Читать книгу Philosophy of Psychology - Lisa Bortolotti - Страница 30

The next study involves the so-called ‘conjunction fallacy’ (also known as the conjunction effect), which is a mistake people make when they assume that a statement describing the conjunction of two states of affairs (e.g., ‘Tomorrow it will be raining and it will be cold’) is more probable than a statement describing one of those states of affairs alone (e.g., ‘Tomorrow it will be raining’).

In one experiment by Tversky and Kahneman (1983), participants first read personality sketches of hypothetical people, and then answered questions about them. Participants were divided into three groups according to their background in probability and statistics: the naïve group (those with no background), the intermediate group (those with basic knowledge of probability and statistics), and the sophisticated group (those with advanced knowledge of probability and statistics). Here is the personality sketch of a hypothetical person, 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 participated in anti-nuclear demonstrations.

Please rank the following statements by their probability, using 1 for the most probable and 8 for the least probable:

1. Linda is a teacher in elementary school.

2. Linda works in a bookstore and takes Yoga classes.

3. Linda is active in the feminist movement.

4. Linda is a psychiatric social worker.

5. Linda is a member of the League of Women Voters.

6. Linda is a bank teller.

7. Linda is an insurance salesperson.

8. Linda is a bank teller and is active in the feminist movement.

The result revealed that statements 3 and 7 were regarded, respectively, as the most probable and the least probable, which is explained by the similarity of Linda’s personality sketch with the stereotypical image of someone politically active, and the dissimilarity between her personality sketch and the stereotypical image of an insurance salesperson. The crucial statements to consider in this experiment are 6 and 8. The mean rank of the former was lower than the mean rank of the latter, which means that Linda being a bank teller and active in the feminist movement was regarded as more probable than her being a bank teller. This, however, is a mistake. Linda being a bank teller and active in the feminist movement is the conjunction of her being a bank teller and of her being active in the feminist movement. And the conjunction cannot be more probable than one of the conjuncts. Thus, this mistake is an obvious violation of the conjunction rule. This fallacy was found in all three groups of participants and background knowledge in statistics and probability did not have a significant effect on the participant’s performance.