LOS ANGELES — What’s the difference between growing up in an affluent family and growing up poor in America?

Yes, upper-middle-class kids have more money. But they also have more attentive parents, stable families, good teachers – and even more friends and acquaintances.

As a result, well-off kids have one more advantage, according to Harvard social scientist Robert D. Putnam: They have air bags in their lives.

Social air bags offer the same kind of protection as car air bags: In a collision, you walk away unharmed. When affluent kids stumble, a parent or someone else often jumps in to help – by hiring a lawyer, paying for therapy, making sure contraceptives are available or merely finding a tutor.

“A lot of poor kids don’t have any access to help,” Putnam told me last week. “Some of the stories are heartbreaking.”

Putnam’s new book, “Our Kids,” is a concise and readable collection of current research on inequality in America – not income inequality but widening inequality of opportunity among the nation’s children.

It’s a powerful challenge to the fundamental premise of American society: the comforting myth of a level playing field for talented, hardworking kids no matter their origins. Its strongest passages are not statistics but stories. And often, the key difference is the presence of those air bags: When a kid hits a roadblock, is anyone there to help?

Take a 21-year-old Santa Ana, California, woman whom Putnam calls Lola. Her parents, both drug addicts, deserted her as an infant; she was raised mostly by an older sister, with no parents in the house.

Lola was a voracious reader in elementary school, but when she arrived at Santa Ana High School she was misassigned to classes for Spanish speakers and lost an entire year. “There were kids with guns in the school,” she told one of Putnam’s researchers. “The teachers would even say out loud … that they’re just there to baby-sit, that they don’t care if we learn or not.”

Lola is trying to make her way through community college, but the odds are against her.

Some of Putnam’s statistics are equally compelling. Here are just three:

n Educational opportunity depends on income more than test scores. “High-scoring poor kids are now slightly less likely to get a college degree than low-scoring rich kids,” Putnam reports. In 2000, 29 percent of poor kids with high test scores got degrees, compared with 30 percent of affluent kids with low scores.

n The achievement gap among children shows up in tests on the first day of kindergarten, at age 5 – and it depends on family income, too. “The class gap among students entering kindergarten (is) two times greater than the racial gap,” Putnam reports.

n A 2014 study found that almost two-thirds of affluent children had access to informal mentors outside their families – family friends, teachers, coaches, church leaders or counselors. Fewer than 40 percent of poor children reported any mentoring.

Are there solutions? Putnam proposes a list, most of them familiar: more tax breaks for families with children, easier access to contraceptives, universal preschool, programs to allow poor kids to transfer to schools in affluent neighborhoods, “combat pay” to attract good teachers to troubled schools. And a modest measure that he’s passionate about: Schools should stop charging kids for participating in athletics and other extracurricular activities.

In a debate that often comes down to a search for a single villain – either economic change driven by Wall Street or the collapse of the traditional family thanks to libertines of the 1960s – he comes down firmly on both sides.

“The first driver was the collapse of the working-class economy … and that eroded families,” he said. “But the collapse of the family also became an independent cause.”

But amid that familiar political division, here’s one last startling statistic: About 95 percent of Americans agree that “everyone in America should have equal opportunity to get ahead,” and 86 percent believe that society should do “whatever is necessary” to make that promise real. It’s hard to find any question these days that many Americans agree on.

Some potential Republican candidates for president, including former Gov. Jeb Bush and Sen. Marco Rubio of Florida have said they want to make equal opportunity, traditionally a Democratic issue, part of their agenda.

They won’t agree with each other, or with their Democratic rivals, on what to do. But they’re right on one count: The problem ought to be at the center of the 2016 presidential campaign.