Tim Harford is a member of a small but dangerous sect: economists whose work is fun to read. Richard H. Thaler and Cass R. Sunstein’s work is just as much fun. But Harford differs from Thaler and Sunstein with regard to a basic question: Are people smart or dumb?

For Thaler and Sunstein, authors of Nudge: Improving Decisions About Health, Wealth, and Happiness, it is obvious that people, if they’re not dumb, are at least poorly wired for making some of life’s most important decisions.

Most folks do a good job of picking out an ice cream flavor because, after all, everyone knows what chocolate tastes like—and if you don’t want chocolate, you can always order a different flavor. Plus, feedback is prompt; you’ll know immediately whether you like your choice or not. But choosing how (or whether) to treat prostate cancer is different, because the consequences of what you decide are both unfamiliar and uncertain, and the feedback is long delayed. When possible, people will simply avoid weighing the choices and do whatever the doctor tells them.

This is why default options govern our fates. How much money do you set aside for your 401(k) plan, and what do you invest it in? Most likely, you apportion funds at whatever rate and asset blend your company started you with. Call it a quirk, but people are hardwired to favor the status quo. Here is another quirk: Most folks are predisposed to favor immediate gains, such as eating ice cream, over deferred ones.

Thaler and Sunstein argue for little “nudges” to provoke people into overcoming their faulty human wiring. The little beep you hear when you fail to buckle a seat belt is a good example. Similarly, when A.T.M.'s remind us to take our cards, they are giving us a nudge without which we are maddeningly likely to leave our cards behind. Humans, the authors observe, are susceptible to “postcompletion” errors. Once we finish a primary task, we are apt to forget the subsidiary one, a fact that’s self-evident to anyone who has driven away from a gas station without replacing the gas cap.

The authors recommend improving the “choice architecture” for a wide variety of everyday situations so that we are nudged in the right direction: smaller plates in cafeterias; plain-English descriptions of mortgage options; stoves with the knobs in two rows of two rather than a single row of four, so that the human eye can automatically infer which knob corresponds to which burner. Sunstein and Thaler don’t want to force us—only to nudge us. But the premise of their so-called libertarian paternalism is that left to our own rather hapless devices, we would often make the wrong choice.

Harford, a columnist for the Financial Times, argues to the contrary in The Logic of Life: The Rational Economics of an Irrational World. “Economists’ faith in people’s rationality is usually about right,” he writes. Although he acknowledges the many studies that have documented human folly, he claims that “it rarely pays to assume that any human being is incapable of weighing the pros and cons of the decision in front of him.” Obviously, he has never seen me try to turn on the stove.

In Harford’s previous book, The Undercover Economist, he plumbed the business world and found unexpected riches, memorably explicating the marketing savvy behind Starbucks' array of menu options. Now, in The Logic of Life, Harford applies rational-choice theory to explain human history writ large and small—everything from why the Industrial Revolution began in England to why prostitutes do not invariably insist that their customers use condoms.

As with Thaler and Sunstein, he wanders from the world of economics into ordinary life situations, many of which involve dating, and attempts to show the economic principles that govern them. But Harford sees sublime rationality everywhere he looks. He aims to entertain and no doubt shock us by using the term blowjob on the very first page, going on to argue that the fad for oral sex among teens is a rational choice that reduces the risk of contracting H.I.V. or getting pregnant.

Likewise, he says, prostitutes who eschew condoms have implicitly deduced that those risks are small relative to the premium they can presumably charge for unprotected sex. These calculations, he says, need not be conscious; after all, an outfielder can put his glove in the path of an arcing fly ball without knowing any physics. This is an elegant line of thought but not a totally convincing one. Instinct may be a good guide for athletes, but—a certain former crusading governor should take note—it will not necessarily foster sound decisions for those involved in the sex trade.

Though his tone is irreverent, Har­ford is no heretic; he only pretends to be. Things happen for a reason, he argues, and the reason is that people make cogent choices. This is mainline economics meshed with determinism. Thus he holds that the Industrial Revolution was no accident. Because En­gland had plenty of coal and a shortage of manual labor, it was in its interest to develop a cheap fuel—coke—for producing iron. So somebody in England did.

Like other true believers, Harford is wont to carry his faith too far. It becomes a truism to say that everything has a reason (well, of course it does), and it defies common sense to think that the underlying decisions are always, or even nearly always, logical. Examples: giving mortgages without proof of income or starting wars you can’t win. Harford goes so far as to say that prospective addicts may be rational in their choice to light up or inject themselves.
This is precisely the sort of neoclassical economics orthodoxy that Thaler, a behavioral economist at the University of Chicago Graduate School of Business, has been debunking for years. He and Sunstein, his colleague at the university’s law school, treat addiction and overeating like what they obviously are—evidence of people’s inability to weigh benefits and costs.

What further distinguishes Nudge—by far my favorite of these two works—is its prescriptive element. For instance, people are subject to groupthink; they switch political candidates (as they do stocks) on the basis of momentum and popular sentiment rather than careful evaluation. These tendencies, once recognized, can be harnessed for the good. It turns out that the best way to get people to pay taxes or conserve energy is not to threaten them with punishment but to inform them that most other people are already doing so. Their innate conformism will do the rest. This is just one of the numerous ways that Thaler and Sunstein reveal a human economic animal that, while highly imperfect, is imperfect in predictable ways. We are flawed, but we can be nudged.