Overconfidence Bias in Investing: Why Trading Is Hazardous to Your Wealth

If loss aversion is the most damaging emotional bias, overconfidence is probably the most common one. And Pompian goes as far as calling it “one of the most detrimental biases that an investor can exhibit.”

Here’s the thing about overconfidence: it feels good. That is what makes it so dangerous. You feel smart, informed, in control. Meanwhile, your portfolio is quietly bleeding money because you are trading too much, diversifying too little, and underestimating risks left and right.

Two Flavors of Overconfidence

Pompian splits overconfidence into two types, and both are bad news.

Prediction overconfidence is when you think your forecasts are more accurate than they actually are. The confidence intervals you set for your predictions are way too narrow. You might think a stock will return somewhere between -10% and +10% this year, but history shows that swings of 30% or 40% happen more often than you think.

Here is a fun example from the book. Researchers asked investors: “The Dow Jones Average was at 40 in 1896. It crossed 9,000 in 1998. If dividends had been reinvested, what would the value be?” Then they asked people to give a range they felt 90% confident about.

Nobody got even close. The actual answer? 652,230. That is not a typo. Six hundred fifty-two thousand. People’s ranges were absurdly narrow because they simply could not imagine numbers that large.

Certainty overconfidence is when you are too sure about the quality of your judgments. You read some articles about a company, maybe get a tip from a friend, and suddenly you “know” it is a good investment. In studies, when people said they were 100% sure of an answer, they were actually right only about 70-80% of the time. At 90% confidence, they were right about 75% of the time.

We are all walking around with an inflated sense of how much we know. And this carries directly into investing decisions.

The Research That Should Scare You

The landmark study Pompian highlights is by Brad Barber and Terrance Odean, charmingly titled “Boys Will Be Boys.” They studied the trading records of 35,000 households from 1991 to 1997 and found some really uncomfortable results.

First, men trade more than women. This is not a surprise if you know anything about gender differences in confidence. Men are generally more overconfident.

Second, more trading means worse returns. The group that traded the least (about 1% annual turnover) earned 17.5% per year, actually beating the S&P 500’s 16.9%. The most active traders (over 9% monthly turnover) earned only about 10% per year.

Let that sink in. The people who did almost nothing beat the market. The people who traded the most underperformed it badly.

Barber and Odean’s conclusion became famous: “Trading is hazardous to your wealth.”

And here is the kicker: both men and women would have been better off if they had just kept their January 1st portfolio unchanged for the entire year. The stocks people sold went on to perform better than the stocks they bought to replace them. Men’s replacement picks underperformed by 20 basis points per month. Women’s by 17 basis points.

So every time these investors made a “smart” move, they actually made things worse.

The Four Deadly Behaviors

Pompian identifies four specific ways overconfidence hurts investors:

1. You think you can pick stocks. Many investors believe they have a special ability to identify good investments. The data says otherwise. After trading costs but before taxes, the average investor underperforms the market by about 2% per year. And mutual fund investors do even worse: from 1984 to 1995, the average stock mutual fund returned 12.3% annually, but the average investor in those same funds earned only 6.3%. Why? Because people kept jumping in and out at the wrong times.

2. You trade too much. Every trade has costs. Commissions, spreads, taxes. And every trade is also a prediction that could be wrong. The more you trade, the more opportunities you create to be wrong and to pay for the privilege.

3. You underestimate downside risk. If you are prediction-overconfident, you think the range of possible outcomes is narrower than it really is. You are not prepared for big losses because you did not think they were possible. Then reality hits.

4. You hold underdiversified portfolios. Think about the former executive who holds 80% of their wealth in their old company’s stock because they “know” the company. Or the tech enthusiast who loaded up on nothing but tech stocks in the late 1990s. Overconfidence makes people think they do not need diversification because they already picked the winners.

The Uncomfortable Truth About Financial Planning

Pompian ends the chapter with a sobering observation. Most parents with school-age children say they have a long-term financial plan. They feel confident about their financial future. But a vast majority of these same households have not actually saved enough for educational expenses, and very few have a real financial plan covering investments, budgeting, insurance, savings, and wills.

This is overconfidence in action on the life-planning level. People feel confident that things will work out, so they do not actually do the work to make sure things work out. Then they are surprised and discouraged when goals are not met.

What To Do About It

Track your trades. If you think you are a great stock picker, prove it to yourself. Look at your actual trading records for the past two years. Calculate your real performance. I suspect most people who do this exercise get a reality check pretty quickly.

Understand market volatility. Read some history. Look at what markets actually do during crashes, corrections, and recoveries. The numbers might shock you into having more realistic expectations.

Question your confidence. When you feel 100% sure about an investment, remember that in studies, 100% confidence correlates with only about 80% accuracy. Build in some humility buffer.

Diversify. If you hold a concentrated position and someone asks you “If you did not own any of this stock today, would you buy as much as you own right now?” and the answer is no, that tells you something important.

My Take

As someone who spent 20 years in IT, I have seen overconfidence everywhere. Engineers who are sure their code is bug-free. Project managers who are sure the deadline is realistic. People who are sure they understand a system they just started looking at yesterday.

Investing is no different. And the worst part is that overconfidence does not feel like a problem. It feels like competence. You think you are just being smart and well-informed. But the data is clear: the more confident you are, the more you trade. The more you trade, the worse you do.

The simplest investment advice might be the hardest to follow: do less.

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This is part of a series retelling “Behavioral Finance and Wealth Management” by Michael M. Pompian. Start from the beginning.