Predictably Irrational

Ariely's argument is that our departures from rational choice are not random noise but systematic, which means they can be measured, predicted and exploited. Each chapter is built around experiments he ran with colleagues, often on MIT students, such as an Economist subscription offer in which a useless print-only option steers buyers to the bundle.

The 2009 revised and expanded edition, used here, adds his reflections on the financial crisis and a more pointed case against rational-agent economics. The New York Times review by David Berreby called it a more radical book than its friendly tone suggests.

The book is easier going than Kahneman's Thinking, Fast and Slow and more experiment-driven than Thaler and Sunstein's Nudge. Two caveats matter. Economists such as Eric Crampton argued that Ariely draws stronger policy conclusions than his lab results support. More seriously, parts of the evidence have not held up. The Ten Commandments cheating study used in the honesty chapters failed a large replication; a 2012 paper he co-wrote was retracted in 2021 over fabricated data, which he denied producing; and in 2026 Psychological Science retracted his 2002 deadlines study with Klaus Wertenbroch, the basis of the procrastination chapter, after a failed replication and signs of data tampering.

Read it for the ideas about anchoring, relative value and ownership, and treat individual findings with appropriate skepticism.

1. Anchors set prices. An arbitrary first number, even an irrelevant one, shapes what people will pay later; watch for it in negotiations and valuation work.
2. Decoys steer choice. Adding an inferior option can push buyers toward the option the seller wants, a pattern behind much product and fee design.
3. Owners overvalue what they hold. The endowment effect helps explain why investors hang on to positions and sellers ask more than buyers will pay.
4. Distance from cash loosens ethics. In Ariely's experiments people cheated more for tokens than for money, a warning about how abstraction in finance can blur judgment.
5. Check the evidence. Several of Ariely's findings, including the Ten Commandments and deadline studies described here, have not held up, so use the book as a map of well-known biases rather than proof of any one study.

The book is a sequence of short chapters, each built around one bias and the experiments behind it.

1. How we judge value. Ariely opens with relativity and decoy pricing, then shows how arbitrary anchors set what people are willing to pay, and why a price of zero distorts choices more than a low price.
2. Social norms and emotion. He contrasts social norms with market norms, showing that paying people can reduce their effort, and tests how arousal changes the decisions people predict they will make.
3. Self-control and ownership. Chapters on procrastination test deadlines with his own students (the underlying 2002 paper was retracted in 2026), then examine the endowment effect and why people pay to keep options open even when doing so costs them.
4. Expectations and price. He shows that expectations change how things taste and feel, and that a more expensive placebo painkiller works better than a cheap one.
5. Honesty. Two chapters study cheating and find that people cheat a little when they can, and more when the reward is a step removed from cash, a point he extends to corporate and financial misconduct.
6. Beer, free lunches and the crisis. A final chapter covers how ordering in a group changes choices, and the expanded edition adds reflections on the 2008 meltdown.

Intelligent, lively, humorous, and thoroughly engaging, The Predictably Irrational explains why people often make bad decisions and what can be done about it.