Why Smart People Make Bad Investment Decisions

Why this matters

Most of this course is about understanding investments. This module is about understanding the person making the decisions. That turns out to matter just as much: two people with the same knowledge and the same investments can end up with very different results, purely because of how they behave.

The same mistakes, again and again

In 2002, the Nobel Prize in economics recognized Daniel Kahneman for bringing insights from psychology into economics, "especially concerning human judgment and decision-making under uncertainty."¹ The research that followed, often called behavioral finance, has found that investors don't make random mistakes. They make the same ones, predictably.

A 2010 report prepared for the SEC by the Library of Congress summarized the pattern: many investors damage their portfolios by not diversifying enough, trading frequently, following the herd, favoring what's familiar, selling winners while holding losers, and giving in to optimism, short-term thinking, and overconfidence.²

None of those depends on how intelligent someone is. They come from the way everyone reacts to uncertainty, money, and other people.

What it costs

Two well-known pieces of research put numbers on it:

• Trading too much. Brad Barber and Terrance Odean studied 66,465 households at a large discount brokerage from 1991 to 1996. The households that traded most earned 11.4% a year while the market returned 17.9%. The average household turned over 75% of its portfolio each year. The authors pointed to overconfidence as an explanation.³
• Buying and selling at the wrong times. Morningstar estimates that the average dollar invested in US mutual funds and ETFs earned 7.0% a year over the 10 years ended December 31, 2024, about 1.2 percentage points a year less than the funds' own 8.2% return. It attributes that gap to the timing and size of investors' purchases and sales, and notes that it's equivalent to about 15% of the funds' total return.⁴

$10,000, invested for	At 8.2% a year	At 7.0% a year	Difference
10 years	$21,992	$19,672	$2,320
30 years	$106,370	$76,123	$30,247

The same funds, held by people who bought and sold at different times. Over 30 years, the gap costs more than a quarter of the ending value. The rates come from Morningstar's estimate; applying them for 30 years is a hypothetical illustration.

Why markets are such a hard teacher

People get good at things like driving or cooking through quick, clear feedback: do something, see the result, adjust. Investing gives feedback that's slow, noisy, and mixed with luck.

• Luck and skill look alike. A bad decision can make money for years, and a good one can lose money for years.
• Feedback arrives late. Whether a long-term investment was wise may not be clear for a decade.
• Emotions run high at exactly the wrong moments. The urge to act is strongest after big moves, which is when acting tends to hurt most.
• Everyone else is watching too. Prices, headlines, and other people's gains are visible all day, every day.

What helps

Trying harder in the moment rarely works, because the moment is the problem. What works is deciding in advance, while calm, and making the good choice the easy one:

• A written plan and allocation, from Part 5's What a Portfolio Is For.
• Automatic contributions, so investing doesn't depend on how you feel that month.
• Fewer decisions: broad funds and scheduled rebalancing, instead of frequent trades. Morningstar found that investors in all-in-one allocation funds, which rebalance for them, kept the largest share of their funds' returns.⁴
• Rules for buying and selling individual stocks, written before you need them.

The rest of this module looks at the main mistakes one at a time: Fear, Greed, FOMO, and Herding; Loss Aversion, Anchoring, and Sunk Costs; Overconfidence, Recency, and Narrative Bias; and how to judge your own decisions fairly in Process vs. Outcome.

Key takeaways

• Investors make the same mistakes predictably: under-diversifying, trading too much, following the herd, selling winners and holding losers, and overconfidence.
• The households that traded most in Barber and Odean's study earned 11.4% a year while the market returned 17.9%.
• Morningstar estimates the average fund dollar earned about 1.2 points a year less than the funds over 10 years, because of the timing of purchases and sales.
• Markets give slow, noisy, luck-filled feedback, so the best defense is rules and habits set in advance.

Check your understanding

Why did the average dollar in US funds earn less than the funds themselves in Morningstar's estimate?

1. Because the funds charged investors extra fees that weren't counted in their returns.
2. Because of when investors bought and sold, adding and pulling money at unfavorable times.
3. Because the funds reported returns before taxes and investors had to pay taxes on them.

An investor made money on a trade made on a whim. What's the most reasonable conclusion?

1. The whim was a good decision, since the result proves the reasoning was sound.
2. The investor has a talent for timing and should trade on instinct more often.
3. Very little: in markets, a single good result can come from luck as easily as skill.

What's next

The strongest emotions in investing are the ones that spread through crowds. Fear, Greed, FOMO, and Herding looks at how they drive panics and bubbles, and what to do when you feel them.