Each week, In Theory takes on a big idea in the news and explores it from a range of perspectives. This week, we’re talking about paid family leave. Need a primer? Catch up here.

Ruth Milkman is distinguished professor of sociology at the CUNY Graduate Center, president of the American Sociological Association and author of the new book “On Gender, Labor, and Inequality.”

The United States is famously exceptional in its failure to guarantee paid family leave to new parents. What is less well-known is that this failure contributes to the growing problem of income inequality, widening the gap in well-being between the haves and the have-nots.

In 21st-century America, paid leave is available to most upper-level employees, especially professionals and managers, when they become parents or need to care for a seriously ill family member. However, the nation’s burgeoning ranks of low-wage workers typically have no access whatsoever to any kind of paid leave. Instead, they are repeatedly forced to choose between earning a day’s pay and providing vital care to their families. When they choose the latter, they fall even further behind.

The only federal legislation addressing this issue is the 1993 Family and Medical Leave Act, the very first bill President Clinton signed into law after his inauguration. It requires many employers to provide up to 12 weeks of unpaid family and medical leave to both men and women who need to care for a new child or attend to the health needs of themselves or a close family member. It also ensures that workers can return to the same job or a similar one after an FMLA leave.

Although it was a major breakthrough when it was passed, the law has severe limitations. FMLA covers only employers with 50 or more workers and stipulates various other requirements. In practice, it covers only about 60 percent of the labor force and less than 25 percent of new mothers. And perhaps most importantly, most workers simply cannot afford to take time off without pay. A 2012 study by the Labor Department found that nearly half (46 percent) of FMLA-eligible workers who needed a family leave but did not take it cited lack of pay as the reason.

Some U.S. employers do provide paid family leave to their employees — but this covers only about 12 percent of those employed in the private sector. In many cases, however, only highly trained full-time professionals and managers are eligible for employer-provided benefits, while low-wage and part-time workers are all too often left out entirely. The Census Bureau reports that 66 percent of new mothers with four-year college degrees have access to some type of paid maternity leave, but only 18 percent of those without a high school diploma do. Yet because less educated workers typically are at the bottom of the labor market, living paycheck to paycheck, they are the ones who can least afford to take time off without pay.

The state-level paid family leave programs in California, New Jersey, Rhode Island and New York state (as of last month) are notable in that they all aim for universal coverage, unlike both the FMLA and employer-provided leave programs. Regardless of employer size and whether workers are part- or full-time, virtually everyone employed in the private sector in these states is (or in the case of New York, will soon be) eligible for partially paid time off to bond with a new baby or care for a seriously ill family member. The FAMILY Act that is languishing in Congress would create a similarly universal program at the national level.

These programs all are designed to help level the playing field, although it turns out that this is not as simple a matter as it first appears. My research on the first decade of California’s paid family leave program (in collaboration with economist Eileen Appelbaum) found that because many low-wage workers remain unaware of the existence of that state’s paid family leave program, it is more often used by those in higher-level, better-paying jobs. On the other hand, the low-wage workers who did know about and use California’s paid family leave program have benefited greatly.

There is a mountain of evidence that babies’ health is enhanced when they are cared for by their own parents in the early months of life; similarly, seriously ill family members recover more quickly when cared for by a close relative. But the importance of paid family leave programs goes beyond these vital benefits for family health and well-being. Workers with access to paid leave have more stable employment and are able to improve their earnings over time. Indeed, paid family leave that is available to everyone, not just the most privileged employees, can help bridge the widening gap between low-wage workers and the upper ranks of the labor force.

Explore these other perspectives:

Anne-Marie Slaughter and Brigid Schulte: A modern family leave policy needs these three things

Carrie Lukas: I live in a country with paid family leave. It’s no magic bullet.

Sarah Jane Glynn: Stop saying businesses can’t afford paid family leave