In her best-selling book, “Grit: The Power of Passion and Perseverance,” Angela Duckworth celebrates grit as the single trait in our complex and wavering nature which accounts for success. Photograph by Ruth Fremson / The New York Times / Redux

Angela Duckworth, in her best-selling book, “Grit: The Power of Passion and Perseverance,” celebrates a man whom she calls a “grit paragon”: Pete Carroll, the coach of the Seattle Seahawks, who led the team to a Super Bowl victory in 2014. It seems that Carroll had seen Duckworth’s TED talk nine months earlier and got in touch, eager to reassure her that building grit was exactly what the Seahawks culture was all about. Two years later, Duckworth visited the Seahawks training camp. She lectured to the team’s players and coaching staff. The subject was . . . grit. Duckworth was impressed by the Seahawks, and she quotes sentiments that are characteristic of the Carroll ethos: “Compete in everything you do. You’re a Seahawk 24-7. Finish strong. Positive self-talk. Team first.” Since the team trains ferociously all the time—going all out, for instance, in bone-crunching intra-squad practice sessions—this conversation may not have been entirely necessary.

Duckworth, a professor of psychology at the University of Pennsylvania, finds grit in the best possible places. Her grit obsession, as she recounts, began at least a decade earlier. As a graduate student, she visited West Point, where each year twelve hundred new cadets go through a gruelling seven-week training regimen (“Beast Barracks”) before entering freshman year. Most make it through, though some do not. Why not? Duckworth could make some guesses. In this same period, eager to find out what made top people successful, she was interviewing “leaders in business, art, athletics, journalism, academia, medicine and law.” She discovered that “the highly successful had a kind of ferocious determination that played out in two ways. First, these exemplars were unusually resilient and hardworking. Second, they knew in a very, very deep way what they wanted.”

Thus armed, Duckworth returned to West Point, a couple of years later, with something called the Grit Scale, a written survey that she asked a fresh batch of cadets to administer to themselves. The survey measured their degree of identification with such statements as “Setbacks don’t discourage me. I don’t give up easily” and “My interests change from year to year.” All the statements were essentially a way of measuring perseverance and passion (by which she means stick-to-itiveness—i.e., perseverance again). The cadets who took the survey were then assigned a grit score. At the end of Barracks Beast, Duckworth was pleased with the results. Seventy-one cadets had dropped out, and “grit turned out to be an astoundingly reliable predictor of who made it through and who did not.”

A happy experimental result, I suppose, but one can’t suppress certain doubts about it. Cadets may drop out of a gruelling training period for many reasons—emotional, physical, even moral, in the sense that they are angered by pain administered as an entry test. To call all the possible reasons simply an absence of grit can be true in no more than the most general way. (Perhaps, in this environment, it took grit to withdraw.) And, even if we agree that, broadly speaking, the survivors had more grit, how do we apply this lesson to the rest of life? To get into West Point in the first place, you must have good grades, athletic ability, and leadership qualities; you might say that the cadets already had grit. The survivors have extra grit, I guess. But how much can be gleaned from the endurance of eighteen-year-olds through seven weeks of torment? The cadets are admirable, of course, but they are way off the charts compared with the rest of us; their experience may be of limited use.

Other social scientists, looking at the West Point situation and many others that Duckworth considers, might have called grit an “independent variable”—one possible factor in a given experimental situation affecting many other factors. But Duckworth decided that grit is the single trait in our complex and wavering nature which accounts for success; grit is the strong current of will that flows through genetic inheritance and the existential muddle of temperament, choice, contingency—everything that makes life life. In recent years, Duckworth has tested all kinds of people on the Grit Scale, and in her book she writes about many other high achievers whom she interviewed or studied from afar—Jeff Bezos, the founder of Amazon; Will Shortz, the Times crossword-puzzle editor; Jamie Dimon, of JPMorgan Chase; Francesca Martinez, the British woman with cerebral palsy who became a famous standup comic; the women’s soccer team at the University of North Carolina. They all have grit.

I’m not sure what we’re learning from any of this. There may be a few champions who get by purely on talent, luck, or family wealth, but we can assume—can’t we?—that most highly successful people are resilient and persevering. It would be news if they weren’t. Grit can be partly inferred from their success itself, which is, of course, what drew Duckworth to these people in the first place. There are no mediocre or moderately successful people in her book, and she has little interest in the myriad ways we hamper ourselves—failure, in this account, is simply owing to a lack of grit.

Tautology haunts the shape of these fervent lessons. “Grittier spellers practiced more than less gritty spellers,” Duckworth assures us. Well, yes. She is looking for winners, and winners of a certain sort: survivors in highly competitive activities in which a single physical, mental, or technical skill can be cultivated through relentless practice. As examples, however, instances of success in soccer, spelling bees, and crossword-puzzle design suffer from the same weakness as success during Barracks Beast—they may not offer much help to people engaged in work that demands more diffuse or improvisatory skills. In many careers, you can grind away for years and get nowhere if you aren’t adaptable, creative, alert. In modern offices, many people work in teams, present ideas to a group, move from one project to another. Grit may be beside the point.

Even so, “Grit” is a pop-psych smash. More than eight million people saw Duckworth’s TED talk before the book came out. Duckworth is in demand in many places as a motivational speaker. But if she were merely battling Norman Vincent Peale for control of the airport book racks, and Tony Robbins for YouTube dominance, her success wouldn’t matter very much. Duckworth’s work, however, has been playing very well with a second audience: a variety of education reformers who have seized on “grit” as a quality that can be located and developed in children, especially in poor children. Some public schools are now altering their curricula to teach grit and other gritty character traits. In California, a few schools are actually grading kids on grit; the practice is widespread in the trendsetting charter-school chain KIPP (Knowledge Is Power Program). The standardized-testing agencies that administer the National Assessment of Educational Progress (NAEP) and the Program for International Student Assessment (PISA) are moving toward the inclusion of character assessment as a measure of student performance. Duckworth, to her credit, has argued against tying such scores to the evaluation of teachers and the funding of schools, but that development may be inevitable.

This snowballing effect among school reformers can’t be understood without recognizing a daunting truth: We don’t know how to educate poor children in this country. (Our prosperous students do fine on international tests.) George W. Bush’s No Child Left Behind program and President Obama’s Race to the Top incentives were designed to raise test scores in general, and in particular to close the gap between affluent and poor children, but neither program, putting it mildly, has succeeded. Despite some success at individual schools, there has been little over-all improvement in the scores of poor children. The gap between white and minority children has actually increased in recent years.