Conjunction fallacy

The conjunction fallacy is a cognitive bias where individuals assume specific conditions are more probable than a single general one. This often occurs when people mistake the conjunction of two events as more likely than one of the events alone, violating the basic rule of probability.

How it works

This bias arises because people often rely on heuristic thinking instead of probabilistic reasoning. When presented with a narrative that seems more representative of real-world knowledge, individuals tend to favor it, despite it being statistically less probable. This simplification caters to the human need to make quick judgements rather than complex analyses.

Examples

• A famous example introduced by Tversky and Kahneman involves a fictional woman named Linda. When participants are asked whether Linda, who is described as a philosophy major and socially active, is more likely to be a bank teller or a bank teller who is also a feminist, the majority incorrectly choose the latter despite it being a less likely conjunction of traits.
• In a medical context, a patient might believe they have a rare disease because their symptoms align with those described for that disease plus their unique traits, rather than the far more common condition with a few overlapping symptoms.

Consequences

This fallacy can lead to poor decision-making in various fields, as the reliance on representative narratives over probabilistic data may result in incorrect judgments and beliefs. It can impact areas such as law, where juries may be swayed by emotional stories rather than factual evidence, and finance, where investors might assume unjustified correlations between market events.

Counteracting

To counteract the conjunction fallacy, individuals should be encouraged to engage in probabilistic thinking, assessing the base rates and statistical likelihood rather than relying on intuitions or narratives. Education and training focused on critical thinking and statistical literacy can help mitigate this bias.

Critiques

Some argue that the conjunction fallacy arises not solely from cognitive errors but also from misunderstandings in the framing of problems. They suggest that some instances attributed to the fallacy may involve different interpretations by respondents, where they focus on relevance or typicality instead of strict probability.

Fields of Impact

Also known as

Relevant Research

• Extensional versus intuitive reasoning: The conjunction fallacy in probability judgment

  Tversky, A., & Kahneman, D. (1983)

  Psychological Review, 90(4), 293

• The 'conjunction fallacy' revisited: How intelligent inferences look like reasoning errors

  Hertwig, R., & Gigerenzer, G. (1999)

  Journal of Behavioral Decision Making, 12(4), 275-305