Please consider Linda, a 31-year-old woman, single and bright. When she was a student, in high school and in college too, she was deeply involved in social justice issues, and also participated in environmental protests. Which is more probable about Linda’s occupation today?
a) Linda works as a TV reporter;
b) Linda is a bank teller;
c) Linda is a bank teller and active in the environmental movement.
Quick, what’s your answer? a) or b) or c)? And in what precise order?
First, ignore how you ranked a), as it is irrelevant to this particular exercise. The key is this: If you ranked c) as more probable than b), you are wrong…and in very good company. That’s what most of my Stanford colleagues and I answered the first time we faced this teaser, and it reflects a very pervasive cognitive bias, technically called a “conjunction fallacy.”
Statistically speaking, by definition, it is more probable that Linda is a bank teller, which is a whole category, that she is both a bank teller AND also active in the environmental movement, which is a subset of that whole category.
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