Planning fallacy
The planning fallacy is the systematic tendency to underestimate how long a task will take, even when you know that similar tasks have overrun in the past. Kahneman and Tversky named it in 1979, and Buehler, Griffin and Ross (1994) showed students predicting thesis completion dates missed their own worst-case estimates. It survives experience because we plan from an idealised inside view of the task rather than from past evidence.
| Field | Cognitive psychology / behavioural economics |
|---|---|
| Key researchers | Daniel Kahneman, Amos Tversky, Roger Buehler |
| Related concepts | Optimism bias, inside vs outside view, reference class forecasting, Parkinson's Law |
The evidence
In Buehler, Griffin and Ross (1994, Journal of Personality and Social Psychology), students estimated when they would finish their honours theses. The average prediction was about 34 days; the average reality was about 56. Fewer than a third finished by the time they had predicted assuming everything went as badly as it possibly could. The bias appears for taxes, renovations, software projects and essays alike, and interestingly, people predict other people's tasks more accurately than their own.
Why it matters for procrastination
Underestimating duration makes delay feel safe: if the report only needs two hours, starting the night before seems fine. When the estimate proves wrong, panic, all-nighters and shame follow, and the shame feeds the next round of avoidance. The proven counter is the outside view: base estimates on how long similar tasks actually took, not on how this one feels. Practical versions include multiplying your gut estimate by 1.5 to 2, or timeboxing work against a real calendar rather than an imagined one.
Related
Frequently asked questions
Why do I always underestimate how long things take?
Because you plan from a best-case mental simulation of the task and ignore the base rate of past overruns. The fix is mechanical, not motivational: look up how long similar tasks actually took and use that number instead of your feeling.