The more I reflect on Daniel Kahneman’s book Thinking Fast and Slow the more I realize just how brilliant it is.

It’s a very well-written, impeccably organized, and astoundingly researched catalog of how our human brain makes decisions, with a focus on the most common ways it fails. Each chapter feels like it could be a book on its own, and the ideas seem to be just the very first of amazing (and sometimes scary) insights into how our brains work.

I understand more by writing so I’ve written down here the key principles or ideas in the book.

1. You think in two distinct ways: quick and effortless, called System 1, and slow and demanding, called System 2.

System 1 is automatic and quick; your default mode. System 2 is what is typically thought of as “thinking,” where you’re actively engaging in mental activities that require effort. System 2 is a limited resource. Imagine multiplying 17 x 24 while making a left turn in heavy traffic. For another example of System 2 failing when taxed, check out The Gorilla Test.

2. System 1 tends to dominate your thinking because System 2 requires effort.

In other words, you tend not to “think” as often as you think you do.

3. System 2 is a limited resource, and consuming it can have consequences, often related to self-control.

Self-control and cognitive effort are forms of mental work. Let’s call using System 2 in a taxing way “thinking hard”—anything that requires you to actively engage a demanding cognitive task. If you think hard, occupying System 2, System 1 has more influence on your behavior.

People who are cognitively busy are also more likely to make selfish choices, use sexist language, and make superficial judgments in social situations.

Kahneman highlights the work of the psychologist Roy Baumeister that “all variants of voluntary effort—cognitive, emotional, or physical—draw at least partly on a shared pool of mental energy.”

If you have to work hard at something, you are less able to exert self-control at something else that follows, an effect known as ego depletion .

Baumeister also discovered that hard thinking led the nervous system to consume large amounts of glucose and showed that consuming sugary drinks could undo some of ego depletion.

4. People vary in their ability to counter the quick, intuitive thoughts emerging from System 1.

Quickly answer the following: A bat and ball cost $1.10. The bat costs $1 more than the ball. How much does the ball cost?

If you said 10 cents, you let System 1 fool you. If you said 5 cents, you engaged System 2, overriding the intuitive answer System 1 threw out.

Another one: Does the conclusion follow from the premises for the following? All roses are flowers. Some flowers fade quickly. Therefore some roses fade quickly.

The answer is no. If you said yes, which seems to make sense, System 1 fooled you.

Kahneman makes a very important point with these tests. Having administered the test to Ivy League students, he points out that many failed the test. Fifty percent of students at Harvard, MIT, and Princeton failed the bat/ball question, and a large majority of college students failed the rose question. These students certainly can answer the questions but for some reason choose not to engage with them properly.

The ease with which they are satisfied enough to stop thinking is rather troubling. “Lazy” is a harsh judgment about the self-monitoring of these young people and their System 2, but it does not seem to be unfair. Those who avoid the sin of intellectual sloth could be called “engaged.” They are more alert, more intellectually active, less willing to be satisfied with superficially attractive answers, more skeptical about their intuitions.

Kahneman points out work done by Keith Stanovich and Richard West who argued that intelligence is distinct from the ability to avoid biases of judgment, which they call rationality.

5. Your brain is very susceptible to ideas and behavior because of its tendency to associate ideas without your realizing.

A key concept is the association of ideas. One idea activates other ideas in your mind.

An idea that has been activated does not merely evoke one other idea. It activates many ideas, which in turn activate others. Furthermore, only a few of the activated ideas will register in consciousness; most of the work of associative thinking is silent, hidden from our conscious selves.

Association can work in unusual ways. One experiment exposed subjects to words associated with the elderly (e.g., Florida, forgetful, bald, gray, wrinkle) and found that the subjects exposed to those words then walked down the hallway much more slowly than those that hadn’t been exposed.

This effect can also work in reverse. When subjects were asked to walk around a room at a third of their normal pace, they were much more likely to recognize words related to old age. Their minds were primed for old age.

Reciprocal priming effects tend to produce a coherent reaction: if you were primed to think of old age, you would tend to act old, and acting old would reinforce the thought of old age.

An amazing experiment: Subjects were asked to listen to a message through headphones and told that they were testing the quality of the audio equipment by moving their heads repeatedly to check for distortion. Half were told to nod their heads up and down; the other half to shake their heads side to side. The half that nodded was much more likely to accept the message they heard than the half that shook their head.

The chapter (Chapter 4, “The Associative Machine”) is among the best. Kahneman goes on to list experiment after experiment with amazing outcomes.

6. How easy (or not) it is for you to mentally process something can have unusual effects.

Kahneman paints the picture of a dial in your head with “easy” on one end and “strained” on the other that measures the extent to which things are going well or not. If things are going well and there are no threats, System 1 can stay in control. If not, as the dial increasingly moves to “strained,” that means System 2 needs to jump in.

Reading a sentence in clear font pushes the dial to cognitive ease. Same with an idea you’e heard before or been prepped for in some way before (i.e., one that has been primed). Same with hearing a speaker when you’re in a good mood. All cognitive ease.

Conversely, poor font, faint colors, difficult wording, or a bad mood lead to cognitive strain.

What’s surprising, though, is the effect this has on your thinking:

The various causes of ease or strain have interchangeable effects. When you are in a state of cognitive ease, you are probably in a good mood, like what you see, believe what you hear, trust your intuitions, and feel that the current situation is comfortably familiar. You are also likely to be relatively causal and superficial in your thinking. When you feel strained, you are more likely to be vigilant and suspicious, invest more effort in what you are doing, feel less comfortable, and make fewer errors, but you are also less intuitive and less creative than usual.

