Drunkard's Walk - Leonard Mlodinow [13]
If the details we are given fit our mental picture of something, then the more details in a scenario, the more real it seems and hence the more probable we consider it to be—even though any act of adding less-than-certain details to a conjecture makes the conjecture less probable. This inconsistency between the logic of probability and people’s assessments of uncertain events interested Kahneman and Tversky because it can lead to unfair or mistaken assessments in real-life situations. Which is more likely: that a defendant, after discovering the body, left the scene of the crime or that a defendant, after discovering the body, left the scene of the crime because he feared being accused of the grisly murder? Is it more probable that the president will increase federal aid to education or that he or she will increase federal aid to education with funding freed by cutting other aid to the states? Is it more likely that your company will increase sales next year or that it will increase sales next year because the overall economy has had a banner year? In each case, even though the latter is less probable than the former, it may sound more likely. Or as Kahneman and Tversky put it, “A good story is often less probable than a less satisfactory…[explanation].”
Kahneman and Tversky found that even highly trained doctors make this error.2 They presented a group of internists with a serious medical problem: a pulmonary embolism (a blood clot in the lung). If you have that ailment, you might display one or more of a set of symptoms. Some of those symptoms, such as partial paralysis, are uncommon; others, such as shortness of breath, are probable. Which is more likely: that the victim of an embolism will experience only partial paralysis or that the victim will experience both partial paralysis and shortness of breath? Kahneman and Tversky found that 91 percent of the doctors believed a clot was less likely to cause just a rare symptom than it was to cause a combination of the rare symptom and a common one. (In the doctors’ defense, patients don’t walk into their offices and say things like “I have a blood clot in my lungs. Guess my symptoms.”)
Years later one of Kahneman’s students and another researcher found that attorneys fall prey to the same bias in their judgments.3 Whether involved in a criminal case or a civil case, clients typically depend on their lawyers to assess what may occur if their case goes to trial. What are the chances of acquittal or of a settlement or a monetary judgment in various amounts? Although attorneys might not phrase their opinions in terms of numerical probabilities, they offer advice based on their personal forecast of the relative likelihood of the possible outcomes. Here, too, the researchers found that lawyers assign higher probabilities to contingencies that are described in greater detail. For example, at the time of the civil lawsuit brought by Paula Jones against then president Bill Clinton, 200 practicing lawyers were asked to predict the probability that the trial would not run its full course. For some of the subjects that possibility was broken down into specific causes for the trial’s early end, such as settlement, withdrawal of the charges, or dismissal by the judge. In comparing the two groups—lawyers who had simply been asked to predict whether the trial would run its full course and lawyers who had been presented with ways in which the trial might reach a premature conclusion—the researchers found that the lawyers who had been presented with causes of a premature conclusion were much more likely than the other lawyers to predict that the trial would reach an early end.
The ability to evaluate meaningful connections among different phenomena in our environment may be so important that it is worth seeing a few mirages. If a starving caveman sees an indistinct greenish blur on a distant rock, it is more costly to dismiss it as uninteresting when it is in reality a plump,