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Summary and Analysis of Predictably Irrational: The Hidden Forces That Shape Our Decisions: Based on the Book by Dan Ariely
So much to read, so little time? This brief overview of Predictably Irrational tells you what you need to know—before or after you read Dan Ariely’s book. Ìý Crafted and edited with care, Worth Books set the standard for quality and give you the tools you need to be a well-informed reader. Ìý This short summary and analysis of Predictably Irrational Historical contextChapter-by-chapter overviewsImportant quotesFascinating triviaGlossary of termsSupporting material to enhance your understanding of the original workAbout Predictably The Hidden Forces That Shape Our Decisions by Dan ÌýPredictably Irrational, the New York Times bestseller by Duke psychology and behavioral economics professor Dan Ariely, challenges the idea that we always make perfectly rational decisions. Featuring examples from daily life alongside results of his fascinating experiments, Ariely explains how emotional, psychological, and social factors can lead to irrational behavior—which can be damaging to ourselves and others. Ìý From the coffee we drink or the medicine we take, to the companies we support and the relationships we value, we make irrational decisions every day that can cost us in the long run. Ìý Ariely reveals not only when and how we tend to act irrationally, but why, so we can learn from our mistakes and design ways to facilitate smarter decision-making. Ìý The summary and analysis in this ebook are intended to complement your reading experience and bring you closer to a great work of nonfiction. Ìý
50 pages, Kindle Edition
Published March 28, 2017
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May 3, 2025
This short book is a great summary of
(2008) by . Predictably Irrational was , and sold well for a nonfiction book about psychology and . It has been highly influential and heavily cited, so if you've done much reading in the social and behavioral sciencies you've probably seen ideas from it. For example, Ariely originated or studied many of the entries that later appeared in the article on the English Wikipedia. Predictably Irrational stands alongside (2011) by as classics in their field.
Given that this summary book is short (obviously), it introduces many concepts with a paragraph or two. If you are seeing them for the first time, this may not be enough. Again, obviously, the next step would be to read Ariely's book, or look up the Wikipedia articles that exist for most of them. For example, the summary briefly mentions the , a topic of such importance and complexity that it warrants at least a full book of its own. The definition given here is:
Tragedies of the commons therefore amount to , the bane of libertarians and free-market fundamentalists. Free marketeers typically have blind faith in Adam Smith's , whereby individuals each acting in their own rational self-interest produce a collectively wonderful result. This only works when all costs of the individual's actions are correctly priced to the individual. But in a tragedy of the commons, the market only pays the individual to take more. The market doesn't by itself penalize the individual for taking too much, since the consumer cannot see or price in the impending collapse of the commons.
The classic example is overfishing a fishery. A fishery consists of a population of fish that can reproduce at a certain rate. "Sustainable fishing" then entails catching no more fish per year than the number that the fishery can replenish indefinitely. And for many species of fish, the most critical fish are the BOFFFFs - big old fat fecund female fish, which produce a hugely disproportionate number of eggs. But big fish tend to have the most commercial value, and so may be deliberately sought by fishermen.
Tragedy can result when fishermen are free to take as many fish as they are able. Thus the individual fisherman is incentivized to invest in more boats and better gear, to catch more fish and earn a higher profit. But when every fisherman follows the same clear logic, the overall catch soon exceeds the sustainable yield. The fishery collapses and may need years or decades to recover, if it can recover at all. The fishermen who depend on the fishery then go bankrupt. This has happened multiple times, e.g. the . In fact it happens predictably when there is a valuable unregulated resource that multiple competing humans have the technology to exploit.
Tragedies of the commons are therefore instances of Collective action problems. These problems feature a conflict of interest between what is rational for the individual vs. what is rational for the group. Immanuel Kant (1724 � 1804) anticipated a moral solution to such problems with his : "Act only according to that maxim whereby you can, at the same time, will that it should become universal law." Applied to the tragedy of the commons it would translate as, "Extract no more than your fair share of the sustainable resource", which would be the sustainable yield divided by the number of individuals taking a share. Of course in practice, voluntary adherence to Kant's categorical imperative rarely works, as anyone who follows it becomes the chump of the next guy who flouts it, and so the leviathan of government must often step in and regulate the commons.
So that's more detail about the Tragedy of the commons than you get from this summary book, and there's a whole lot more to learn about it. But this should give some idea of the subject density of this summary book. Therefore while the summary book is valuable for an introduction or a refresher, and is far better than reading nothing at all, it by no means replaces more detailed reading.
