Our Kids: The American Dream in Crisis. By Robert Putnam. Simon & Schuster; 386 pages; $28 and £18.99.

THE most important divide in America today is class, not race, and the place where it matters most is in the home. Conservatives have been banging on about family breakdown for decades. Now one of the nation’s most prominent liberal scholars has joined the chorus.

Robert Putnam is a former dean of Harvard’s Kennedy School of Government and the author of “Bowling Alone” (2000), an influential work that lamented the decline of social capital in America. In his new book, “Our Kids”, he describes the growing gulf between how the rich and the poor raise their children. Anyone who has read “Coming Apart” by Charles Murray will be familiar with the trend, but Mr Putnam adds striking detail and some excellent graphs (pictured). This is a thoughtful and persuasive book.

Among the educated elite the traditional family is thriving: fewer than 10% of births to female college graduates are outside marriage—a figure that is barely higher than it was in 1970. In 2007 among women with just a high-school education, by contrast, 65% of births were non-marital. Race makes a difference: only 2% of births to white college graduates are out-of-wedlock, compared with 80% among African-Americans with no more than a high-school education, but neither of these figures has changed much since the 1970s. However, the non-marital birth proportion among high-school-educated whites has quadrupled, to 50%, and the same figure for college-educated blacks has fallen by a third, to 25%. Thus the class divide is growing even as the racial gap is shrinking.

Upbringing affects opportunity. Upper-middle-class homes are not only richer (with two professional incomes) and more stable; they are also more nurturing. In the 1970s there were practically no class differences in the amount of time that parents spent talking, reading and playing with toddlers. Now the children of college-educated parents receive 50% more of what Mr Putnam calls “Goodnight Moon” time (after a popular book for infants).

Educated parents engage in a non-stop Socratic dialogue with their children, helping them to make up their own minds about right and wrong, true and false, wise and foolish. This is exhausting, so it helps to have a reliable spouse with whom to share the burden, not to mention cleaners, nannies and cash for trips to the theatre.

Working-class parents, who have less spare capacity, are more likely to demand that their kids simply obey them. In the short run this saves time; in the long run it prevents the kids from learning to organise their own lives or think for themselves. Poor parenting is thus a barrier to social mobility, and is becoming more so as the world grows more complex and the rewards for superior cognitive skills increase.

Mr Putnam’s research team interviewed dozens of families to illustrate his thesis. Some of their stories are heart-rending. Stephanie, a mother whose husband left her, is asked if her own parents were warm. She is “astonished at our naïveté”. “No, we don’t do all that kissing and hugging,” she says. “You can’t be mushy in Detroit...You gotta be hard, really hard, because if you soft, people will bully you.” Just as her parents “beat the hell” out of her, so she “whups” her own children. She does her best, but her ambitions for them go little further than not skipping school, not becoming alcoholic and not ending up on the streets.

At every stage, educated families help their kids in ways that less educated ones do not or cannot. Whereas working-class families have friends who tend to know each other (because they live in the same neighbourhood), professional families have much wider circles. If a problem needs solving or a door needs opening, there is often a friend of a friend (a lawyer, a psychiatrist, an executive) who knows how to do it or whom to ask.

Stunningly, Mr Putnam finds that family background is a better predictor of whether or not a child will graduate from university than 8th-grade test scores. Kids in the richest quarter with low test scores are as likely to make it through college as kids in the poorest quarter with high scores (see chart).

There are no obvious villains in this story. Mr Murray suggested that the educated classes preach the values they practise by urging the poor to get married before they have children. But the record of those who tell other people how to arrange their love lives is hardly encouraging. Both George W. Bush and Barack Obama preached the virtues of responsible fatherhood, to no obvious effect.

Mr Putnam sees “no clear path to reviving marriage” among the poor. Instead, he suggests a grab-bag of policies to help poor kids reach their potential, such as raising subsidies for poor families, teaching them better parenting skills, improving nursery care and making after-school baseball clubs free. He urges all 50 states to experiment to find out what works. A problem this complex has no simple solution.