Test each component probability separately

Before judging a joint claim, estimate each element on its own, then check whether the conjunction is lower.

Why it works

The conjunction fallacy arises because the mind evaluates scenarios by how well they fit a mental prototype, not by the arithmetic of probability. Forcing yourself to assign a probability to each component independently imposes the logical constraint that P(A and B) ≤ P(A) — a rule our intuition routinely violates when B makes A "make more sense." Explicit decomposition activates deliberate reasoning and dislodges the representativeness anchor.

How to do it

1. Identify the conjunction: write out the two or more claims being combined ("she is a teller AND an activist").
2. Estimate the probability of each element independently, without letting one inform the other.
3. Apply the logical constraint: the joint claim cannot be more likely than the least likely element.
4. If your initial intuition violates this, treat that gap as a prompt to revisit the evidence.

Evidence

Tversky and Kahneman (1983) demonstrated the conjunction fallacy across many populations and scenarios, including medically trained subjects; the error is robust and not explained by misunderstanding the question. (observational)

Some researchers argue that subjects interpret "probable" as "plausible" or that conversational pragmatics shift the meaning; the core finding is nonetheless replication-stable.

Sources

• Tversky & Kahneman (1983), Extensional versus intuitive reasoning: The conjunction fallacy in probability judgment, Psychological Review

Common mistake

Decomposing the conjunction into components but still letting the narrative connection between them inflate the joint estimate — the components must be assessed truly independently.