At least, it makes no sense to Head. To Gut, it makes perfect sense. One of Gut’s simplest rules of thumb is that the easier it is to recall examples of something, the more common that something must be. This is the “availability heuristic,” which I call the Example Rule.
Kahneman and Tversky demonstrated the influence of the Example Rule in a typically elegant way. First, they asked a group of students to list as many words as they could think of that fit the form _ _ _ _ _ n _. The students had 60 seconds to work on the problem. The average number of words they came up with was 2.9. Then another group of students was asked to do the same, with the same time limit, for words that fit the form _ _ _ _ ing. This time, the average number of words was 6.4.
Look carefully and it’s obvious there’s something strange here. The first form is just like the second, except the letters “i” and “g” have been dropped. That means any word that fits the second form must fit the first. Therefore, the first form is actually more common. But the second form is much more easily recalled.
Armed with this information, Kahneman and Tversky asked another group of students to think of four pages in a novel. There are about 2,000 words on those four pages, they told students. “How many words would you expect to find that have the form _ _ _ _ ing?” The average estimate was 13.4 words. They then asked another group of students the same question for the form _ _ _ _ _ n _. The average guess was 4.7 words.
This experiment has been repeated in many different forms and the results are always the same: The more easily people are able to think of examples of something, the more common they judge that thing to be.
Note that it is not the examples themselves that influence Gut’s intuitive judgment. It is not even the number of examples that are recalled. It is how easily examples come to mind. In a revealing study, psychologists Alexander Rothman and Norbert Schwarz asked people to list either three or eight behaviors they personally engage in that could increase their chance of getting heart disease. Strangely, those who thought of three risk-boosting behaviors rated their chance of getting heart disease to be higher than those who thought of eight. Logically, it should be the other way around—the longer the list, the greater the risk. So what gives? The explanation lies in the fact—which Rothman and Schwarz knew from earlier testing—that most people find it easy to think of three factors that increase the risk of heart disease but hard to come up with eight. And it is the ease of recall, not the substance of what is recalled, that guides the intuition.
The Rothman and Schwarz study also demonstrated how complex and subtle the interaction of Head and Gut can be. The researchers divided people into two groups: those who had a family history of heart disease and those who didn’t. For those who did not have a family history, the results were as outlined above. But those who did have a family history of heart disease got precisely the opposite results: Those who struggled to come up with eight risk-boosting behaviors they engage in rated their chance of getting heart disease to be higher than those who thought of three examples. Why the different result? People with no family history of heart disease have no particular cause for worry and nothing to base their judgment on, so they are more casual in their judgment and they go with the estimate that Gut comes up with using the Example Rule. But people with a family history of heart disease have a very compelling reason to think hard about this, and when they do, Head tells them that Gut is wrong—that, logically, if you engage in eight risk-boosting behaviors, your risk is higher than if you engage in three such behaviors. A similar study by different researchers—this time quizzing women about the risk of sexual assault—got similar results: Those who did not think the risk was personally relevant went with Gut’s estimate based on the Example Rule, while those who did corrected their intuition and drew a more logical conclusion.
As a rule of thumb for hunter-gatherers walking the African savanna, the Example Rule makes good sense. That’s because the brain culls low-priority memories: If time passes and a memory isn’t used, it is likely to fade. So if you have to think hard to remember that, yes, there was a time when someone got sick after drinking from that pond, chances are it happened quite a while ago and a similar incident hasn’t happened since— making it reasonable to conclude that the water in the pond is safe to drink. But if you instantly recall an example of someone drinking that water and turning green, then it likely happened recently and you should find somewhere else to get a drink. This is how Gut makes use of experience and memory.
The Example Rule is particularly good for learning from the very worst sort of experiences. A snake coils and hisses inches from your hiking boot. An approaching truck slips onto the shoulder of the highway and then weaves into your lane. A man presses a knife to your throat and tells you not to resist. In each case, the amygdala, a lump of brain shaped like an almond, will trigger the release of hormones, including adrenaline and cortisol. Your pupils dilate, your heart races, your muscles tense. This is the famous fight-or-flight response. It is intended to generate a quick reaction to immediate threats but it also contains one element intended to have a lasting effect: The hormones the amygdala triggers temporarily enhance memory function so the awful experience that triggered the response will be vividly encoded and remembered. Such traumatic memories last, and they are potent. Long after calm has returned, even years later in some cases, they are likely to be recalled with terrifying ease. And that fact alone will cause Gut to press the alarm that we experience as an uneasy sense of threat.
Even in circumstances much less dramatic than those that trigger the fight-or-flight response, the amygdala plays a key role. Neuroscientists have found that the amygdalas of people sitting in a quiet, safe university laboratory will suddenly spark to life when frightening or threatening images are shown. The level of activity corresponds with the level of recall people have later. As psychologist Daniel Schacter recounts in his book The Seven Sins of Memory, people who are shown a sequence of slides ranging from the ordinary—a mother walking her child to school—to the dreadful—the child is hit by a car—will remember the negative images far more readily than the others.
An image doesn’t have to be as awful as a car hitting a child to have this effect, however. A face with a fearful expression will do. Neuroscientist Paul Whelan even found that flashing an image of a fearful face for such a short time that people aren’t consciously aware that the face is fearful—they report that it looks expressionless—will trigger the amygdala. And that makes the memory more vivid, lasting, and recallable.
Fear is certainly the most effective way of gluing a memory in place, but there are others. Any emotional content makes a memory stickier. Concrete words—apple, car, gun—do better in our memories than abstractions like numbers. Human faces are particularly apt to stick in our minds, at least if they’re expressing emotions, because scientists have found such images stir the amygdala just as frightening images do. And all these effects are cumulative. Thus, a visually striking, emotion-drenched image—particularly one featuring a distraught person’s face—is almost certain to cut through the whirl of sensations we experience every moment, grab our full attention, and burrow deep into our memories. A fallen child clutching her knee and heaving agonized sobs may just be a stranger on the sidewalk but I will see her and remember, at least for a while—unlike the boring conversation about taxes I had at that dinner party with a man whose name I forgot almost the moment I heard it.