Other items: while reading the summary, I was struck by the quaintness of Ariely's examples of irrational behavior. Many of them are more in the realm of mild quirks than catastrophic errors, although the latter are always possible. In particular, reading the summary might make you wonder if Ariely had ever gotten a taste of some seriously real irrationality. Such as the kind studied in (2016) by (see my review of the summary book for that book).
Well, it turns out that Ariely's relatively sheltered life (of apparently dealing mostly with folks at elite universities, who had to pass stringent selection for cognitive capacity to get there) came to a rather jolting end. See his latest (at the time of this review) book, (2023). Boy howdy, does Ariely confront some irrationality there!
If you have been following any contentious topics (politics, religion, the science of human intelligence differences, evolution, climate change, etc.) you've been dealing with irrational people for a long time. That Ariely had managed to avoid them until so relatively late in life seems remarkable to me. As I write this in 2025, Trump just finished the first 100 days of his second term, and it's pretty clear that trying to ignore disinformation doesn't make it go away. It's not enough for scientists to just "discover" science for each other. Rationality doesn't trickle down efficiently to hoi polloi on its own. Unless scientists prioritize the problem of convincing the science deniers that science is real (as real as science has to be to churn out steadily improving technologies like smartphones), we might eventually not have any science. That is, it's time for people who understand the value of science to stop denying that there's a war against science. A well-funded war that has been doing its groundwork for decades.
Another potential issue is that Predictably Irrational came out shortly before the broke. Thus it's hard to be sure how firm the various findings in the book are. We now know (or should know) that a single study may not be enough to adequately support a novel psychological finding. For a long cautionary treatment, see (2020) by . Note: I'm not accusing Ariely of any misconduct, I'm just pointing out that we now know psychological studies are prone to certain kinds of errors and there are ways to guard against them, which weren't always followed in the past. So basically you should be on alert when reading any pop psychology book that pre-dates the replication crisis. Some findings in the book might not replicate. One potential red flag is where a study tested "college students." College students are said to be the most heavily psychologically tested subpopulation on the planet, due to their convenient availability to psychology researchers, most of whom are based at universities. But college students are a select group, and their behavior may not translate to other population groups.
I'll close this review with a peculiar experiment:
I'd like to repeat the experiment with a "premier pianist" in place of the violinist. Because a piano is a polyphonic instrument with a wide pitch range, an unaccompanied piano performance can come closer to being a complete song as opposed to a single line in a song. My guess is that passersby would be better able to recognize when they are hearing a premier pianist as opposed to a premier violinist. If a "premier pianist" is more recognizable as such, even when out of their usual performative context, then the reaction of the passersby would be better explained by the quality and completeness of the product itself rather than by passerby expectation.
Given that this summary book is short (obviously), it introduces many concepts with a paragraph or two. If you are seeing them for the first time, this may not be enough. Again, obviously, the next step would be to read Ariely's book, or look up the Wikipedia articles that exist for most of them. For example, the summary briefly mentions the , a topic of such importance and complexity that it warrants at least a full book of its own. The definition given here is:
Tragedy of the commons: The phenomenon in which individual demand for a shared resource overwhelms the supply of that resource; greedy participants can consume more than their fair share, leading to an overall negative effect on everyone else who uses the resource.While the description is accurate, the reader seeing this term for the first time could use a bit more. In particular, the tragedy results from each participant acting in his own rational near-term self-interest. From the standpoint of the individual, who is helpless to restrain the other participants, self-restraint would just mean he gets nothing while the other parties take everything. And this is what makes tragedies of the commons so pernicious - the only way to solve them is to either persuade or coerce all the participants to be less rationally selfish.
Tragedies of the commons therefore amount to , the bane of libertarians and free-market fundamentalists. Free marketeers typically have blind faith in Adam Smith's , whereby individuals each acting in their own rational self-interest produce a collectively wonderful result. This only works when all costs of the individual's actions are correctly priced to the individual. But in a tragedy of the commons, the market only pays the individual to take more. The market doesn't by itself penalize the individual for taking too much, since the consumer cannot see or price in the impending collapse of the commons.
The classic example is overfishing a fishery. A fishery consists of a population of fish that can reproduce at a certain rate. "Sustainable fishing" then entails catching no more fish per year than the number that the fishery can replenish indefinitely. And for many species of fish, the most critical fish are the BOFFFFs - big old fat fecund female fish, which produce a hugely disproportionate number of eggs. But big fish tend to have the most commercial value, and so may be deliberately sought by fishermen.
