Predictably Irrational

Chapter-by-chapter summary

The book is a series of experiments rather than a story, so this section follows its main topics in order. Chapter titles and numbering differ between editions, so each part is labelled by topic.

Part 1: Relativity and the decoy

We rarely judge things on their own. We compare. Ariely shows this with a magazine offer: web only for $59, print only for $125, print plus web for $125. Of 100 MIT students, 16 chose web only, none chose print only, and 84 chose the bundle. When he removed the useless print-only option, 68 chose web only and just 32 chose the bundle. The print-only plan was a decoy that made the bundle look like a bargain. It matters because sellers, and our own minds, steer us by what sits beside an option.

Part 2: Anchors and arbitrary coherence

The first price we see sticks. Ariely asked students to write down the last two digits of their Social Security number, then to bid on items such as wine and chocolate. Students with higher numbers bid noticeably more. Once a first price is in our heads, later prices feel consistent with it, even though the starting point was random. He calls this arbitrary coherence. The story of black pearls, made to look precious by displaying them beside costly gems, shows the same mechanism. Prices tell us less about value than we think.

Part 3: The pull of zero

Free is not just a very low price. When students chose between a Lindt truffle at 15 cents and a Hershey's Kiss at 1 cent, most took the truffle. Cut each price by one cent, so the Kiss was free, and most switched to the Kiss. The real gap between the two had not changed. Zero removes the feeling of a possible loss, so a free item feels like pure gain. This helps explain the pull of free shipping and of freebies we hardly want. Zero deserves extra suspicion.

Part 4: Social norms and market norms

We live by two rulebooks. In social exchanges people help out of goodwill. In market exchanges they expect to be paid. Lawyers asked to advise older people in need for about $30 an hour mostly refused, yet agreed to help for free. Mixing the rulebooks causes trouble: when an Israeli day care fined parents for picking up children late, more parents arrived late, not fewer, because the fine turned a duty into a price. Money, Ariely says, is often the costliest way to motivate people.

Part 5: Emotion and arousal

When calm, we cannot imagine how we will act when excited. Ariely and his colleague George Loewenstein asked male Berkeley students questions about sexual preferences and risky choices twice, once in a calm state and once while aroused by a laptop. Their answers changed sharply, and they accepted options they had earlier rejected. The topic matters less than the gap between the two states. We plan for our calm selves and then fail to prepare for our heated ones. Anger, hunger and excitement work the same way.

Part 6: Procrastination and self-control

We mean to save, study and exercise, and then we put it off. In one class experiment, students wrote three papers under different deadline rules. Those given fixed, evenly spaced deadlines earned the best grades. Those who chose their own deadlines did worse than that, and those with one deadline at the end did worst. Choosing deadlines at all showed they knew their weakness. The lesson is that commitments we cannot easily change let today's calm self protect us from tomorrow's tempted one.

Part 7: The high price of ownership

Owning something raises its value in our eyes. At Duke University, tickets to a popular basketball game were given out by lottery. Students who won asked more than ten times as much to sell as students without tickets were willing to pay. Owners focus on what they would lose by selling, while buyers focus on what they would gain. This helps explain why people cling to old stocks, houses and clutter, and why free trials turn into purchases. Asking what you would pay if you did not own it yet brings some calm.

Part 8: Keeping doors open

Options feel free, but they cost attention and effort. In a computer game, players earned money by clicking on one of three doors. A door left unvisited for a while slowly vanished, so players burned clicks racing back to keep every door open, even when the doors paid little, and they earned less overall. Ariely sees the same habit in people who hesitate to commit to one path. Closing a door is sometimes the wisest move, because it frees energy for the goal that matters most.

Part 9: The effect of expectations

What we expect shapes what we experience. At an MIT pub, students compared two beers: an ordinary one and a version with a few drops of balsamic vinegar, nicknamed MIT Brew. Students told about the vinegar before tasting mostly disliked it. Those who learned of it only afterward, or never, often preferred it. The taste was identical; the information that arrived first changed the experience. Brand names, reviews and descriptions quietly write the experience we later report as our honest opinion.

Part 10: The power of price and placebo

Price works as a signal of quality. In a test where people received electric shocks, participants were given a pill described as a new painkiller. Those told it cost full price reported more relief than those told it had been discounted, though both pills were placebos. A higher price raised expectations, and expectations changed how much pain people felt. Ariely points out that placebos can bring real relief, which is why a label or a price tag can matter far more than it should.

Part 11: Dishonesty and the fudge factor

Most people are not criminals, yet many cheat a little. In puzzle tests that paid for each correct answer, some participants could shred their answer sheets and report their own scores. They claimed a bit more than they had earned, but only a bit, and changing the chance of being caught made little difference. Asking people to recall the Ten Commandments before the test reduced cheating. Ariely calls this the personal fudge factor: we cheat only as far as we can still see ourselves as honest.

Part 12: What to do with all this

The closing pages argue that predictable mistakes are an opportunity. If we know where people go wrong, we can design better savings plans, rules, products and classrooms around real human behavior instead of an imaginary rational customer. Ariely also urges readers to test their own habits, because experiments, not assumptions, show how we actually behave. That curious, experimental attitude holds the whole book together.

Key people and terms

Dan Ariely

The author, a behavioral economist. Badly burned as a teenager, he became curious about how patients and doctors make choices about pain, and that curiosity grew into the experiments in this book. Most of his studies use university students as participants.

George Loewenstein

A behavioral economist and Ariely's collaborator on the arousal studies. His work on how emotion bends judgment sits behind the book's argument that calm people cannot predict how they will act when excited.

Decoy effect

An option nobody picks that makes another option look better. The print-only magazine plan is the book's best example: it exists mainly to make the print-plus-web bundle look like a bargain, and it works.

Arbitrary coherence

The idea that a first price, even a random one, becomes the anchor for later judgments. Once we accept a starting price, we compare everything to it and feel consistent, although the starting point meant nothing.

Social norms and market norms

Two sets of rules for exchange. Social norms run on goodwill and favors; market norms run on prices and pay. Mixing them, for example by offering a small payment for a favor, can damage the relationship.

Personal fudge factor

The amount of cheating a person allows while still feeling honest. People cheat a little, not a lot, so they can keep seeing themselves as decent. Reminders of morality shrink the fudge factor.

Ownership effect

People value what they own more than what they could buy. In the Duke ticket lottery, owners asked far more to sell than other students would pay to buy.

Writing about this book

Predictably Irrational works well for book reports and essays because it invites you to compare the experiments with your own habits. Start by picking the one experiment that stuck with you most, then ask whether you would have reacted the same way. If you would not, explore why; that tension makes a stronger essay than simple agreement.

Questions to think about:

• What does “rational” mean here, and does Ariely show that people are foolish, or only that they are human?
• Which of your own habits looks like procrastination, ownership or the pull of free?
• If mistakes are predictable, what should schools, shops or governments do differently?
• Most studies used university students. How might that shape the conclusions?

Keep the retelling short. Choose one or two experiments, report the numbers accurately, and spend most of your words on your own thinking. A closing paragraph explaining how the book changed one choice you make, such as how you shop or plan your study time, gives the piece a clear ending.