Conjunction fallacy

posted Nov 11, 2011, 11:02 AM by Abhishek Ojha   [ updated Nov 11, 2011, 11:02 AM ]

I always mention this in one way or another in discussions but I didn't know what it is called ! While reading this article by Michael Lewis. I learned about 'conjunction fallacy'. Here is what Wikipedia says about it.  I am glad to know about another principle which I always thought about and is very well known in psychological academic world...

The conjunction fallacy is a logical fallacy that occurs when it is assumed that specific conditions are more probable than a single general one.

The most often-cited example of this fallacy originated with Amos Tversky and Daniel Kahneman:

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 is more probable?

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

85% of those asked chose option 2. However the probability of two events occurring together (in "conjunction") is always less than or equal to the probability of either one occurring alone—formally, for two events A and B this inequality could be written as \Pr(A \and B) \leq \Pr(A), and \Pr(A \and B) \leq \Pr(B).

 ...Tversky and Kahneman argue that most people get this problem wrong because they use the representativeness heuristic to make this kind of judgment: Option 2 seems more "representative" of Linda based on the description of her, even though it is clearly mathematically less likely.

More details on Wikipedia