Most readers of The New York Times probably subscribe to what Paul Tough calls “the cognitive hypothesis”: the belief “that success today depends primarily on cognitive skills — the kind of intelligence that gets measured on I.Q. tests, including the abilities to recognize letters and words, to calculate, to detect patterns — and that the best way to develop these skills is to practice them as much as possible, beginning as early as possible.” In his new book, “How Children Succeed,” Tough sets out to replace this assumption with what might be called the character hypothesis: the notion that noncognitive skills, like persistence, self-control, curiosity, conscientiousness, grit and self-confidence, are more crucial than sheer brainpower to achieving success.

“Psychologists and neuroscientists have learned a lot in the past few decades about where these skills come from and how they are developed,” Tough writes, and what they’ve discovered can be summed up in a sentence: Character is created by encountering and overcoming failure. In this absorbing and important book, Tough explains why American children from both ends of the socioeconomic spectrum are missing out on these essential experiences. The offspring of affluent parents are insulated from adversity, beginning with their baby-proofed nurseries and continuing well into their parentally financed young adulthoods. And while poor children face no end of challenges — from inadequate nutrition and medical care to dysfunctional schools and neighborhoods — there is often little support to help them turn these omnipresent obstacles into character-enhancing triumphs. The book illuminates the extremes of American childhood: for rich kids, a safety net drawn so tight it’s a harness; for poor kids, almost nothing to break their fall.

Though Tough examines at length the travails of both groups, it’s the plight of disadvantaged children that compels his interest and emotions. In his previous book, the well-received “Whatever It Takes,” Tough followed the efforts of the educator Geoffrey Canada to turn his social service organization, the Harlem Children’s Zone, into a “conveyor belt” that would reliably carry the neighborhood’s children from infancy through primary and secondary school, into college and the middle class. In Canada’s story, Tough found a deep and complicated character fighting to accomplish a valiant goal in the face of terrific odds. In “How Children Succeed,” Tough is working in miniature, sketching a handful of poor children and their mentors, and these depictions sometimes lack the force and distinctiveness of his portrait of Canada. But they are keenly and sensitively observed, and occasionally even whimsical, as in his captivating account of Kewauna Lerma, a Chicago teenager. Growing up in the erratic care of a feckless single mother, “Kewauna seemed able to ignore the day-to-day indignities of life in poverty on the South Side and instead stay focused on her vision of a more successful future.” Kewauna tells Tough, “I always wanted to be one of those business ladies walking downtown with my briefcase, everybody saying, ‘Hi, Miss Lerma!’ ”

Here, as throughout the book, Tough nimbly combines his own reporting with the findings of scientists. He describes, for example, the famous “marshmallow experiment” of the psychologist Walter Mischel, whose studies, starting in the late 1960s, found that children who mustered the self-control to resist eating a marshmallow right away in return for two marshmallows later on did better in school and were more successful as adults.