Thinking, Fast and Slow

In chapter 2, Kahneman advises readers to try an exercise: write out several strings of four digits, and, while keeping a steady beat, report each string, wait two beats, and then report the same string but add one to each digit. So, for example, if the string is 5294, a person should read out that number, and then say aloud 6305. Most people have a difficult time with this exercise.

In experiments, Kahneman and a colleague—Jackson Beatty—found that people’s eyes dilated the harder they worked during this exercise. People’s eyes dilated most when they were asked to add three to each digit; with anything more demanding, people simply gave up. This also led Kahneman to observe that in casual conversation, people’s eyes did not dilate at all. Mental life is normally conducted without much effort.

The Add-3 exercise also reveals that we cannot expend more energy on a mental task than we need. One would never be able to spend more energy memorizing four digits than in completing the Add-3 exercise, because we simply do not need as much energy to do so, and will always use the least amount of energy possible.

Additionally, as a person becomes skilled in a task, its demand for energy diminishes. Talent has similar effects. Highly intelligent individuals need less effort to solve the same problems, which we know both from monitoring both pupil size and brain activity.

Kahneman then questions what makes certain tasks more demanding than others. He believes that more effort is required to maintain several ideas that require separate actions, or in which information has to be combined to make decisions—like choosing between two options at a restaurant. Time pressure, as experienced in Add-3, is another driver of effort.

Switching between tasks is also difficult, because we train our brains to accomplish a particular task when we focus on it. For example, if we are asked to count all the instances of the letter f on a page, it would not come naturally, but gradually we would train ourselves to focus on the letter f. But if we were then asked to count the commas in a page, we would have to overcome our newly acquired tendency to focus on the letter f.

Kahneman finishes the chapter by commenting that very few things in our lives force us to expend as much mental effort as Add-3. We avoid mental overload by breaking up work into easy steps, or by relieving our working memory when we use pencil and paper rather than trying to hold a variety of information in our head. We take our time and try to expend as little energy as possible.