Patricia Arquette knows moms aren't having it all right now. (Photo by Matt Sayles/Invision/AP)

Last night at the Academy Awards, actress Patricia Arquette brought down the house in her acceptance speech for some socioeconomic commentary on wage inequality.

“To every woman who gave birth, to every taxpayer and citizen of this nation, we have fought for everybody else’s equal rights,” she proclaimed. “It is our time to have wage equality once and for all, and equal rights for women in the United States of America!”

But she didn't just say "women." She said "every woman who gave birth." And that speaks to a pretty important distinction in the wage inequality research: There's not so much a gender pay gap as there is a motherhood pay gap. And there's new research all the time explaining why it persists.

Women make 82 cents for every dollar a man makes, according to the most recent figures from the Bureau of Labor Statistics. But when you break that down, the situation looks a lot better for women who've never married -- those women only make five percent less than single men. Childless women, meanwhile, make seven percent less than single men. University of Massachusetts sociology professor Michelle Budig displayed the data like this in a recent report for the centrist think tank Third Way:


Married mothers make the least. (Michelle Budig, Third Way)

What's more, while the overall gender wage gap has been shrinking in the United States, the discrepancy for mothers has been growing, and it gets wider with every additional child. One reason for this: The cost of day care for two kids can make it more cost-effective for one parent to stay home, according to the Organization for Economic Cooperation and Development, and it's usually not going to be the father. The effect may be more pronounced among low-income mothers, who show a larger wage gap than high-earning women.

There's pretty extensive literature on why the motherhood pay gap is still so vast. Perhaps most obviously, women have to take time off for pregnancies and often return to reduced hourly schedules after giving birth -- that accounted for a large difference in earnings for graduates of a top U.S. business school.

More subtly, there's evidence that women are more likely to accept non-monetary compensation for jobs -- predominantly, good health insurance -- which lowers the wage number. And discrimination plays a role, too: A psychological experiment found that people are likely to rate mothers as less competent and committed to their work than non-mothers in ways that affect hiring and promotion decisions (fathers, on the other hand, received no such skepticism).

Just last week, however, we learned just how thoroughly the wage gap is wired into the nature of parenthood. Looking at the decline in fertility in China after the imposition of the one-child policy, researchers found that the gender wage gap closed, as well. They theorize that since women have a shorter fertility window than men, they tend to marry older guys if they want to have more time to bear children. With age comes experience, which creates a relationship of comparative advantage: Since men have relatively more education and are more advanced in their careers, they have higher earning potential, and so they shoulder the responsibility of financial support. That builds on itself, leaving women far behind.

Potential solutions to the motherhood penalty are well-known: Paid parental leave, universal pre-kindergarten, and child tax credits, to name a few. Arquette probably didn't have time to go into them in her speech (and backstage, might have undermined her goodwill with unfortunate remarks about how gay people and people of color "we’ve all fought for to fight for us now"). But maybe next time, an actress -- or actor -- could use that platform to go beyond the problem and ask for something specific to fix it.