The great psychologist William James was Gertrude Stein's teacher and mentor. As legend tells it, James once posed a single question on a final examination: "What is risk?"
Stein wrote, "This is," walked out of the examination room and went about her business. Supposedly James gave Stein an A.
After a tragedy such as the one in Boston, people have a heightened sense of risk. If a flood, an earthquake, a violent crime or a terrorist attack has occurred in the recent past, people tend to have a feeling of vulnerability, captured in the alarming idea that "you can't be safe anywhere." Often that feeling is far greater than reality warrants. This is so because of two facts about how human beings respond to risk.
The first is that we often assess probabilities not by looking at statistics, but by asking what events come readily to mind. If you are unable to think of a case in which a crime occurred in your neighborhood, or of a situation in which an accident resulted from talking on a mobile phone while driving, you might not much worry about crime or distracted driving. But if your neighbor was recently robbed, or if a friend was badly injured in a crash caused by distracted driving, you might think that the risk is pretty high.
People use the "availability heuristic," which means that we assess risks by asking whether a bad (or good) event is cognitively "available." It is hardly unreasonable to use the availability heuristic, yet we can be misled by it, and far more frightened than we need to be.
A bad event may have occurred in the recent past, but it might have been a fluke, and the risk might be really low. Even if there was a robbery in your neighborhood last month, there might be no reason for alarm.
When a terrible event produces widespread fear, it is often because of the availability heuristic. A tragic event becomes so public, and so memorable, that people feel at risk whether or not they really are.
The second problem is that for some risks, we tend to focus mostly on the possible outcome, and not so much on the likelihood that it will actually come to fruition. Much of the time, of course, we really care about probability. If you are asked how much you would pay to buy a 1 percent chance of winning $500, you will say a lot less than if you are asked how much you would pay to buy a 99 percent chance of winning $500.
But when people's emotions are running especially high, the outcome is the dominant consideration, and it can crowd out consideration of probability. Studies show that when people are asked how much they would pay to avoid a 1 percent chance of getting a painful electric shock, their answer is only slightly lower than what it is when they are asked how much they would pay to avoid a 99 percent chance of getting such a shock.
The lesson is straightforward. In situations that trigger strong negative emotions, people tend to focus on the very worst that might happen, and the question of probability turns out to be secondary. Those who sell life insurance exploit this feature of human nature. You might be able to persuade people to buy insurance if you can get them to think about the economic risks faced by their family.
When terrorists succeed in generating widespread fear, it is also because they get people to focus on terrible outcomes, and not on the likelihood that they will come about. Because strong emotions are produced by the prospect of a terrorist attack, people might well become more frightened than reality warrants.
It is hardly irrational to be scared in the immediate aftermath of an attack, when people may not know the scope of the threat and the extent of the danger. An elevated sense of vulnerability is hard to avoid. Yet even when individuals are highly unlikely to be at risk, they might remain fearful, simply because the horrible outcome is so vivid.
Nonetheless, it is possible for people to have a realistic sense of what risk is and what it isn't. Going about your business can be a good way to reduce individual, social and economic harm - and it can be a forceful answer to those who seek to frighten us.
Cass R. Sunstein, a professor at Harvard Law School, is a Bloomberg View columnist. He is the former administrator of the White House Office of Information and Regulatory Affairs.