Rationality: What It Is, Why It Seems Scarce, Why It Matters

by Steven Pinker

Kahneman and Tversky singled out a major ineptitude in our Bayesian reasoning: we neglect the base rate, which is usually the best estimate of the prior probability.5 In the medical diagnosis problem, our heads are turned by the positive test result (the likelihood) and we forget about how rare the disease is in the population (the prior). The duo went further and suggested we don’t engage in Bayesian reasoning at all. Instead we judge the probability that an instance belongs to a category by how representative it is: how similar it is to the prototype or stereotype of that category, which we mentally represent as a fuzzy family with its crisscrossing resemblances (chapter 3). A cancer patient, typically, gets a positive diagnosis. How common the cancer is, and how common a positive diagnosis is, never enter our minds. (Horses, zebras, who cares?) Like the availability heuristic from the preceding chapter, the representativeness heuristic is a rule of thumb the brain deploys in lieu of doing the math.6 Tversky and Kahneman demonstrated base-rate neglect in the lab by telling people about a hit-and-run accident by a taxi late at night in a city with two cab companies: Green Taxi, which owns 85 percent of the cabs, and Blue Taxi, which owns 15 percent (those are the base rates, and hence the priors). An eyewitness identified the cab as Blue, and tests showed that he correctly identified colors at night 80 percent of the time (that is the likelihood of the data, namely his te...