Behaviour File 02 · The research behind Over-Optimism
The plan that slipped wasn’t written by fools.
Updated 5 October 2026
Psychology students were asked how long their final thesis would take.
Average guess: 33.9 days. Average reality: 55.5 days. Their own worst case: 48.6 days.
They still finished about a week later than their own worst case. The plan wasn’t written by fools. It was written by humans.
Researchers call this the planning fallacy. Bent Flyvbjerg’s 2021 review ranks optimism bias and the planning fallacy among the ten biases that matter most in projects.
It’s the published science behind one of the patterns in the KOZMO model, Over-Optimism. Knowing about the bias, the students showed, isn’t enough on its own to escape it.
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Sources: Buehler, Griffin & Ross (1994), Journal of Personality and Social Psychology 67(3); Flyvbjerg (2021), Project Management Journal 52(6), open access at arxiv.org/abs/2202.00125.