Base Rate Fallacy

If you’ve ever been a college student, you probably know that there are certain stereotypes attached to different majors. For example, students in engineering are often viewed as hardworking but cocky, students in business are stereotypically preppy and aloof, and arts students are activists with an edgy fashion sense. These stereotypes are wide generalizations, which are often way off the mark. Yet, they are frequently used to make projections about how individuals might act.

Renowned behavioral scientists Daniel Kahneman and Amos Tversky once conducted a study where participants were presented with a personality sketch of a fictional graduate student named Tom W. They were given a list of nine areas of graduate studies and told to rank them in order of likelihood that Tom W. was pursuing studies in that field. At the time, far more students were enrolled in education and the humanities than in computer science. However, 95% of participants said it was more likely that Tom W. was studying computer science than education or humanities. Their predictions were based purely on the personality sketch—the individuating information—with total disregard for the base rate information.¹

As much as that one person in your history elective course might look and act like the stereotypical medical student, the odds that they are actually studying medicine are very low. There are typically only a hundred or so people in that program, compared to the thousands of students enrolled in other faculties like management or science. It is easy to make these kinds of snap judgments about people since specific information often overshadows base rate information.