The Linda Problem

Linda is described as thirty-one, outspoken and deeply concerned about social justice. People are asked which is more likely: that she is a bank teller, or that she is a bank teller active in the feminist movement. Most choose the second.

That cannot be right. Every feminist bank teller is a bank teller, so the second group sits inside the first and cannot be larger. Kahneman and Tversky called this the conjunction fallacy: judging two things together as likelier than one of them alone.

The error comes from resemblance. The longer description fits the picture of Linda better, so it feels more probable. A reliable check is to ask of any rich story: which plain part of it would have to be true anyway, and is that part likely?

The bank teller

A friend tells Ona about a stranger on the train, quiet, carrying paint-stained boots and a sketchbook. Which is likelier, she asks: that he is a teacher, or that he is a teacher who paints on weekends?

Ona feels drawn to the second. Then she sees the pattern. Teachers who paint are some of the teachers, so the plain answer has to be at least as likely. The extra detail made the story nicer, not likelier.

Try this today Next time a story seems very probable, strip it to its plainest version and compare.

More on this fallacy: Conjunction fallacy →