Illusion of Explanatory Depth: Knowing Less Than You Think

Key Takeaways

• The illusion of explanatory depth is believing you understand how something works far better than you really do.
• Rozenblit and Keil showed the illusion is strongest for explanatory knowledge, not facts or procedures.
• The common mistake is investing on a thesis you cannot explain step by step to someone else.
• Trying to write the full mechanism out, before you buy, exposes and closes the gap.

What It Is

Yale researchers Leonid Rozenblit and Frank Keil named the effect in their 2002 Cognitive Science paper The misunderstood limits of folk science. Across a series of studies they asked people to rate how well they understood everyday objects such as a zipper or a flush toilet, then to write a detailed mechanistic explanation, then to rate their understanding again.

The pattern was consistent. Initial confidence was high, the written explanations were thin, and the second self-rating dropped sharply. People discovered, only when forced to explain, that they had confused familiarity with genuine understanding.

The Intuition

Recognizing something and explaining it are different skills. You see a stock chart, a balance sheet, or a product every day, and that familiarity feels like knowledge. The feeling holds up until someone asks you to walk through the actual mechanism, at which point the missing pieces appear.

Rozenblit and Keil found the illusion is far stronger for explanatory knowledge, the kind involving causal mechanisms, than for facts, procedures, or stories. That is exactly the kind of knowledge investing relies on. Why does this business compound, how does this product make money, what causes this instrument to move, are all causal questions where the illusion runs deepest.

How It Works

The effect runs in three steps. First, familiarity with a topic creates a feeling of understanding. Second, that feeling is rarely tested, because you seldom have to explain the mechanism out loud. Third, when an explanation is finally demanded, the gap surfaces and confidence collapses.

In a portfolio the danger is that the test usually comes too late, after a position goes wrong. An investor who "understood" a complex product or a high-flying business often turns out to have held a label, not a mechanism. The illusion also feeds overconfidence, since a strong sense of understanding inflates conviction and position size. The practical defense is to run the explanation test before committing money, not after.

Worked Example

Suppose an investor is drawn to a fast-growing software company and rates their understanding of the business as a clear 8 out of 10. The story is familiar: strong product, happy customers, rising revenue.

Now apply the Rozenblit-Keil test. Write out, in plain mechanism, how the company actually earns and keeps a dollar. How does it acquire a customer, at what cost, how long does that customer stay, what does it cost to serve them, where do margins come from, and what could break the loop. Many investors who felt like an 8 find they reach a 4 once they try to write it down.

That drop is not failure, it is information. The exercise either fills the gaps with real research or reveals that the conviction was borrowed from a narrative. Either way the investor sizes the position to what they truly understand, not to how familiar it felt.

Common Mistakes

1. Mistaking familiarity for understanding. Seeing a name often is not knowing how it works. Test comprehension by explaining the mechanism, not by how comfortable the name feels.

2. Investing in instruments you cannot explain. Complex products reward the illusion because the label sounds understood. If you cannot describe how it pays off and what breaks it, you do not understand it yet.

3. Skipping the write-it-down test. Confidence stays high until you try to explain. Writing the full causal story before buying is the cheapest way to find the holes.

4. Letting conviction set position size. A feeling of understanding inflates conviction and then size. Anchor size to demonstrated knowledge, not to felt certainty.

5. Outsourcing the thesis to a story. A compelling narrative can substitute for mechanism. Borrowed confidence collapses exactly when you need it most, during a drawdown.

Frequently Asked Questions

What is the illusion of explanatory depth in simple terms? It is thinking you understand how something works much better than you actually do. The gap stays hidden until you try to explain the thing in detail.

How does the illusion of explanatory depth affect investment decisions? It lets you hold confident views and large positions on businesses or products you cannot really explain. As the worked example shows, an investor who feels like an 8 on a company can drop to a 4 when forced to write out how it makes money.

What is a real-world example of the illusion of explanatory depth? Rozenblit and Keil found people confidently claimed to understand a zipper or a toilet, then could not explain the mechanism when asked. The same overconfidence applies to complex financial products.

How can investors avoid the illusion of explanatory depth effectively? Before buying, write out the full causal mechanism of how the investment makes and loses money. The act of explaining exposes the gaps and tells you whether to research more or pass.

How is the illusion of explanatory depth different from overconfidence bias? Overconfidence bias is overrating your judgment in general. The illusion of explanatory depth is the specific overrating of how well you understand how something works.

Sources

1. Rozenblit, L. & Keil, F. (2002). "The misunderstood limits of folk science: an illusion of explanatory depth." Cognitive Science 26(5), 521-562. https://pmc.ncbi.nlm.nih.gov/articles/PMC3062901/
2. The Decision Lab. "The Illusion of Explanatory Depth." https://thedecisionlab.com/biases/the-illusion-of-explanatory-depth
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. "Overconfidence Bias." https://corporatefinanceinstitute.com/resources/career-map/sell-side/capital-markets/overconfidence-bias/

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.