By Claire Cain Miller, Aug. 8, 2014, Nytimes.com

When measured by who holds a job, American women are falling behindwomen in other developed countries. Yet the American women who areworking tend to have more high-achieving careers. Within that paradox lies a puzzle about how to shape maternity leave.

The United States is the only country besides Papua New Guinea that mandates maternity leave but does not require that it be paid. Other developed countries, meanwhile, offer paid leave (in the United Kingdom, for instance, mothers get a year off, 39 weeks of which are paid) along with other family-friendly benefits like publicly funded child care and the right to demand part-time work.

Economists say paid leave is essential to making it possible for women to work. Beyond benefiting babies and families, it helps the economy by increasing the chances that women continue to work, even years later. The White House Council of Economic Advisers said last month that women’s declining labor force participation is impeding a full economic recovery, and that mandatory paid leave would be a powerful tool to fix it.

But it turns out that long paid leaves can also hold back women. Researchers are finding that even as family leave boosts labor force participation by women, it can have negative effects on their job opportunities.