Cognitive Scaffold

Preparing your thinking workspace

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MENTAL MODEL · M4984

Disjunction Fallacy

Disjunction Fallacy
SystemsmediumComplexity Science
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Updated 2026-08-11

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INTRODUCTION

English translation pending.

CORE DEFINITION

The disjunction fallacy, studied in probability judgment research, concerns errors in estimating the chance that at least one of several events occurs. Its core claim is that people underestimate how failure probability compounds across many components: with a one percent failure rate per part, a system of one hundred parts fails about sixty-three percent of the time. The key qualification is that overlapping events must not be double-counted when adding probabilities.

SCAFFOLDING EFFECT

psychology

Reduce cognitive load

- Count the links: multiply the success rates along a long chain instead of admiring each step. - Shorten the chain: remove steps or handoffs so fewer things have to go right. - Add parallel paths: build redundancy so one failure does not end the whole sequence.

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Anchor fast decisions

Failure probability compounds because a chain succeeds only if every step succeeds, so the overall rate is the product of the individual rates and falls faster than any single one. People instead evaluate each step separately and judge the sequence by its typical step, which hides the accumulation. Conversely, when several events can each cause an outcome, their probabilities overlap, and adding them naively overstates the total.

MINIMUM ACTION

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Source support: Explicit

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