Instead of fighting a phony mommy war over what Hilary Rosen said about Ann Romney, we should face the fact that most families these days cannot afford to have one parent stay home with the kids. This is not about “lifestyle” or “values.” This is an economic struggle highlighting yet again the social costs arising from decades of stagnating or declining wages and growing income inequality.

There is a profound class bias in our discussion of what mothers should or should not do. The public debate seems premised on the idea that all two-parent families have a choice as to whether one or both work. That’s still true for the better-off. But this choice is denied to most American families. They have had to send two people into the workforce whether they wanted to or not.

Thus the importance of a study released this week by the Center for American Progress that deserves wide attention. The report demonstrates conclusively that the ruckus over Ann Romney’s decisions is 30 years out of date. Its core conclusion: “Most children today are growing up in families without a full-time, stay-at-home caregiver.”

“In 2010, among families with children,” the study notes, “nearly half (44.8 percent) were headed by two working parents and another one in four (26.1 percent) were headed by a single parent. As a result, fewer than one in three (28.7 percent) children now have a stay-at-home parent, compared to more than half (52.6 percent) in 1975, only a generation ago.”

And these changes are driven more by economics than by any of the mommy-war issues that provide so much fodder for television and radio brawls. “Breadwinning wives are even more common in families with lower incomes,” according to the CAP report. “Seven in 10 (69.7 percent) working wives earn as much or more than their husbands in the bottom 20 percent of income distribution for all families. And about half (45.3 percent) of working wives are breadwinners in families in the middle of the income distribution, up from four in 10 (39.1 percent) in 2007 and only 15.2 percent in 1967.”

So here’s the deal: If you want more households in which one parent can stay home with the kids, you need to boost the incomes of average American families — and especially of poorer families. For millions of American moms and dads, debates about “feminism” or “social conservatism” are irrelevant. It’s about money.

The timing of the report was not driven by the Romney-Rosen kerfuffle. Written by Sarah Jane Glynn, it was an update of an earlier study by CAP senior economist Heather Boushey that was part of a project on working women organized by Maria Shriver. Tuesday was “Equal Pay Day,” and Boushey said that the new study sought to underscore that equal pay “isn’t just about women, it’s about their families, because women are the breadwinner or co-breadwinner.”

We need to look at both sides of the work-family equation. There are, indeed, as Boushey notes, many families in which “women are working because they want to.” That decision should be respected no less than the one Ann Romney made.

But there are many others where the woman “is a single parent, or her husband is unemployed, or her husband isn’t seeing the kind of wage growth that his father did and can’t afford to support the family on his own.”

This points to a contradiction that few conservatives want to confront. When trying to win votes from religious and social traditionalists, conservatives speak as if they want to restore what they see as the glory days of the 1950s family. But they are reluctant to acknowledge that it was the high wages of (often unionized) workers that underwrote these arrangements.

Yet on the right, economic conservatism almost always trumps social conservatism, and market imperatives almost always get priority over family imperatives. As a result, the United States has the weakest family-leave laws in the industrialized world. We have done far less than other well-off countries to accommodate the difficult work-family dilemmas that most moms and dads deal with in the new economy.

It’s good that Ann Romney had choices. She made them honorably and raised a great family. Now let’s debate what should matter in a presidential campaign: which policies will relieve the economic pressures on millions of parents who are equally determined to do right by their kids but have far less room for maneuver. Pro-family rhetoric doesn’t pay the bills.

ejdionne@washpost.com