What is optimism bias?

The belief that you’re less likely than average to experience bad outcomes and more likely to enjoy good ones — overestimating your personal odds of success and safety.

What is the planning fallacy?

A close relative of optimism bias: underestimating how long a task will take and what it will cost, because we picture the smooth best-case path.

How do you counter optimism bias?

Anchor estimates on the base rates of similar cases, get an outside view, and plan deliberately for the realistic — not the hoped-for — scenario.

Optimism bias

Estimation & judgement

Optimism bias is the tendency to overestimate the likelihood of good outcomes and underestimate the bad — believing you’re personally less at risk than others of negative events, from illness to project overruns.

✦ Widely referenced — cross-referenced 18× across this reference (12 related ideas · 2 comparisons · 4 hubs) · The State of Thinking 2026 →

By the ReadGlobe Editors · Reviewed 2026-05-29

Why it happens

A positive self-image and an inflated sense of control lead people to see their own future as rosier than the base rates justify. It feels motivating and protective, so the mind under-weights realistic risk.

'This time is different' is the most expensive sentence in finance.

Examples

Assuming your project will finish on time though most similar ones overran — the planning fallacy.
Underestimating your personal risk of accident or illness.
Newlyweds certain they won’t divorce, despite the known rate.

How to counter it

Use base rates and the track record of similar cases, not hope.
Ask a neutral outsider for their estimate.
Plan for the realistic case, with a buffer, rather than the best case.

The deeper point

It’s why "this time is different" is the most expensive sentence in finance, and why nearly every project runs late. We plan for the single path we imagine, not the base rate of all the paths like it.

Frequently asked

What is optimism bias?: The belief that you’re less likely than average to experience bad outcomes and more likely to enjoy good ones — overestimating your personal odds of success and safety.
What is the planning fallacy?: A close relative of optimism bias: underestimating how long a task will take and what it will cost, because we picture the smooth best-case path.
How do you counter optimism bias?: Anchor estimates on the base rates of similar cases, get an outside view, and plan deliberately for the realistic — not the hoped-for — scenario.

Dunning–Kruger effect Self-serving bias Margin of safety Circle of competence

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Where it’s shows up in

See this alongside the other thinking tools of building a startup and software engineers.

Where it bites

This bias distorts planning & estimation and product decisions.

The books behind better thinking

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Prefer to read? The canonical picks:

Thinking, Fast and Slow — Daniel Kahneman
The Art of Thinking Clearly — Rolf Dobelli
The Great Mental Models, Volume 1 — Shane Parrish
Poor Charlie’s Almanack — Charlie Munger
Super Thinking — Gabriel Weinberg & Lauren McCann
Seeking Wisdom — Peter Bevelin

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Cite this page

APA

ReadGlobe. (2026). Optimism bias. https://readglobe.com/bias/optimism-bias/

MLA

"Optimism bias." ReadGlobe, 29 May 2026, readglobe.com/bias/optimism-bias/.

Primary source: Wikipedia

Editorial synthesis © ReadGlobe 2026, drawing on Kahneman’s Thinking, Fast and Slow, the Tversky–Kahneman research program, and the primary cognitive-science literature. · Last reviewed 2026-05-29.