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.
The favorite-longshot bias is probably the most durable finding in sports-betting research: underdogs get over-bet, so they pay back far less than their odds suggest. Everyone cites it. We wanted to see it in our own database before trusting it, so we took 21,917 ATP and WTA matches with market prices, flat-backed every selection in each odds band, and counted what came back.
| 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% |
There is no kink anywhere in that table: the longer the price, the more you lose. Heavy favourites cost you 3.2% over thousands of bets. Longshots eat a third of every stake. A gradient that clean isn't variance. Bookmakers keep favourite prices sharp because that end is where informed money shops, and they load the bulk of the margin onto underdogs. Casual bettors chase the big payout anyway, so the market lets them pay for it.
Our table also matches the academic record, which is reassuring in both directions. Forrest and McHale found the same bias across the whole range of odds in men's tennis two decades ago [1], and later work confirmed it on fresh tennis data while testing competing explanations for why it refuses to die [2]. This is one of the most replicated findings in betting-market research, and our database reproduces it band for band.
Every band is negative, because every price carries the bookmaker's margin. You cannot beat the market by blindly backing any band, favourites included. The point is relative. The structural tax is thin on short prices and brutal on long ones, so a genuine edge is far easier to realise near the favourite end, where the overround is small, than on longshots, where you start 15–35% underwater before your edge counts for anything.
"Value on the underdog" is usually a trap: a 6.00 shot that "should be 4.50" still sits inside a band that loses 35% on average, and your read has to be extraordinary to clear that. Short prices are where sharp money lives, not because favourites always win but because that's where the market's tax is lowest. Chasing longshot payouts feels smart and bleeds slowly.
This is why our model runs a minimum-odds floor and treats long-priced "value" with suspicion: our own data says that's the segment where an apparent edge most often turns out to be the tax in disguise. We'd rather pass than feed it.
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.