Quantum BiasHow to Stop Overtrading: What 66,465 Brokerage Accounts Reveal
The bias that turns every idea into a trade, and what it costs you.
In this article
In one of the most cited studies in finance, the households that traded the most earned 11.4% a year after costs, while the market earned 17.9%. The average household earned 16.4% and turned over more than 75% of its portfolio every year. The authors' explanation was overconfidence: people trade more when they overestimate what they know.
Overtrading is rarely one big decision. It's the sum of a lot of small decisions that each felt justified at the time. Here's what the research measured, how overconfidence shows up for futures traders, and how to compare how often you trade with your results.
| Question | Data | Answer |
|---|---|---|
| Do investors who trade more earn more? | 66,465 households at a U.S. discount broker, 1991–1996 (Barber and Odean, 2000) | No: the most active group earned 11.4% a year, against 17.9% for the market |
The study
Brad Barber and Terrance Odean published Trading Is Hazardous to Your Wealth: The Common Stock Investment Performance of Individual Investors in the Journal of Finance in April 2000. They studied 66,465 households holding stocks at a big U.S. discount brokerage, from 1991 to 1996.
Here are the results, after trading costs:
| Group | Annual return |
|---|---|
| Households that traded most | 11.4% |
| Average household | 16.4% |
| The market | 17.9% |
Exact values from table 2 in this article. Bar lengths share a zero baseline; no new calculation or simulated price path. Read the table and its stated sample and limits in this article.
The average household turned over more than 75% of its portfolio every year, and the more a household traded, the further behind it fell. The authors concluded that overconfidence could explain both the heavy trading and the bad performance, and their verdict became the title of the paper.
Day traders: same story, just more extreme
If regular investors pay for trading too much, day traders, who trade nonstop by definition, should be the extreme case. The biggest study of persistent day traders confirms it.
Fernando Chague, Rodrigo De-Losso and Bruno Giovannetti studied every individual who started day trading Brazilian equity index futures between 2013 and 2015 and kept at it for at least 300 days. In their 2019 working paper, Day Trading for a Living?, they report that 97% lost money and only 0.4% made more than a bank teller's daily wage. The best performer made about US$310 a day, with a standard deviation of about US$2,560 a day. The authors found no evidence that traders got better with experience.
So sticking with it didn't produce skill. The people who kept going for 300 days or more didn't, as a group, learn their way to profits.
What overconfidence looks like at the screen
Overconfidence isn't arrogance. It's usually quiet:
- Too many setups. Every move looks like an opportunity because you think you can read it.
- Ranges that are too narrow. You expect the market to stay in a tighter range than it does, and you size positions that way.
- Explaining wins, excusing losses. Wins prove your skill. Losses are bad luck or the market "being irrational".
- Ignoring base rates. Most day traders lose money, and overconfidence is believing that stat applies to someone else.
It also feeds off other biases. Hindsight bias makes past setups look obvious, which pumps up your confidence, while the disposition effect keeps losing trades open, which hides how often you were wrong.
The winning-streak trap
Overconfidence isn't constant. It tends to swell after a run of wins, exactly when you're most tempted to size up and trade more. A few good days feel like proof of skill, even though a short streak is well within what pure luck produces for a strategy with no edge at all.
A practical rule: only change position size on a schedule, for example once a month, based on a record long enough to mean something, and never in the middle of a streak. The same rule protects you after a losing streak, when the temptation is to size up to win it back.
Why trading more costs more
Three costs grow with every trade:
- Commissions and exchange fees. Small per trade, big over a year.
- The spread and slippage. Every round trip gives up part of a tick, more in thin markets or fast moves.
- Decision quality. Your tenth trade of the day is rarely as well prepared as your first, as fatigue and frustration build up.
A strategy with a small edge per trade can make money at ten trades a week and lose money at fifty, without any change in the setups themselves.
A simple cost example
Round numbers, just to show the idea. Say one MNQ round trip costs about 1.5 points in commissions, fees and slippage combined, and a trader's average edge before costs is 2 points per trade.
| Trades per week | Gross edge per week | Costs per week | Net per week |
|---|---|---|---|
| 10 | 20 points | 15 points | +5 points |
| 25 | 50 points | 37.5 points | +12.5 points |
| 25, with the edge falling to 1.4 points on the extra trades | 41 points | 37.5 points | +3.5 points |
The third line is the realistic one, because extra trades are rarely as good as the first ones. When the average edge of the added trades drops even a little, trading more gets you less, with more risk and more screen time. The exact numbers depend on your broker and your setups. What matters is the shape: costs grow with the number of trades, and edge usually doesn't.
How to measure your own overconfidence
You don't need a psychologist, just your journal and an afternoon.
1. Calibration check. Before each trade, write down the odds you give it of hitting its take-profit. After 50 trades, group them: of the trades you rated at 70%, how many actually hit the take-profit? If the answer is 45%, your confidence is running ahead of your results.
2. Frequency check. Group your trades by their order in the day (first, second, third and so on) and compare the average result of each group. A lot of traders find their results get worse as the day's trade count goes up.
3. Range check. Before the open, write down the high and low you expect for the session. At the close, see how often the actual range stayed inside yours. If it breaks out more often than you expected, you're underestimating volatility.
4. Planned vs unplanned. Tag each trade as planned before the open or taken on impulse, and compare the two groups.
Structural defenses
- A max number of trades per day, set before the session.
- A written, time-stamped plan, so unplanned trades stand out as unplanned. A daily bias with levels and an invalidation gives that plan a structure.
- A fixed session window. Our research covers 9:30–12:00 ET, and a fixed window limits how many decisions you make.
- A monthly review of frequency against results, using the checks above.
Limits
- Barber and Odean studied stock investors from 1991 to 1996, when commissions were way higher than today. Lower costs shrink one part of the penalty, not the behavior.
- The Brazilian study covers one market and one period, and defines persistent traders as people who traded on at least 300 days.
- In these studies, overconfidence is inferred: trading frequency and returns are observed, and the bias is the authors' explanation.
Sources
- Barber, B. M., & Odean, T. (2000). Trading is hazardous to your wealth: The common stock investment performance of individual investors. Journal of Finance, 55(2), 773–806. Author's PDF
- Chague, F., De-Losso, R., & Giovannetti, B. (2019). Day Trading for a Living? (Working Paper 2019_47). Department of Economics, University of São Paulo (FEA-USP). IDEAS/RePEc
FAQ
What is overtrading? Trading more often than your edge justifies, so costs and lower-quality decisions eat into your results.
Does trading more often lower returns? In Barber and Odean's study of 66,465 households, the most active traders earned 11.4% a year, against 17.9% for the market.
What percentage of day traders lose money? In a study of persistent day traders in Brazilian equity index futures, 97% lost money.
How do I know if I'm overconfident? Compare the odds you give your trades with how often they actually work, and compare your results on busy days with your results on quiet days.
How many trades per day is too many? There's no universal number. Measure your results by trade number within the day, and set a limit where they start to drop.
Is overconfidence always bad for traders? You need some confidence to act at all. The problem is miscalibration: confidence that runs higher than your record supports, which calibration checks help fix.
Do lower commissions solve overtrading? They cut one cost, but not the spread, slippage or the drop in decision quality with each extra trade.
Research and education only. Not financial advice. Futures trading involves substantial risk of loss.
How this article was made: drafted with AI assistance from the research cited above; facts and figures checked by Gabriel Décarie and Karel Panneton.
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