Conjunction fallacy
Cognitive biases
Hey all,
Welcome to Human Nature, the illustrated psychology newsletter.
In this penultimate article in the series of cognitive biases, we explore the conjunction fallacy.
Conjunction fallacy
What it is: The conjunction fallacy is the error in judgement we make when we estimate the probability of two events occurring at the same time to be higher than the probability of any one of those events occurring alone.
For example, Gail works in a bookshop. Which is more likely to be true? a) Gail wears glasses, or b) Gail wears glasses and is an avid reader. If you answered b, then you committed a conjunction fallacy.
The reason b is the wrong answer in the above scenario has to do with fundamental rules of probability. The probability of two separate events occurring at the same time is always lower than or equal to the probability of one of those events occurring alone. If you think of the probabilities of the two events as two circles forming a Venn diagram, the part where they overlap is the probability of the two of them occurring together.
How its was discovered: In 1983, Tversky and Kahneman presented undergraduate psychology students with the following description:
“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 antinuclear demonstrations.”
They then asked the students to estimate the probability of the following statements:
a) Linda is a bank teller.
b) Linda is active in the feminism movement.
c) Linda is a bank teller and is active in the feminism movement.
87% of the students said it was more likely for Linda to be a bank teller and a feminist (c) than for Linda to only be a bank teller (a).
How it works: When we make inferences about the world, we rely on the representativeness heuristic. Based on the description above, Linda fits the bill of someone who would be active in the feminist movement, but less so of a bank teller. Therefore, when faced with the choice of Linda being a bank teller or a bank teller and active in the feminist movement, participants go with the less probable option because it feels more representative based on the information they have about Linda and their prior experiences.
Thank you for reading, see you next time!
Sources:
Extensional versus intuitive reasoning: The conjunction fallacy in probability judgment. (Tversky & Kahneman, 1983).
The Conjunction Fallacy? (Wolford, Taylor & Beck, 1990).
Cognitive biases are systematic errors in thinking that we all make and that affect our judgement and behaviours. We humans like to believe we’re rational thinkers, but our brains aren’t quite wired that way. Cognitive biases are just one of the ways in which our thinking is flawed, so it’s good to be aware of them.