Predictably Irrational: The Hidden Forces That Shape Our Decisions is a 2008 book by Dan Ariely, in which he challenges readers' assumptions about making decisions based on rational thought. Ariely explains, "My goal, by the end of this book, is to help you fundamentally rethink what makes you and the people around you tick. I hope to lead you there by presenting a wide range of scientific experiments, findings, and anecdotes that are in many cases quite amusing. Once you see how systematic certain mistakes are—how we repeat them again and again—I think you will begin to learn how to avoid some of them".

The book has been republished in a "revised & expanded edition", and has been adapted as the 2023 television series The Irrational.

Ariely discusses many modes of thinking and situations that may skew the traditional rational choice theory. There are 15 chapters in total, and the following outline the main points.

In chapter 1, Ariely describes the ways in which people frequently regard their environment in terms of their relation to others; it is the way that the human brain is wired. People not only compare things, but also compare things that are easily comparable. For example, if given the following options for a honeymoon—Paris (with free breakfast), Rome (with free breakfast), and Rome (no breakfast included), most people would probably choose Rome with the free breakfast. The rationale is that it is easier to compare the two options for Rome than it is to compare Paris and Rome. Ariely also explains the role of the decoy effect (or asymmetric dominance effect) in the decision process. The decoy effect is the phenomenon whereby consumers will tend to have a specific change in preference between two options when also presented with a third option that is asymmetrically dominated. This effect is the "secret agent" in many decisions. In the example with the honeymoon options, Rome without free breakfast is the decoy. (It makes Rome with breakfast look superior to Rome without breakfast. Comparing Rome and Paris is difficult, so the easy comparison of Rome makes it more likely to choose Rome over Paris.) It makes Paris look inferior when compared to Rome with the free breakfast. Relativity helps people make decisions but it can also make them miserable. People compare their lives to those of others, leading to jealousy and inferiority. Ariely finishes the chapter by saying "the more we have, the more we want" and his suggested cure is to break the cycle of relativity. To break the cycle, people can control what goes on around them. The focus on smaller "circles" can boost relative happiness, as can changing this focus from narrow to broad. When considering upgrading a phone, the consumer could think about what else they could buy with the money they would spend on the upgrade.

The chapter also explores the independence of irrelevant alternatives and the idea of menu dependence.

In chapter 2, consumers purchase items based on value, quality or availability—often on all three. The methods of appointing a value to an object with no previous value, like the Tahitian black pearl, is susceptible to irrational pricing. A value can be as easily (arbitrarily) assigned as by having a fancy ad with "equally" precious items and a high price tag in a window of a store on Fifth Avenue. When consumers buy a product at a certain price, they become "anchored" to that price, i.e. they associate the initial price with the same product over a period of time. An anchor price of a certain object, say a plasma television, will affect the way they perceive the value of all plasma televisions henceforth. Other prices will seem low or high in relation to the original anchor. In other words, decisions about future LCD television purchases become coherent after an initial price has been established in the consumer's mind. A person's self value for services rendered can also be affected by anchor prices; one can irrationally price his/her abilities or services based on an anchor price proposed. Using the concepts of anchor price and arbitrary coherence, Ariely challenges the theory of supply and demand. He states that demand, the determinant of market prices, can be easily manipulated. Furthermore, supply and demand are dependent on each other (manufacturer's suggested retail prices affect consumers' willingness to pay). Finally, the author claims that the relationships between supply and demand are based on memory rather than on preferences.

In chapter 3, Ariely explains how humans react to the words "free" and "zero". Humans make decisions without rationalizing the outcomes of their choices. To illustrate this point, Ariely conducted multiple experiments. The outcome was consistent: when faced with multiple choices, the free option was commonly chosen. With the opportunity to receive something for free, the actual value of the product or service is no longer considered. Ariely claims, "Most transactions have an upside and a downside, but when something is FREE! we forget the downside. FREE! gives us such an emotional charge that we perceive what is being offered as immensely more valuable than it really is."

Ariely's concept of "FREE!" applies not only to monetary and quantitative costs, but also to time. We forgo some of our time when we wait in line for free popcorn or to enter a museum on a free-entrance day. We could have been doing something else at that time. Ultimately, he demonstrates how such a simple concept can be used to drive business and social policy. For example, to reduce health cost, companies could offer free regular checks. Employees would be more willing to get them at zero cost rather than paying some amount of money. Ariely recommends the consideration of the net benefits of the choices we make regarding both preference and money. Perhaps we would get the better deal and even save money if we did not react to free the way we do.