Have you met Linda?

Here's an interesting factoid you probably didn't know about yourself. Let me introduce you to Linda.

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.

Go ahead, pick one.

If you're like most people, you picked the second one. However, that's not correct. Why? Answer this similar question.

Consider of a room containing 100 people.

How many of those people are:

1. bank tellers?
2. bank tellers who are also active in the feminist movement?

Which set of people is larger? Of course, option one contains all bank tellers, both feminist and non-feminist.

This is an example of the conjunction fallacy, in which people answer the probability question by instead thinking of plausibility. But when you're asked the same question in terms of concrete numbers (instead of percentages), you are more likely to answer correctly.

I got this from Thinking Fast and Slow, by Daniel Kahneman.