When trading NBA games on Polymarket, perhaps you've had the same experience as many others: before the game, watching a certain team with a clearly higher win probability than their opponent, only to see them collapse in the fourth quarter and get swept away (like the recent game between the Hornets and the Heat; this bet has made me seriously question my life choices).

Since everyone says Polymarket is a "truth machine," does that mean if I just mindlessly buy the team with the higher pre-game win probability, I can easily make money?

To verify this hypothesis, I backtested 1,096 regular-season games from the NBA 2025-26 season, and the data revealed the truth—

Mindlessly following the market won't earn you money, but you won't lose much either; the pre-game probabilities have been fully priced in.

Mindlessly following the market, guaranteed

The backtesting strategy used is very simple:

• Use the average probability from 3 minutes before the game as the benchmark
• Trade $100 per game
• Always buy the side with the "higher win probability"

The results:

• A total expenditure of $109,600, return of $107,545.2, resulting in a loss of $2,054
• ROI of -1.87%

This indicates that Polymarket's prices are quite efficient, and the market has fully priced the teams' win probabilities, leaving no "arbitrage" opportunities.

The difference in ROI may stem from transaction costs, emotional premiums, and other dimensions. It would be better to "buy against the market" rather than mindlessly buying, which could yield a 1.87% return.

The true value: Teams should not be generalized

The above backtest is an overall review of one thousand games, and I tried breaking it down from multiple angles to find parts that could break free from market gravity:

• Divided by week: random walk
• Divided by probability: still a random walk, meaning betting on 50%, 60% pre-game win probabilities makes no difference in returns from betting on 70% or 80%
• Divided by teams shows significant differences

Some teams truly live up to the market's trust—

As long as the market believes they will win, they have a higher probability of really winning.

• POR (Trail Blazers): ROI 19%
• PHI (76ers): ROI 14%
• SAS (Spurs): ROI 12%
• LAL (Lakers): ROI 11%
• CHA (Hornets): ROI 9%

Why do these teams have such differences? Since the author previously did not know much about NBA teams, an initial assumption was made:

Are they the strongest or weakest teams, thus having high consistency in expectations?

However, upon investigation, the facts were not as such; aside from SAS (Spurs), the other four teams are only ranked slightly above average.

What about the teams with the best records? In fact, the market has already fully priced them in, with a mindless follow-up purchase yielding an average ROI of only 2.16%, indicating that the pre-game win probabilities are completely valid.

• DET (Pistons): ROI 1%
• BOS (Celtics): ROI 4%
• NYK (Knicks): ROI 3%
• OKC (Thunder): ROI -2%
• DEN (Nuggets): ROI -5%

What about the weakest teams?

This is where extreme polarization occurs; such teams often have very few games where the market believes they can win, like the Nets (BKN), who only had 7 games with a win probability greater than 50%, winning 5 of those games, resulting in a high ROI of 21%; while the Pacers (IND) only had 8 games above 50%, winning 4, leading to an ROI of -20%. The sample size is too small to serve as a trading reference.

In other words, theoretically (only theoretically!), POR (Trail Blazers), PHI (76ers), SAS (Spurs), LAL (Lakers), and CHA (Hornets) are the ranges delineated by existing data for following the market.

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