Other people are frequently irrational, and infuriating. But how do we tell, really, when they’re irrational? After all, they often get pretty angry when we point their irrationality out to them.

Examples of irrationality include thinking illogically, engaging in behaviour that is not in our best interests, or having emotional responses that are not warranted by circumstances. If I believe that it is raining and not raining at the same time, this is irrational. But how dare you suggest my emotional reaction is unwarranted? I’m always perfectly reasonable. Psychologists have also uncovered biases in people’s reasoning processes that seem to show that we do not think clearly, or rationally. Many of these biases were uncovered by asking people to respond to scenarios. One of these famous examples is ‘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.

What did you pick?

‘2’ is the wrong answer. I picked ‘2’ when first given this problem and am consequently really keen to prove that I’m not an idiot.

Psychologists have also uncovered biases in people’s reasoning processes that seem to show that we do not think clearly, or rationally. 

Daniel Kahneman and Amos Tversky, who developed this example and tried it out on people, argue that picking ‘2’ is a conjunction error. We don’t realise that the probability of two events occurring together is always less than or equal to the probability of either one occurring alone. If we compare two groups: all female bank tellers, and all female bank tellers who are active in the feminist movement, the numbers of feminist bank tellers will be equal or less than the group of all female bank tellers. So, it is more probable that Linda is a bank teller than that she is a feminist bank teller.

What’s going on here though? Somehow, this doesn’t feel right. If your response is ‘But that wasn’t the question I was answering’ you might be on to something.

Saving Linda

Probability is a term used in statistics, but without some prompt to guide us towards dredging up our secondary school statistics knowledge, ‘probable’ can be interpreted in a number of ways. In the context of the Linda example it can mean ‘Which is a better description of Linda?’ or ‘Which statement best characterises what kind of person Linda is?’. If we are to answer the question in terms of probabilities, then the descriptive information about Linda …

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