Amid all the debate over American income inequality, a bigger threat to fairness and social cohesion has gone unrecognized: namely, the widening divide between haves and have-nots when it comes to nurturing children and preparing them for adulthood. In Our Kids, Robert D. Putnam, the author of Bowling Alone (2000), a best seller about the decline of American civic life, argues that children’s access to the core institutions that foster their development—strong families, strong schools, strong communities—is increasingly separate and unequal.

For the well-educated, the phrase “our kids” may well bring to mind conditions of relative affluence, in which children grow up in a family with two married and attentive (even overattentive) parents; attend high-performing schools; and feel themselves embedded in a network of friends and mentors ready to help them navigate life’s challenges. By contrast, “their kids”—the kids of poor and working-class parents—face a world in which social capital is in short supply. As Mr. Putnam shows powerfully and poignantly—combining reporting with empirical analysis—the disparity results in too many children in nonaffluent circumstances feeling alone, emotionally stunted and unable to summon the will to climb today’s economic ladder into the middle or upper class.

From among Mr. Putnam’s incisive portraits, consider two young African-American men living in Atlanta: Desmond, the son of affluent married parents, and Elijah, whose working-class parents, in his own words, “couldn’t live together for nothing.” Growing up, Desmond’s parents took him to “Mommy and Me” classes at the library, superintended soccer games and piano practice, steered him clear of soda, and plugged him into a local church. The fruits of this “neotraditional” model are clear: Desmond is now a college graduate, an intern at the Centers for Disease Control and close to both of his parents.

Elijah, by contrast, endured a tumultuous family life. He grew up amid a revolving cast of caretakers in a world without consistent attention and affection. As an adolescent, he was shuttling not between soccer and the youth symphony but between taking care of his mother’s year-old twins and spending time in “juvie” for arson. Now he is barely holding down a job bagging groceries and acknowledges struggling to rein in his temper: “I just love beating up somebody and making they nose bleed and just hurting them.” Elijah’s social and economic prospects are bleak, and they are emblematic not only of poor black children but of a growing swath of working-class children of all racial backgrounds.

Our Kids offers considerable empirical support to its claims of institutional inequality, reinforcing the story told by Charles Murray in Coming Apart (2012), his own exploration of class division. We learn that the percentage of children living in single-parent homes has been falling in college-educated circles since the mid-1990s even as it has been rising in homes headed by parents with a high-school diploma or less. Mr. Putnam reports that, by the time they start kindergarten, children from professional families hear 19 million more words than children from working-class families. Even the religious gap between the rich and the poor—traditionally rather narrow—is widening. These days, Mr. Putnam laments, “poor families are generally less involved in religious communities than affluent families,” which is unfortunate, he notes, given that churchgoing is associated with better performance in school, less drinking and drug use, and less delinquency. The class divide in institutional access translates into dramatically different chances that children will flourish later in life.

America did not used to be so socially segregated by class. Mr. Putnam, a professor of political science at Harvard University, points the finger at shifts in the economy: declining real wages and greater job instability for the less-educated. Along the way, he does acknowledge some of the cultural changes and harmful policies that have deepened this divide: e.g., the post-1960s triumph of expressive individualism and drug laws that have put so many lower-income fathers behind bars.

But Our Kids doesn’t give adequate weight to the toxic effects of a Dionysian popular culture and other social transformations that have eroded the values and the vitality of families and churches, hitting working-class and poor communities especially hard. Nor does Mr. Putnam give sufficient consideration to the possibility that the modern welfare state has supplanted the basic functions of the family and of civil society; it has also undercut the effectiveness and affordability of our schools and colleges with a welter of regulations.

Mr. Putnam concludes Our Kids by calling for a raft of civic initiatives and public policies, many of which seek to add to services for poor and working-class children and their parents. Some seem wise: for instance, expanding proven vocational programs such as Career Academies. Others less so, like widening the scope of public preschool education, a perennial source of high hopes but one that has yielded, so far, only meager results at the national level.

We know from the government’s record over the past 50 years that programs can do more harm than the good they intend. More important: No social services designed in Washington will substitute for homes with two devoted parents and communities replete with PTO moms and soccer dads. At the very least, we need a public conversation about the importance of marriage and parenting, one that is conducted in such a way that it will have as much purchase in Bethlehem, Pa., as in Bethesda, Md. Otherwise we can expect to continue living in a world in which “our kids” do just fine and “their kids” do not.

This review first appeared in the Wall Street Journal.