Thinking, Fast and Slow, in bytes

The idea

Your mind runs on two tracks. System 1 is fast, automatic and effortless: it reads faces and finishes “2 + 2”. System 2 is slow and deliberate: it does long division and checks arguments. Most of the time System 1 is in charge.

In real life

You read an angry email and feel defensive before you've finished the first line. That's System 1. Rereading it calmly is System 2.

The idea

System 2 has a small budget of attention. When it's busy, you literally stop seeing things that would otherwise be obvious.

In real life

In a famous experiment, about half of viewers counting basketball passes completely missed a person in a gorilla suit walking through the scene.

The idea

System 2 can check System 1's work, but it prefers not to. It accepts a plausible answer rather than spend effort verifying it.

In real life

A bat and ball cost $1.10; the bat costs $1 more than the ball. Most people say the ball costs 10 cents. It's 5 cents, and many students at top universities got it wrong.

The idea

One idea triggers a web of related ideas, feelings and actions, mostly without your awareness. Your context quietly shapes what you think next.

In real life

After talking about a stressful project, a neutral comment from your manager can sound like criticism. Note: Kahneman later said several famous priming studies failed to replicate.

The idea

When information feels easy to process, because it's familiar, clear or repeated, we tend to believe it and like it. Strain makes us more careful but less comfortable.

In real life

A claim you've heard many times starts to feel true, even if it's false. Clear fonts and simple words make messages more believable.

The idea

System 1 automatically looks for causes and builds a story to explain events, even random ones. It sees intention and pattern everywhere.

In real life

Markets rise and the news says it's “because of” a policy announcement. Markets fall the next day, and the same news is the cause again.

The idea

System 1 builds the most coherent story from the information at hand, and ignores what's missing. Less information can make you more confident, because there's less to contradict the story.

In real life

Hearing only one side of a dispute, people feel sure of their verdict, even when they know they haven't heard the other side.

The idea

If you like one thing about a person, you tend to like everything about them, including things you haven't observed. First impressions colour all that follows.

In real life

Kahneman found his grading of student essays was swayed by how well each student did on the first essay. He switched to grading one question across all students at a time.

The idea

Faced with a hard question, System 1 quietly answers an easier one instead, and you don't notice the swap.

In real life

Asked “How happy are you with your life these days?”, people often report their current mood instead, which is a much easier question.

The idea

Your feelings about something shape what you believe about its risks and benefits. If you like it, it seems low-risk and high-benefit; if you dislike it, the opposite.

In real life

People who favour a technology tend to rate it both safer and more beneficial, even though risk and benefit are separate questions.

The idea

Small samples produce extreme results more often than large ones, purely by chance. Our intuition treats small samples as if they were as reliable as big ones.

In real life

US counties with the lowest kidney cancer rates are mostly small and rural. So are the counties with the highest rates. Small populations just swing more.

The idea

An initial number, even an irrelevant one, pulls your estimate toward it. You adjust away from the anchor, but rarely far enough.

In real life

Experienced German judges who first rolled loaded dice showing a higher number gave longer sentences in the same hypothetical shoplifting case.

The idea

We judge how common something is by how easily examples come to mind. Vivid, recent or personal events feel more frequent than they are.

In real life

Ask both partners what share of the housework they do, and the answers usually add up to more than 100%. Your own efforts are simply easier to recall.

The idea

A small worry, amplified by media coverage and public reaction, can snowball into a widespread panic far out of proportion to the actual risk.

In real life

Media stories about a minor chemical risk in food can trigger public alarm, then policy changes, even when the scientific risk is tiny.

The idea

We judge probability by how much something matches a stereotype, and ignore how common it actually is.

In real life

A shy, tidy person who loves detail “sounds like” a librarian. But there are far more salespeople than librarians, so the person is more likely in sales.

The idea

When we get specific information about a case, we tend to ignore the underlying statistics, even when those statistics matter more.

In real life

85% of a city's cabs are green, 15% blue. A witness who's right 80% of the time says the cab was blue. The real chance it was blue is only about 41%.

The idea

A more detailed story feels more likely, even though adding details can only make it less probable.

In real life

Linda is described as a former activist. Most people rate “Linda is a bank teller and a feminist” as more likely than “Linda is a bank teller”, which is logically impossible.

The idea

People rarely change their view of themselves or others based on statistics. A single vivid case moves us far more than a dataset.

In real life

Students told that most people in a study failed to help a stranger still predicted that individuals would help. Hearing about two pleasant people who didn't help changed their minds.

The idea

Extreme performances are usually followed by more average ones, simply because luck played a part. We invent causes for what is just statistics.

In real life

Flight instructors believed praise made pilots worse and criticism made them better. In fact, great and terrible landings were just followed by more ordinary ones.

The idea

Intuitive predictions are too extreme. They match the impression of the evidence without allowing for how weak the evidence is.

In real life

A child who read early feels like a future top student. But early reading only loosely predicts university grades, so the forecast should be much closer to average.

