on winning horses.

Using a novel dataset, which allows comparisons across heterogeneous sub-groups of pari-mutuel bettors, we demonstrate significant behaviour and performance distinctions between recreational and professional investors. Professionals’ ability to earn abnormal returns on short odds horses in...