Across 21,917 ATP and WTA matches, blindly backing heavy favourites lost 3.2% while blindly backing longshots at 5.00+ lost 34.8% — the bookmaker's margin is loaded onto underdogs, and it gets steadily worse the longer the price.
The single most durable finding in sports-betting research is the favorite-longshot bias: underdogs are systematically over-bet and over-priced, so they return far less than their odds suggest. We tested it on our own database — 21,917 ATP and WTA matches with market prices — by flat-backing every selection in each odds band and measuring the return.
| 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% |
The pattern is perfectly monotone: the longer the price, the more you lose. A heavy favourite loses you 3.2% over thousands of bets; a longshot torches a third of your stake. This isn't variance — it's structural. Bookmakers keep favourite prices sharp to attract informed money and load the bulk of their margin onto underdogs, where casual bettors chase the big payout.
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 smallest on short prices and brutal on long ones. So if you have a genuine edge, it is far easier to realise near the favourite end, where the market is efficient and the overround is thin, than on longshots, where you must overcome a 15–35% structural drag before your edge even counts.
This is why our model applies a minimum-odds floor and treats long-priced "value" with deep suspicion: the data says that's exactly the segment where an apparent edge most often evaporates. We'd rather pass than feed the longshot tax.
Method: 21,917 ATP/WTA singles matches (2022–present) with recorded market odds; flat 1-unit stake on every selection in each band.
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