How common is it?
In a 1994 study by Buehler and colleagues, students predicted they'd finish their senior theses in about 34 days; it actually took about 55. Fewer than half finished even by their "worst-case" estimate. The Sydney Opera House opened a decade late at roughly 14 times its budget. We focus on the best-case scenario and ignore interruptions and past experience.
How to beat it
Kahneman recommends the outside view: instead of picturing your plan's details, ask how long similar projects actually took. Bent Flyvbjerg formalized this as reference class forecasting for megaprojects. Breaking work into pieces and adding buffers also helps. Combined with procrastination, the planning fallacy is a recipe for all-nighters.
FAQ
- Why does it repeat even with experience?
- We blame past delays on unusual circumstances and assume this time is different.
- Do we underestimate others' tasks too?
- Observers tend to be more realistic, sometimes even pessimistic.
Sources reviewed: 2026-10-26
References
- Kahneman, D., & Tversky, A. (1979). Intuitive prediction: Biases and corrective procedures. TIMS Studies in Management Science, 12, 313–327.
- Buehler, R., Griffin, D., & Ross, M. (1994). Exploring the "planning fallacy". Journal of Personality and Social Psychology, 67(3), 366–381.
- Flyvbjerg, B. (2006). From Nobel Prize to project management: Getting risks right. Project Management Journal, 37(3), 5–15.