Plausible denial

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.

Also think about why you think the option you chose is more probable.

Cognitive psychologists Tversky and Kahneman posed this question in an experimental study and found that 90% of respondents chose the second option. However the correct answer is the first option. The reason it is correct is that two conditions occurring (being a bank teller and being a feminist) is less likely than for one of the conditions to occur (being a bank teller regardless of whether she is a feminist or not). Tversky and Kahneman coined this the Conjunction Fallacy.

What is true is that the second option is a more representative, more coherent and more plausible match with Linda's personality description. Linda doesn't really seem the bank teller type but being a feminist would certainly be in character. Thus, instead of making a judgment about probability, most people substitute a judgment about similarity instead.

Does this actually matter in practice? Yes, it matters when important life choices are made based on similarities rather than probabilities. For example, a doctor may diagnose a patient's illness due to symptoms being representative of a particular disease (or even of having no disease at all) when in fact other diseases are more likely.