Planning Fallacy: Why Forecasts Run Long and Over

Key Takeaways

• The planning fallacy is underestimating the time, cost, and risk of future actions while overstating the benefits.
• It comes from taking an "inside view" of a plan rather than comparing it to similar past cases.
• The common mistake is forecasting returns and timelines from a best-case story, not historical base rates.
• The fix is the "outside view": anchor estimates to how comparable situations actually turned out.

What It Is

Daniel Kahneman and Amos Tversky introduced the planning fallacy in 1979 to describe predictions about task completion that lean too optimistic. In 1994, Roger Buehler, Dale Griffin, and Michael Ross explored it in detail, showing that people underestimate their own completion times even while admitting that past projects of the same type ran long.

A later, broader definition by Lovallo and Kahneman frames it as the tendency to underestimate the time, costs, and risks of future actions while overestimating their benefits. That wider version maps directly onto investing, where every forecast bundles a timeline, a cost, and an expected payoff.

The Intuition

When you plan something, your attention goes to the specific steps in front of you, not to the long history of similar efforts. This is the inside view: you build the estimate from the details of this particular case, imagining each step going smoothly.

The outside view does the opposite. It ignores the specifics and asks how comparable cases actually turned out. People rarely take it on their own because the inside view feels more informed and more personal. The catch is that the inside view systematically omits the surprises, delays, and setbacks that the base rate already captures.

How It Works

The fallacy operates by substituting an imagined smooth path for the historical distribution of outcomes. You picture the plan working as designed, attach a timeline and a return to that picture, and treat the result as your forecast. The unglamorous reality, that things usually take longer and deliver less, never enters the calculation.

In investing this distorts three things at once. Timelines for a thesis to play out are set too short, so patience runs out before the payoff arrives. Cost and effort, such as taxes, fees, and the work of monitoring, are understated. Expected returns are pinned to the success scenario rather than to the spread of plausible results. The remedy is reference class forecasting: identify a class of similar past situations and start your estimate from their actual record, then adjust only with strong reason.

Worked Example

Suppose an investor buys into a turnaround thesis and projects the stock will double within 18 months as a new strategy takes hold. The estimate is built from the inside view: management's plan, the product roadmap, and a clean sequence of milestones.

The outside view asks a different question. Across a reference class of comparable turnarounds, how many doubled inside 18 months, and how long did the typical success actually take? If the base rate says most such stories take three years or more and many fail outright, the original forecast is far too optimistic on both timeline and probability.

Anchoring to that reference class, the investor sizes the position smaller, sets a longer horizon, and is not surprised when progress is slow. The thesis can still be right, but the plan no longer depends on the smoothest possible path.

Common Mistakes

1. Forecasting from the best case. Building a return estimate on the success scenario ignores the full range of outcomes. Start from the base rate, then adjust.

2. Taking only the inside view. Focusing on the specifics of your idea hides the delays that history would have warned you about. Deliberately seek a reference class.

3. Underestimating costs and frictions. Taxes, fees, slippage, and the time to monitor a position all eat into plans. Build them in rather than discovering them later.

4. Setting timelines too short. Theses usually take longer to play out than imagined, which exhausts patience. Pad the horizon toward what comparable cases needed.

5. Treating one example as a class. A single inspiring success is not a base rate. Use a genuine group of similar past cases, including the failures.

Frequently Asked Questions

What is the planning fallacy in simple terms? It is assuming a task or plan will go faster, cost less, and pay off better than it usually does. People expect the smooth path even when similar past efforts went off track.

How does the planning fallacy affect investment decisions? It leads to forecasts with timelines that are too short and returns that are too high, based on the success story rather than the odds. As the worked example shows, projecting a quick double can ignore a base rate that says such turnarounds take years.

What is a real-world example of the planning fallacy? Buehler and colleagues found people predict they will finish projects far sooner than they actually do, even knowing past projects ran long. Investors do the same when they assume a thesis will resolve quickly.

How can investors avoid the planning fallacy effectively? Use the outside view, also called reference class forecasting: estimate from how comparable past situations turned out, not from your plan's details. Then size positions and timelines to that record.

How is the planning fallacy different from optimism bias? Optimism bias is a general tilt toward expecting good outcomes. The planning fallacy is the specific form that distorts estimates of time, cost, and risk for a particular plan.

Sources

1. Buehler, R., Griffin, D. & Ross, M. (1994). "Exploring the planning fallacy: Why people underestimate their task completion times." Journal of Personality and Social Psychology 67(3), 366-381. https://web.mit.edu/curhan/www/docs/Articles/biases/67_J_Personality_and_Social_Psychology_366,_1994.pdf
2. Society for Personality and Social Psychology. "The Planning Fallacy: An Inside View." https://spsp.org/news-center/character-context-blog/planning-fallacy-inside-view
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. "Cognitive Bias." https://corporatefinanceinstitute.com/resources/capital-markets/list-top-10-types-cognitive-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.