The idea

We build simple, tidy stories about the past that exaggerate skill and intention and underplay luck. These stories make the world seem more predictable than it is.

In real life

Success stories of famous companies sound inevitable in hindsight. Many similar companies made similar choices and failed.

The idea

Once we know an outcome, we believe we “knew it all along”. This makes us judge decisions by results rather than by the quality of the thinking at the time.

In real life

A sensible decision that turns out badly gets blamed; a reckless gamble that works gets praised.

The idea

We feel confident in our judgments when the story is coherent, even when we know our track record is poor.

In real life

Kahneman's army team assessed officer candidates with total confidence, while knowing their past predictions had been barely better than guessing.

The idea

In many fields, results that look like skill are mostly luck, yet professionals and their clients still believe in skill.

In real life

When Kahneman analysed eight years of results for a firm's investment advisers, their rankings from one year to the next were essentially uncorrelated.

The idea

In noisy, unpredictable fields, simple scoring rules often predict outcomes better than expert judgment, because humans are inconsistent.

In real life

A simple formula using weather data predicts the future price of fine wines better than many expert tasters. The Apgar score assesses newborns in seconds.

The idea

Intuition is reliable only when the environment is regular enough to learn, and the expert has had long practice with quick, clear feedback.

In real life

Experienced firefighters and chess masters build genuine intuition. Long-range political and economic forecasters mostly don't.

The idea

We base plans on best-case scenarios and ignore how long similar projects actually took. The fix is the “outside view”: look at the track record of comparable projects.

In real life

Kahneman's team expected to finish a textbook in about two years. Similar projects took seven to ten, if they finished at all. Theirs took eight.

The idea

Optimism drives action, but makes us underestimate risks. A premortem helps: imagine the project has failed, then write the story of why.

In real life

Before go-live, a team imagines it's six months later and the rollout has failed. In ten minutes they list risks nobody wanted to raise.

The idea

We don't judge outcomes by total wealth, but by changes from a reference point, usually where we are now or what we expected.

In real life

Two people each have $5 million today. One had $1 million yesterday and is thrilled; the other had $9 million and is devastated.

The idea

Kahneman and Tversky's Nobel-winning model: we value gains and losses relative to a reference point, feel losses more than gains, and become less sensitive as amounts grow.

In real life

The difference between $100 and $200 feels big. The difference between $1,100 and $1,200 feels small, though it's the same $100.

The idea

Losses hurt roughly twice as much as equivalent gains feel good. So we work harder to avoid losing than to gain.

In real life

Offered a coin flip where you lose $100 on tails, most people want to win about $200 on heads before they'll accept.

The idea

Once we own something, we value it more than we would pay to get it, simply because giving it up feels like a loss.

In real life

In experiments, people given a mug asked roughly twice as much to sell it as others were willing to pay for the same mug.

The idea

We overweight small chances and underweight near-certainties. That makes us risk-averse in some situations and risk-seeking in others.

In real life

People buy lottery tickets and insurance at the same time. A side likely to lose a lawsuit gambles on trial rather than accept a fair settlement.

The idea

Vivid rare events get overweighted, while abstract ones get ignored. How a probability is described changes how we react.

In real life

“1 in 1,000 people affected” feels more dangerous than “0.1% risk”. Many people even picked a jar with 8 winning marbles in 100 over one with 1 in 10.

The idea

Judging each risky choice on its own makes us too cautious. Looking at many similar decisions together reveals that small favourable bets add up.

In real life

A colleague of Paul Samuelson turned down a single favourable coin-flip bet, but said he'd happily take a hundred of them.

The idea

We keep money and effort in separate mental accounts and hate closing one at a loss, so we throw good resources after bad.

In real life

Investors sell winning shares and keep losers to avoid “realising” a loss. People drive through a blizzard to a game because they've already paid for the ticket.

The idea

We feel more regret about bad outcomes that came from action than from inaction, and when we broke from the norm.

In real life

An investor who switched shares and lost feels worse than one who stayed put and lost the same amount.

The idea

Our preferences can flip depending on whether we see options side by side or one at a time.

In real life

Alone, a dictionary with 10,000 words in perfect condition is valued more than one with 20,000 words and a torn cover. Seen together, the bigger one wins.

The idea

The same facts, worded differently, lead to different choices. “What you see” is shaped by how it's presented.

In real life

Countries where organ donation is the default have far higher sign-up rates than countries where people must opt in.

The idea

An experiencing self lives each moment; a remembering self keeps score and makes decisions. They often disagree, and the remembering self wins.

In real life

People who kept a hand in cold water for 60 seconds, then 30 seconds more with the water slightly warmer, chose to repeat the longer trial, despite more total pain.

The idea

We remember experiences mostly by their most intense moment and how they ended, while largely ignoring how long they lasted.

In real life

Patients remembered a medical procedure as less painful when its final minutes were milder, even though it lasted longer overall.

The idea

Whatever you're thinking about seems far more important to your happiness than it really is.

In real life

People imagine Californians are happier because of the weather. Studies found their life satisfaction was about the same as people living in colder places.