We Americans believe in progress, and yet progress is often a double-edged sword. The benefits and adventures of change often vie with the shortcomings and disruptions, leaving us in a twilight zone of ambiguity and doubt about the ultimate outcome. Few subjects better illustrate this than the decline of marriage, as Isabel Sawhill shows in her sobering book “Generation Unbound: Drifting into Sex and Parenthood without Marriage.”

Even those who know marriage is on the skids — presumably, most of us — may be surprised by the extent of its decline. A little history helps. To Americans coming of age in the 1950s, the expectation was that most would marry. It was part of society’s belief structure. And most did. Now these powerful social pressures have faded and, for many, disappeared.

Consider. In 1960, only 12 percent of adults ages 25 to 34 had never married; by the time they were 45 to 54, the never-married share had dropped to 5 percent. Now fast forward. In 2010, 47 percent of Americans 25 to 34 had never married. Based on present trends, this will still be 25 percent in 2030 when they’re 45 to 54, estimates the Pew Research Center.

The stranglehold that marriage had on middle-class thinking and behavior began to weaken in the 1960s with birth control pills, publication of Betty Friedan’s “The Feminine Mystique” — an assault on women’s traditional housecleaning and child-rearing roles — and the gradual liberalization of divorce laws.

The resulting expansion of personal choice has been breathtaking. Those liberalized divorce laws have freed millions of women and men from unsatisfying or abusive marriages. (From 1960 to 1980, the divorce rate rose nearly 150 percent; it has since reversed about half that gain.) Taboos against premarital sex and cohabitation have virtually vanished. So has the stigma of out-of-wedlock birth or, for married couples, of not having children. With more job opportunities, women flooded the labor market.

Naturally, there was a backlash. These changes spawned new discontents.

Balancing home life and work is stressful and often guilt-ridden. Women and men alike worry they are not devoting enough time to children and/or their jobs. Women resent men who don’t do enough around the house, even though they do much more than they once did. In 1965, wives spent almost seven times as much time on household and child-rearing as husbands; now women do only twice as much, while men still spend more time on the job.

The flight from marriage may also have subtracted from personal happiness. Sawhill quotes from one respected study that “married women and men live longer; they are less likely to be disabled. . . . [They] have better sex than the unmarried and they are less likely to be lonely.”

But the biggest social cost of less marriage involves children. “New choices for adults,” Sawhill writes, “have not generally been helpful to the well-being of children.” Single-parent families have exploded. In 1950, they were 7 percent of families with children under 18; by 2013, they were 31 percent. Nor was the shift isolated. The share was 27 percent for whites, 34 percent for Hispanics and 62 percent for African Americans. By harming children’s emotional and intellectual development, the expansion of adult choices may have reduced society’s collective welfare.

It is not (as Sawhill repeatedly says) that all single-parent households are bad or that all two-parent families are good. But the advantage lies with the approach that can provide children more financial support and personal attention. Two low-income paychecks, or two good listeners, are better than one. With a colleague, Sawhill simulated the effect today if the marriage rates of 1970 still prevailed. The result: The child poverty rate would drop by about 20 percent — a “huge effect” compared with most government programs.

There are other drawbacks. Social commentators Jonathan Rauch and Charles Murray, among others, have argued that marriage is becoming “the great class divide,” as college graduates continue to marry and less-educated couples increasingly don’t. More than 40 percent of births now go to the unwed. Some of these mothers, says Sawhill, will have multiple partners and subject their children “to a degree of relationship chaos and instability that is hard to grasp.”

Sawhill doubts that either liberals or conservatives have workable remedies for these problems. More social services (the liberals) could be “very expensive” and ignore Bill Clinton’s dictum: “Governments don’t raise children; parents do.” Reviving marriage (conservatives) presumes — unrealistically, she says — that many anti-marriage norms can be reversed. Her own preference is that young women better use contraceptives to prevent unwanted pregnancies. This, too, sounds wishful.

Along with the budget deficit, we have a family deficit. It explains some stubborn poverty and our frustrations in combating it. We’ve learned that what good families provide cannot easily be gotten elsewhere. For the nation, this is the deficit that matters most.

Read more from Robert Samuelson’s archive.