Conjunction Fallacy
The conjunction fallacy occurs when someone judges two events occurring together as more probable than one of those events alone. A detailed combination can feel more representative or coherent even though it cannot be more probable than its broader category.
THE IDEA
DISCUSSION
In the course of this project we studied the extension rule in a variety of domains; we tested more than 3,000 subjects on dozens of problems, and we examined numerous variations of these problems. The results reported in this article constitute a representative though not exhaustive summary of this work.
The data revealed widespread violations of the extension rule by naive and sophisticated subjects in both indirect and direct tests. These results were interpreted within the framework of judgmental heuristics. We proposed that a judgment of probability or frequency is commonly biased toward the natural assessment that the problem evokes. Thus, the request to estimate the frequency of a class elicits a search for exemplars, the task of predicting vocational choice from a personality sketch evokes a comparison of features, and a question about the cooccurrence of events induces an assessment of their causal connection. These assessments are not constrained by the extension rule. Although an arbitrary reduction in the extension of an event typically reduces its availability, representativeness, or causal coherence, there are numerous occasions in which these assessments are higher for the restricted event than for the inclusive event. Natural assessments can bias probability judgment in three ways: The respondents (a) may use a natural assessment deliberately as a strategy of estimation, (b) may be primed or anchored by it, or (c) may fail to appreciate the difference between the natural and required assessments.
When to use it
Diagnosis: check whether a proposed combination of diseases has been rated more probable than one of its component diseases occurring with or without the other.
Legal reasoning: compare the probability of guilt with the probability of guilt plus a particular motive. Adding a plausible motive to the story cannot make that conjunction more probable than guilt alone.
Investment analysis: check a forecast that rates low inflation plus high growth as more likely than low inflation regardless of growth.
FROM THEORY TO EVERYDAY LIFE
In practice
Linda the Bank Teller
Participants are presented with a description: 'Linda is 31 years old, single, outspoken, and very bright. She majored in philosophy. As a student, she was very concerned with issues of discrimination and social justice, and also participated in anti-nuclear demonstrations.' Then asked which is more probable: (1) Linda is a bank teller, or (2) Linda is a bank teller and is active in the feminist movement. Despite the conjunction rule, most people choose (2) because it seems more representative of the description.
A MOMENT TO REFLECT
Put it to work.
Recall a personal decision where you assumed a specific combination of events was more likely than a general category. How did representativeness influence your judgment?
In what professional fields (e.g., medicine, law) might the conjunction fallacy lead to costly errors? Provide an example.
How can awareness of the conjunction fallacy improve your interpretation of statistical data or risk assessments?
Often confused with
References
- Extension versus intuitive reasoning: The conjunction fallacy in probability judgment — Tversky, A., & Kahneman, D. (1983)
CONTINUE THE CONVERSATION
Source texts
- Heuristics and Biases The Psychology of Intuitive Judgment (Thomas Gilovich, Dale Griffin, Daniel Kahneman)