Posted by: Gabriele Paolacci | October 6, 2009

Conjunction fallacy

We submitted the classic “Linda problem” from Tversky and Kahneman (1983) to fifty workers (23 women, 46% women, mean age = 32.9). Participants were paid $0.05 for a HIT that comprised other unrelated brief tasks. Participants were given the following information and the following question:

Linda is 31 years old, single, outspoken, and very bright. She majored in philosophy. As a student, she was deeply concerned with issues of discrimination and social justice, and also participated in anti-nuclear demonstrations.

Which of these two alternatives is more probable?

  • Linda is a bank teller.
  • Linda is a bank teller and is active in the feminist movement.

43 participants (86%) indicated the second alternative as more probable, thus exhibiting the classic conjunction fallacy. Such result is consistent with previous research.


Tversky, A., Kahneman, D. (1983). Extensional versus intuitive reasoning: The conjunction fallacy in probability judgment. Psychological Review, 90, 293–315.


  1. Okay, I know that this is not principally an interpretation ‘blog, but (as Gigerenzer would note, albeït with much obfuscating pirouetting) the natural language “and” doesn’t mean the same thing as “∧”. It often carries a sense of subsequence or even consequence. The second statement could be read as a of form (X|Y). And, while prob(X) ≥ prob(X∧Y); prob(X) may be less than prob(X|Y).

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