Every March, when a new semester begins, I repeat the same mistake. “This semester, I’ll prepare my lesson materials well in advance.” The result is always the same — by the end of the semester, I’m up all night scrambling to finish materials before deadlines.
At first I thought it was a willpower problem. But it turned out this wasn’t just my issue. It’s a cognitive bias called the Planning Fallacy.
What Is the Planning Fallacy?
The Planning Fallacy is a cognitive bias first named in 1979 by Daniel Kahneman and Amos Tversky[1]. The core insight: people systematically underestimate how long their projects will take and how much they will cost.
Related: optimize your sleep
What makes this bias peculiar is that we can predict other people’s projects fairly accurately. “She’s going to write a book? That’ll take at least a year.” But when we ourselves sit down to write a book, we think: “Three months should be enough.”
Kahneman explains why this happens: when we plan, we anchor on the best-case scenario. We imagine everything going smoothly, no obstacles, working focused every single day. But reality is never the best-case scenario[2].
The Planning Fallacy in History
The Planning Fallacy isn’t just a personal problem — it appears repeatedly in large-scale public projects.
Danish transport infrastructure researcher Bent Flyvbjerg analyzed 258 major infrastructure projects worldwide and found that 90% exceeded their budgets, with an average overrun of 28%[3]. What’s more striking is that this pattern has been repeating for decades. We are not learning from history.
A famous example: the Sydney Opera House. Originally scheduled to be completed in 1963 with a budget of $7 million. It was actually completed in 1973 at a cost of $102 million — ten years late and 15 times over budget.
Planning Fallacy Patterns I Discovered as a Teacher
After five years in the classroom, I’ve identified several recurring patterns of the Planning Fallacy in my own work.
Pattern 1: Underestimating lesson material prep time. “This slide deck will take an hour.” In reality, it takes three — searching for images, reviewing content, tweaking layouts adds up fast.
Pattern 2: Underestimating administrative task time. “Writing one memo should take 30 minutes.” Add in approval system errors, file format issues, and revision requests from supervisors — suddenly it’s three hours.
Pattern 3: Underestimating student comprehension time. “Students can grasp this concept in 10 minutes.” Reality is always more complex: students with ADHD, different learning paces, students who didn’t sleep the night before.
Overcoming the Planning Fallacy: The Outside View
Kahneman says the most effective way to overcome the Planning Fallacy is to take the outside view[2] — treating your project not as your own unique special case, but as a statistical instance among similar projects.
The practical approach:
- Find a reference class: “How long does it typically take someone like me to do a project like this?”
- Establish a baseline: Use the statistical average as your anchor point.
- Adjust for internal factors: Adjust slightly to reflect what’s unique about your situation — but minimize optimistic adjustments.
For example, I now think like this when making lesson materials: “This type of material typically takes me two hours. This one’s a bit more complex, so I’ll budget three hours.” Doing this dramatically reduced the gap between my estimates and actual time spent.
ADHD and the Planning Fallacy: A Double Vulnerability
People with ADHD are especially vulnerable to the Planning Fallacy — for two reasons.
First, time blindness. One of the core symptoms of ADHD is the inability to intuitively feel the passage of time. You know “there’s a meeting in 30 minutes,” but you can’t viscerally feel how short that 30 minutes actually is.
See also: ADHD time blindness
Second, optimistic planning bias. Due to differences in the dopamine system, people with ADHD experience a particularly strong dopamine surge when making new plans. The feeling of “This time I can really do it!” hits harder — and that feeling gets in the way of realistic planning.
To overcome this, I started time tracking — recording the actual time every task takes, and consulting those records when making future plans. My past records were shocking at first. “I thought I could do this in one hour — it actually took three, every time.”
Practical Strategies to Resist the Planning Fallacy
Philip Tetlock identifies systematic records of past errors as one of the hallmarks of good forecasters[4]. They compare their predictions to actual outcomes, find patterns, and improve future predictions.
Practical strategies:
- The 1.5x rule: Budget 1.5 times your initial estimate. (It may feel excessive at first, but it’s statistically accurate.)
- Explicit buffer time: Don’t lump “buffer” vaguely into your plan — build in concrete buffer blocks.
- Milestone breakdown: Split large projects into small units and estimate each one separately — accuracy improves.
- Post-project review: Record actual time spent after each project and incorporate it into future plans.
Eliezer Yudkowsky wrote on LessWrong: “Record the times you were wrong. And record why you were wrong. This is the core of rationality training[5].”
The Planning Fallacy never goes away. But you can recognize it and prepare for it. And that alone changes things considerably. Writing this very post took twice as long as I planned — but at least I knew it would.
Disclaimer: This article is for educational and informational purposes only. It is not a substitute for professional medical advice, diagnosis, or treatment. Always consult a qualified healthcare provider with any questions about a medical condition.
Your Next Steps
- Today: Pick one idea from this article and try it before bed tonight.
- This week: Track your results for 5 days — even a simple notes app works.
- Next 30 days: Review what worked, drop what didn’t, and build your personal system.
Last updated: 2026-03-16
About the Author
Written by the Rational Growth editorial team. Our health and psychology content is informed by peer-reviewed research, clinical guidelines, and real-world experience. We follow strict editorial standards and cite primary sources throughout.
References
- Kahneman, D., & Tversky, A. (1979). Intuitive prediction: Biases and corrective procedures. Management Science, 12, 313-327.
- Kahneman, D. (2011). Thinking, Fast and Slow. Farrar, Straus and Giroux.
- Flyvbjerg, B., Holm, M. K. S., & Buhl, S. L. (2002). Underestimating costs in public works projects. Journal of the American Planning Association, 68(3), 279-295.
- Tetlock, P., & Gardner, D. (2015). Superforecasting: The Art and Science of Prediction. Crown Publishers.
- Yudkowsky, E. (2008). Making Beliefs Pay Rent. LessWrong.