Tragedy can result when fishermen are free to take as many fish as they are able. Thus the individual fisherman is incentivized to invest in more boats and better gear, to catch more fish and earn a higher profit. But when every fisherman follows the same clear logic, the overall catch soon exceeds the sustainable yield. The fishery collapses and may need years or decades to recover, if it can recover at all. The fishermen who depend on the fishery then go bankrupt. This has happened multiple times, e.g. the . In fact it happens predictably when there is a valuable unregulated resource that multiple competing humans have the technology to exploit.
Tragedies of the commons are therefore instances of Collective action problems. These problems feature a conflict of interest between what is rational for the individual vs. what is rational for the group. Immanuel Kant (1724 � 1804) anticipated a moral solution to such problems with his : "Act only according to that maxim whereby you can, at the same time, will that it should become universal law." Applied to the tragedy of the commons it would translate as, "Extract no more than your fair share of the sustainable resource", which would be the sustainable yield divided by the number of individuals taking a share. Of course in practice, voluntary adherence to Kant's categorical imperative rarely works, as anyone who follows it becomes the chump of the next guy who flouts it, and so the leviathan of government must often step in and regulate the commons.
So that's more detail about the Tragedy of the commons than you get from this summary book, and there's a whole lot more to learn about it. But this should give some idea of the subject density of this summary book. Therefore while the summary book is valuable for an introduction or a refresher, and is far better than reading nothing at all, it by no means replaces more detailed reading.
Other items: while reading the summary, I was struck by the quaintness of Ariely's examples of irrational behavior. Many of them are more in the realm of mild quirks than catastrophic errors, although the latter are always possible. In particular, reading the summary might make you wonder if Ariely had ever gotten a taste of some seriously real irrationality. Such as the kind studied in (2016) by (see my review of the summary book for that book).
Well, it turns out that Ariely's relatively sheltered life (of apparently dealing mostly with folks at elite universities, who had to pass stringent selection for cognitive capacity to get there) came to a rather jolting end. See his latest (at the time of this review) book, (2023). Boy howdy, does Ariely confront some irrationality there!
If you have been following any contentious topics (politics, religion, the science of human intelligence differences, evolution, climate change, etc.) you've been dealing with irrational people for a long time. That Ariely had managed to avoid them until so relatively late in life seems remarkable to me. As I write this in 2025, Trump just finished the first 100 days of his second term, and it's pretty clear that trying to ignore disinformation doesn't make it go away. It's not enough for scientists to just "discover" science for each other. Rationality doesn't trickle down efficiently to hoi polloi on its own. Unless scientists prioritize the problem of convincing the science deniers that science is real (as real as science has to be to churn out steadily improving technologies like smartphones), we might eventually not have any science. That is, it's time for people who understand the value of science to stop denying that there's a war against science. A well-funded war that has been doing its groundwork for decades.
Another potential issue is that Predictably Irrational came out shortly before the broke. Thus it's hard to be sure how firm the various findings in the book are. We now know (or should know) that a single study may not be enough to adequately support a novel psychological finding. For a long cautionary treatment, see (2020) by . Note: I'm not accusing Ariely of any misconduct, I'm just pointing out that we now know psychological studies are prone to certain kinds of errors and there are ways to guard against them, which weren't always followed in the past. So basically you should be on alert when reading any pop psychology book that pre-dates the replication crisis. Some findings in the book might not replicate. One potential red flag is where a study tested "college students." College students are said to be the most heavily psychologically tested subpopulation on the planet, due to their convenient availability to psychology researchers, most of whom are based at universities. But college students are a select group, and their behavior may not translate to other population groups.
I'll close this review with a peculiar experiment:
In another experiment, Ariely asked Joshua Bell, one of the world’s premier violinists, to pose as a street performer at a Washington, DC, metro station during morning rush hour. Only 2.5% of people dropped change in his open violin case, and, when later surveyed, a majority of passersby did not remember hearing music at all. Given their previous experience, they did not expect a world-class musician to be playing in the DC metro, and, consequently, didn’t hear one.I question the conclusion, because what the passersby did not hear was an actual performance. The violin is mostly a monophonic instrument with a limited pitch range. It doesn't play "songs" so much as melodies. Accordingly, a premier violinist like Joshua Bell will rarely perform unaccompanied before an audience. One could argue that Joshua Bell without his orchestral backup is not the same Joshua Bell, "premier violinist".
I'd like to repeat the experiment with a "premier pianist" in place of the violinist. Because a piano is a polyphonic instrument with a wide pitch range, an unaccompanied piano performance can come closer to being a complete song as opposed to a single line in a song. My guess is that passersby would be better able to recognize when they are hearing a premier pianist as opposed to a premier violinist. If a "premier pianist" is more recognizable as such, even when out of their usual performative context, then the reaction of the passersby would be better explained by the quality and completeness of the product itself rather than by passerby expectation.
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