The error of supposing that a joint probability can be higher than the probability of the separate individual probabilities. Tversky and Kahneman (1983) illustrate it thus: 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 the more probable:
(1) Linda is a bank teller;
(2) Linda is a feminist;
(3) Linda is a bank teller and is active in the feminist movement?
Tversky and Kahneman (1983) report that 85 percent of respondents indicated that (3) is more likely than (1), thereby assigning a higher probability to the conjunction than its constituents. In describing and explaining behaviour, it seems wise to allow for such widespread departures from ' rationality '; when prescribing what ought to be done, however, the proper course is less clear.