In a sense, the entire brand advertising industry rests on this idea. Familiarity leads to pleasurable feelings. Kahneman describes an experiment by the psychologist Robert Zajonc on what is known as the mere exposure effect.

For a period of a few weeks, Zajonc ran an ad on the front page of the student papers at the University of Michigan and Michigan State University. The words themselves were nonsense and Turkish-sounding (to make them foreign and unusual): kadirga, saricik, biwonjni, nansoma, and iktitaf. The extent to which the words were shown was varied. One was shown only once, and other others were shown, two, five, ten, or twenty-five times.

Kahneman:

The results were spectacular: the words that were presented more frequently were rated much more favorably than the words that had been shown only once or twice.

7. Our brains are hardwired to see causality, even where there may be none.

Kahneman describes research done by the psychologist Albert Michotte:

Michotte…argued that we see causality, just as directly as we see color. To make his point, he created episodes in which a black square drawn in paper is seen in motion; it comes into contact with another square, which immediately begins to move. The observers know that there is no real physical contact, but they nevertheless have a powerful “illusion of causality.” If the second object starts moving instantly, they describe it as having been “launched” by the first. Experiments have shown that six-month-old infants see the sequence of events as a cause-effect scenario, and they indicate surprise when the sequence is altered. We are evidently ready from birth to have impressions of causality, which do not depend on reasoning about patterns of causation. They are products of system 1.

Kahneman points out that this element of our psychology is incredibly significant:

The prominence of causal intuitions is a recurrent theme in this book because people are prone to apply causal thinking inappropriately, to situations that require statistical reasoning. Statistical thinking derives conclusions about individual cases from properties of categories and ensembles. Unfortunately, System 1 does not have the capability for this mode of reasoning; System 2 can learn to think statistically, but few people receive the necessary training.

8. You have a bias to believe and confirm.

Whitefish eat candy.

Kahneman offers that phrase as an example of how our brains, in order to unbelieve something, have to first believe it. When we read that statement, the associative machinery in our brain that automatically makes connections began to associate the two ideas. System 1 kicked in, tying the two ideas together and making us believe them for a short time, before System 2 allowed us to unbelieve the idea.

Kahneman also details experiments by the psychologist Daniel Gilbert that show how if System 2 is taxed, we’ll believe more of these nonsensical statements, concluding: “The moral is significant: when System 2 is otherwise engaged, we will believe almost anything.”

[As a side note here, it’s worth mentioning another book that blew my mind: But Wait…There’s More by Remy Stern, which details the $100 billion—yes, $100 BILLION—infomercial industry. The relevant aspect here outlined by Stern is that these infomercials often sell products with no value at all at high prices because the commercials catch people when they’re tired and vulnerable.]

9. You have a tendency to either fully like or dislike someone or something.

Kahneman points out that if you like the president’s politics, you likely feel positively about his voice and appearance as well.

He gives the example of you meeting someone named Joan at a party and having a pleasant conversation with them. You’ll later think of them as someone generous, when asked about their likelihood to contribute to a charity. In fact, you don’t know that they’re generous. But you know you like her. You also know you like generous people. Therefore, you conclude Joan is probably generous—and, in fact, like her more after you’re asked the question about her generosity because you added generosity to her list of pleasant qualities.

Kahneman describes an experiment by the psychologist Solomon Asch.

What do you think of Alan and Ben?
Alan: intelligent—industrious—impulsive—critical—stubborn—envious
Ben: envious—stubborn—critical—impulsive—industrious—intelligent

More specifically: whom do you view more favorably? Most people say Alan. But notice that they have the same qualities, just presented in the opposite sequence. For Alan, you minimize the negative qualities or attribute them in such a way that they are positive (e.g., “Alan’s stubborn because he’s so intelligent”).

Kahneman goes on to tie this tendency to evaluations and group discussions. You want independent judgments of things, where errors can be decorrelated. For example, when he graded tests, he used to grade one student’s essays from start to finish. But he found he let his first grades influence the later grades so he switched to blindly grading one question for all students at a time. He also suggests that at meetings in which an important issue has to be discussed that all participants write down a brief summary of their position ahead of time.

10. You tend to think that what you see is all there is.

Kahneman comes back to the idea of associative memory frequently, with one of its key features being that it only represents active ideas. In other words, your brain automatically (via System 1) activates many, many ideas when it encounters something. But anything that isn’t activated is completely ignored in that unconscious process.

Kahneman: “System 1 excels at constructing the best possible story that incorporates ideas currently activated, but it does not (cannot) allow for information it does not have.” … “When information is scarce, which is a common occurrence, System 1 operates as a machine for jumping to conclusions.”

Kahneman touches upon a key idea: “The measure of success for System 1 is the coherence of the story it manages to create. The amount and quality of the data on which the story is based are largely irrelevant.” In other words, System 1 is a horrible analyst.”

System 2 can override this, of course, but its important to realize that this is happening. “System 1 is expected to influence even the more careful decisions. Its input never ceases.”

This idea is so central that Kahneman came up with the abbreviation WYSIATI: what you see is all there is.

System 1 uses a series of shortcuts to lead you to make a judgment: basic assessments, sets and prototypes, intensity matching, and mental shotgun.

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You have a tendency to answer an easier question when presented with a harder question.

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(As you can see, this is still in progress. Stay tuned!)