Across 21,917 ATP and WTA matches, blindly backing heavy favourites lost 3.2% of stake while blindly backing longshots at 5.00 or above lost 34.8%. The bookmaker's margin is loaded onto underdogs, and it gets steadily worse the longer the price, so no flat staking plan on either end is profitable.
Underdogs get over-bet and pay back less than their odds suggest. This favorite-longshot bias is one of the most replicated results in betting research. We checked it in our own database: 21,917 ATP and WTA matches with market prices, a flat stake on every selection in each odds band.
| Price band | Actual win rate | Flat-bet ROI |
|---|---|---|
| Heavy favourite (<1.30) | 85.1% | −3.2% |
| Favourite (1.30–1.60) | 66.7% | −4.5% |
| Slight favourite (1.60–2.10) | 52.1% | −7.2% |
| Underdog (2.10–3.00) | 37.7% | −9.7% |
| Underdog (3.00–5.00) | 23.8% | −14.7% |
| Longshot (5.00+) | 9.6% | −34.8% |
Losses grow steadily with the price: 3.2% of stake on heavy favourites, 34.8% on longshots. Bookmakers keep favourite prices sharp, where informed money bets, and load most of the margin onto underdogs.
This matches the academic record. Forrest and McHale found the bias across the whole range of odds in men's tennis [1], and later work confirmed it on newer tennis data while testing explanations for why it persists [2].
Every band is negative, because every price carries the bookmaker's margin. No band is profitable to back blindly, favourites included. What differs is how much an edge has to overcome: little near the favourite end, 15–35% on longshots.
A 6.00 shot that "should be 4.50" still sits in a band that loses about 35% on average. This is why our model uses a minimum-odds floor and treats long-priced value with suspicion.
Method: 21,917 ATP/WTA singles matches (2022–present) with recorded market odds; flat 1-unit stake on every selection in each band. Re-verified 2026-08-07 on the grown sample (23,330 priced matches): every band reproduces within normal drift, longshots at 5.00+ now return −37%.
References:
[1] D. Forrest and I. McHale, "Anyone for tennis (betting)?" The European Journal of Finance, vol. 13, no. 8, pp. 751–768, 2007.
[2] J. Lahvička, "What causes the favourite-longshot bias? Further evidence from tennis," Applied Economics Letters, vol. 21, no. 2, pp. 90–92, 2014.