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Kahneman introduces the two ways of thinking—two systems, as he metaphorically calls them—that give his book its title. System 1 “operates automatically and quickly, with little or no effort and no sense of voluntary control.” Recognizing anger in a facial expression is a System 1 operation.

System 2 handles mental activities that require deliberate effort and sustained concentration. Examples of System 2 operations include multiplying two-digit numbers in one’s head, looking for a particular person in a crowd, and maneuvering into a small parking space. Kahneman cites studies suggesting that, like other effortful behaviors such as staying away from a tempting snack, System 2 thinking depletes mental and physical resources and can be impaired when those resources run low. The consequences can be dire. For example, one study found that Israeli parole judges’ decisions were affected by how long it had been since the judges had eaten.

Systems 1 and 2 should not be thought of as processes in different parts of the brain; they are just different styles of thinking—different gears for the same engine, so to speak. Kahneman sometimes speaks of the two systems, metaphorically, as though they were each persons in their own right. System 2 likes to think of itself as being in charge and is occasionally identified by Kahneman as one’s true self, but because System 2 is effort-intensive and consumes resources, it often gets lazy and lets System 1 operate unchecked.

System 1 is subject to an effect called “priming.” Kahneman offers several illustrations. Someone who just saw the word EAT is more likely to complete SO_P as SOUP rather than as SOAP. More strikingly, someone who has recently seen words associated with old age (Florida, forgetful, bald) will be primed to walk more slowly—and conversely, someone who walked more slowly than usual for several minutes will have an easier time recalling words associated with old age. (The replicability of this study was later questioned. See Context: A Collaboration Between Differing Types.)

 Finally, someone putting money in a cash jar will put more in when the jar sits beneath a picture of a pair of watchful eyes. System 1 promotes cognitive ease, the sense that all is well with one’s perceptions and opinions, and this sense of ease in turn encourages continued use of System 1. In so doing, cognitive ease makes people susceptible to many kinds of errors. For example, they are more likely to believe a statement if it is printed in boldface type. Also, they are more likely to interpret foreign words (or nonsense words) as having positive meanings if repeated exposure has made the words familiar. This is not to say that System 1 is simply a set of bad mental habits. On the contrary, System 1 is the source of our sense of what is normal and what is surprising. It is also the source of our immediate judgments about causal relationships.

For better or worse, then, System 1 is “a machine for jumping to conclusions,” as Kahneman puts it in Chapter 7. It is biased toward believing things, whereas System 2 is more skeptical. System 1 readily forms judgments based on available evidence, even when contrary evidence is known to exist. In one study, participants were asked to judge the guilt or innocence of a person arrested after a verbal confrontation at a drug store. Participants tended to agree with whoever presented the case, the prosecution or the defense, even though the participants knew that there was another side to the story, which they had not been given. This is the What You See Is All There Is (WYSIATI) effect. System 1 is better at some kinds of judgments than at others. It is notably better at recognizing when something is an average member of a collection than at combining many parts to arrive at a whole or total.

One device System 1 often resorts to is substituting an easy question for a hard one. Example: When asked, “How much should I contribute to save an endangered species?” System 1 will set the contribution amount based on the answer to the question, “How much do I feel like giving when I look at a picture of an endangered animal?”

Next section Part II. Heuristics and Biases (Chapters 10–18)

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