Sunday, November 16, 2003

Hey Dick! (It's the aftermath, Stupid.)
Steven Jay Gould in The New York Review, from 1988.

- Amos Tversky, who studied "hot hands," has performed a series of elegant psychological experiments with Daniel Kahneman. These long-term studies have provided our finest insight into "natural reasoning" and its curious departure from logical truth. To cite an example, they construct a fictional description of a young woman: "Linda is thirty-one 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." Subjects are then given a list of hypothetical statements about Linda: they must rank these in order of presumed likelihood, most to least probable. Tversky and Kahneman list eight statements, but five are a blind, and only three make up the true experiment:

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

Now it simply must be true that the third statement is least likely, since any conjunction has to be less probable than either of its parts considered separately. Everybody can understand this when the principle is explained explicitly and patiently. But all groups of subjects, sophisticated students who ought to understand logic and probability as well as folks off the street corner, rank the last statement as more probable than the second. (I am particularly fond of this example because I know that the third statement is least probable, yet a little homunculus in my head continues to jump up and down, shouting at me—"but she can't just be a bank teller; read the description.")

Why do we so consistently make this simple logical error? Tversky and Kahneman argue, correctly I think, that our minds are not built (for whatever reason) to work by the rules of probability, though these rules clearly govern our universe. We do something else that usually serves us well, but fails in crucial instances: we "match to type." We abstract what we consider the "essence" of an entity, and then arrange our judgments by their degree of similarity to this assumed type. Since we are given a "type" for Linda that implies feminism, but definitely not a bank job, we rank any statement matching the type as more probable than another that only contains material contrary to the type. This propensity may help us to understand an entire range of human preferences, from Plato's theory of form to modern stereotyping of race or gender.-

More on this later.

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