Optimism Bias: Why You Expect Better Than Average

Key Takeaways

• Optimism bias is overestimating your chance of good outcomes and underestimating your chance of bad ones.
• Neuroscientist Tali Sharot found people update beliefs more readily from good news than from bad news.
• The common mistake is projecting recent gains forward and ignoring base rates for failure.
• It pushes investors toward concentrated bets, thin cash buffers, and unrealistic return assumptions.

What It Is

Optimism bias is the inclination to expect that the future will be more favorable than the evidence supports. Neuroscientist Tali Sharot described it in her 2011 Current Biology review The optimism bias, drawing on brain-imaging work showing that people update their beliefs asymmetrically. When the news is better than expected, they revise their estimates strongly. When the news is worse than expected, they barely move.

That asymmetry matters for investing. It means optimistic forecasts get reinforced by good outcomes while bad outcomes are quietly discounted, so beliefs drift upward over time regardless of reality.

The Intuition

People generally rate their own chances above average, expecting more career success, better health, and longer lives than the statistics allow. The same applies to portfolios. Surveys repeatedly find that investors expect their own returns to exceed both the market and other investors, which is arithmetically impossible in aggregate.

The bias is comfortable because it feels like confidence and ambition. The problem is that it shifts your sense of probability without shifting the actual odds. You plan around the good case, prepare lightly for the bad case, and are then surprised when the average outcome arrives.

How It Works

The mechanism has two parts. First, you set an expectation that is more favorable than the base rate. Second, the asymmetric updating Sharot documented keeps that expectation high: confirming evidence is absorbed, disconfirming evidence is downweighted.

In a portfolio this produces predictable distortions. Return assumptions are set too high, so saving and spending plans rest on numbers the market rarely delivers. Cash buffers are too thin because a serious drawdown feels unlikely to happen to you. Positions become concentrated because the favored thesis seems almost sure to work. Each choice is reasonable if the optimistic forecast is right, and each is fragile if it is not.

Optimism bias also overlaps with the planning fallacy, the tendency to underestimate how long and how costly a plan will be, and with the illusion of control, the sense that you can steer outcomes you cannot.

Worked Example

Suppose an investor builds a retirement plan assuming 10 percent annual returns because the last few years delivered that. The long-run average for a diversified stock portfolio is closer to a more modest figure once inflation and weak decades are included.

Planning at 10 percent, the investor saves less today, confident the high return will close the gap. Reality delivers, say, 6 percent over the period. The shortfall compounds across decades into a meaningfully smaller balance than planned.

A more careful approach runs the plan at a conservative return, then treats any excess as a bonus rather than a foundation. The same logic applies to position sizing: assume your favorite idea has roughly the base-rate odds of any active bet, not a special exemption from failure.

Common Mistakes

1. Extrapolating recent gains forever. A strong few years feels like a new baseline. Use long-run averages that include weak periods, not the best stretch you can remember.

2. Underweighting the downside. Optimism bias makes losses feel like they happen to other people. Size positions and cash buffers as if the bad case is genuinely possible, because it is.

3. Over-concentrating in a favorite idea. Believing one thesis is almost certain leads to oversized bets. Diversify as if you might be wrong, which the base rates say you often will be.

4. Setting unrealistic return assumptions. Plans built on optimistic numbers quietly require you to save too little. Stress-test with a lower return and check whether the plan still holds.

5. Ignoring disconfirming evidence. Because bad news barely moves an optimistic mind, write down what would prove your thesis wrong in advance, then act when it appears.

Frequently Asked Questions

What is optimism bias investing in simple terms? It is expecting your own results to be better than average, so you assume your picks will win and your plan will go smoothly. The trouble is that the real odds do not give you that special treatment.

How does optimism bias affect investment decisions? It leads you to assume high returns, hold too little cash, and concentrate in favorite ideas. As the worked example shows, planning at 10 percent when reality delivers 6 percent leaves a large gap over time.

What is a real-world example of optimism bias? Most investors in surveys expect to beat the market and other investors, which cannot all be true at once. Sharot also noted that broad optimism helped many ignore evidence before the 2008 collapse.

How can investors avoid optimism bias effectively? Plan with conservative return assumptions and treat any excess as a bonus, not the base case. Write down in advance what would prove a thesis wrong, since optimistic minds resist bad news.

How is optimism bias different from overconfidence bias? Optimism bias is expecting good outcomes to favor you. Overconfidence bias is overrating the accuracy of your own knowledge and forecasts. They reinforce each other but describe different errors.

Sources

1. Sharot, T. (2011). "The optimism bias." Current Biology 21(23), R941-R945. https://www.sciencedirect.com/science/article/pii/S0960982211011912
2. The Decision Lab. "Optimism Bias." https://thedecisionlab.com/biases/optimism-bias
3. CFA Institute. "The Behavioral Biases of Individuals." Refresher Readings. https://www.cfainstitute.org/insights/professional-learning/refresher-readings/2026/the-behavioral-biases-of-individuals
4. Corporate Finance Institute. "Pessimist vs. Optimist Investors." https://corporatefinanceinstitute.com/resources/knowledge/trading-investing/pessimist-vs-optimist-investors/

Disclaimer

This article is educational content only and is not financial advice. Nothing here is a recommendation to buy, sell, or hold any security. Consult a licensed advisor before making investment decisions.