Reasoning error · probability statistics

Base-rate neglect

Base-rate neglect occurs when a prior frequency or prevalence is ignored or underweighted relative to vivid, specific or seemingly diagnostic information about an individual case.

Also known as: base-rate fallacy

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How to recognize it

  • A probability judgment is made without asking how common the candidate explanations were before the new evidence.
  • A vivid case description outweighs large differences in prevalence.
  • Test accuracy is discussed without the underlying rate of the condition or event.

How to reduce it

  • Write down the prior/base rate before interpreting case-specific evidence.
  • Use natural frequencies or concrete counts when possible.
  • Compare posterior possibilities, not just whether one clue is compatible with a hypothesis.

Base rates are not always the final answer. Strong diagnostic evidence can legitimately overwhelm a prior. The error is skipping the prior entirely.

This matters especially when the candidate explanation is rare. A clue can be ‘consistent with’ a rare explanation and still leave a common explanation far more probable.

Research basis

Research & reasoning sources

  1. The base-rate fallacy in probability judgments Acta Psychologica · 1980 · peer reviewed study · DOI 10.1016/0001-6918(80)90046-3

    Classic empirical account of underweighting base rates when more specific or apparently causal information is available.

  2. Judgment under Uncertainty: Heuristics and Biases Science · 1974 · academic resource · DOI 10.1126/science.185.4157.1124

    Foundational review of heuristics affecting judgment under uncertainty, including the use and neglect of prior probability information.

Investigation ↔ Learn

Claims using this reasoning error

9
Unsupported Reviewed Oct 15, 2026

Do ships and aircraft disappear unusually often in the Bermuda Triangle?

Famous disappearances in the Bermuda Triangle are real historical incidents, but memorable cases do not establish a higher risk per voyage or flight. NOAA says there is no evidence of unusually frequent mysterious losses. A 2022 analysis examined maritime and aviation incident records but lacked the traffic-exposure denominators needed to compare disappearance rates with other regions. The elevated-rate claim is unsupported; the exact comparative rate remains uncertain.

Unsupported Reviewed Oct 10, 2026

Do most people with bloating, fatigue or skin symptoms have hidden intestinal parasites?

Parasitic infections are common in many endemic regions and can be asymptomatic or produce nonspecific gastrointestinal, skin or systemic symptoms. But risk is strongly shaped by geography, sanitation, travel, food, water and exposure, and major public-health guidance does not support assuming that most people with ordinary nonspecific symptoms have parasites. Diagnosis is based on exposure context and appropriate stool, blood or other testing.

Contradicted Reviewed Oct 8, 2026

Do vaccines cause autism?

The vaccine-autism claim has been tested in large cohort studies, case-control studies, meta-analyses and evidence reviews. A 2019 Danish cohort of 657,461 children found no increased autism risk after measles–mumps–rubella (MMR) vaccination, including among children with elevated baseline risk, and a 2014 meta-analysis summing more than 1.2 million cohort study enrollments found no increased autism risk associated with vaccination, MMR, thimerosal or mercury exposure; overlap among contributing national cohorts means this is not a verified count of unique children. The 1998 paper that catalyzed the MMR claim was a 12-child case series, did not establish causation and was later retracted.

Misleading Reviewed Oct 7, 2026

Can a polygraph reliably determine whether someone is lying?

Polygraph comparison-question tests often classify deception above chance in controlled studies, and a 2021 meta-analysis reported substantial discrimination. But the measured physiological responses are not specific to lying, field validation is difficult, accuracy varies by protocol and population, and screening applications are especially vulnerable to false positives. Calling the polygraph a generally reliable individual lie detector overstates what the evidence supports.

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