There’s a moment in Dale Maharidge and Michael S. Williamson’s “Someplace Like America,” a documentary account of three decades of American downward mobility, when Maharidge spontaneously decides to phone up Charles Murray. It’s around the year 2000, welfare reform is on the books, and Maharidge wants to know what Murray, the author of “Losing Ground” and the country’s harshest critic of the welfare state, has to say about the growing phenomenon of the working poor—Americans who have jobs but still can’t make ends meet.

“Give me an example,” Murray says. Maharidge begins to describe a woman named Maggie Segura, employed by the state of Texas, whom Maharidge met, along with her daughter, at a food bank. “Is she a single mother?” Murray demands. Guilty as charged. Maggie Segura shouldn’t have had a child with the wrong guy—point Murray. He asks for another example, and Maharidge describes an intact family: three kids, mom, and dad, Obie, who works as a janitor but has to sell his blood plasma to make up for shortfalls in the family budget. Murray is undeterred. “What is the appropriate success for working families? The guy is making ten bucks an hour, the wife is working part time. They’ve got three kids. Should we feel bad?” Murray does some quick calculations. “If I had to, I could figure out ways to live on $550 a week with three kids. I probably wouldn’t live in Austin. I’d go someplace else, where it was a lot cheaper. I’d make choices.”

Point Murray again. Whatever stories from around the country Maharidge hits his way, Murray kills with the return. You can win every point when your social theory tells you that the only poor people in America—especially the America of the nineties boom, when “the general trajectory is up and away. You can make a decent living without the government helping you”—are ones undeserving of help. Maggie Segura had a child by the wrong man. Obie should have moved his family to Appalachia. Everybody screws up.

I was reminded of this scene from “Someplace Like America” while watching a new documentary film, “Two American Families,” which will air next Tuesday night, July 9th, on the PBS series “Frontline.” The film, produced by Tom Casciato and Kathleen Hughes (friends of mine), and narrated by Bill Moyers, follows the lives of two families in Milwaukee, the Stanleys and the Neumanns—the former black, the latter white—over the past two decades, starting in 1991. Both come out of the solid working class, and their fates are familiar ones. Jackie Stanley and Tony Neumann had factory jobs at the huge engine maker Briggs & Stratton, while Claude Stanley worked for A. O. Smith, a leading maker of chassis frames. All were union jobs, and all disappeared around 1990 as manufacturing went overseas. That’s when we meet the Stanleys and the Neumanns—just as both families are beginning to sink. The only work the men can find pays half the factory wage, without benefits—Claude waterproofs basements, Tony retrains and works the overnight shift doing light manufacturing. Jackie Stanley tries to sell real estate; Terry Neumann gets into a cosmetics-selling scheme, works at a school cafeteria, then drives a forklift. Without unions to support them, they are all at the mercy of indifferent employers and the harsh vagaries of the post-industrial economy.

The Stanley kids pick up odd jobs to help their parents. The Neumann kids start to struggle in school as their parents’ work lives leave a vacuum at home. There are never enough hours of work, or hours together at home, or dollars to pay the bills. Vacations disappear; health crises become disasters. The oldest Stanley boy, Keith, goes to college at Alabama State on a credit card. Neither family can get back to where they were before their economic slide began—a slide that coincides with those booming nineties.

This outline barely does justice to “Two American Families,” which will take its place among the central documents of our time. We all know the gist of the story, but its power lies in what we didn’t know, in the details of these American lives: Tony Neumann trying to hold back his tears at Sunday mass, the hardening of Terry Neumann’s features as she feeds a severely disabled boy in her care, Jackie Stanley’s sense of failure, Claude Stanley’s undaunted laugh even as his eyes flash with anger. The white family splits up; the black family stays together. Terry Neumann loses her house, which JPMorgan Chase sells at a fraction of the debt she owed on it. Of the eight kids between the two families, only Keith Stanley gets a four-year college degree, and with it a good job at city hall. The other children survive in varying degrees of dependency on their parents; some become parents themselves, too soon.

If you screened “Two American Families” for Charles Murray and other social critics who believe that the decline of America’s working class comes from a collapse of moral values, social capital, personal responsibility, and traditional authority, they would probably be able to find the evidence they’d need to insulate themselves against the sorrow at the heart of the film. None of the four parents finished college. The Neumanns’ divorce leaves Terry and the children in worse straits than ever. The Stanleys don’t move to rural Mississippi, where life is cheaper. The kids make plenty of their own mistakes. None of them thinks of inventing Napster. The Stanleys and Neumanns are punished to the fullest extent of the economic law for every mistake made, and for all the mistakes they didn’t make.

But the intellectually honest response to this film is much less comforting, for the overwhelming impression in “Two American Families” is not of mistakes but of fierce persistence: how hard the Stanleys and Neumanns work, how much they believe in playing by the rules, how remarkable the cohesion of the Stanley family is, how tough Terry Neumann has to become. Both families devoutly attend church. Government assistance is alien and hateful to them. Keith Stanley says, “I don't know what drugs or even alcohol looks like.” In the words of Tammy Thomas, whose similar story is told in my new book, “The Unwinding: An Inner History of the New America,” these people do what they’re supposed to do. They have to navigate this heartless economy by themselves. And they keep sinking and sinking.

On Sunday, the Times reported that C.E.O. pay in 2012 increased by sixteen per cent over the previous year, with the median compensation package now at $15.1 million. The blessings at the top grow more fruitful year by year, in good times and bad. There must be a social or economic theory somewhere that explains why all this is necessary and just.