Single mothers’ entry into work in the 1990s was also driven by a thriving economy and low unemployment, in addition to major policy changes. The Personal Responsibility and Work Opportunity Reconciliation Act of 1996, known as welfare reform, required many recipients to work , and set an expiration date on benefits. The Clinton administration also expanded the earned-income tax credit for low earners, which becomes more generous the more people work.

The employment rate of single mothers climbed, and the share on public assistance dropped. But work requirements also left a significant number without jobs or federal aid — a group researchers describe as disconnected. They tend to have less education and younger children, and are more likely to be facing physical or mental health issues, substance-abuse problems or domestic violence.

By 2016, there were roughly 3.3 million jobless single mothers and just 1.3 million families receiving welfare, known as Temporary Assistance for Needy Families, the lowest number since welfare reform was enacted, according to LaDonna Pavetti, vice president for family income support policy at the Center on Budget and Policy Priorities.

Even people eligible for TANF have been applying for it at lower rates, largely because of the reputation that it is difficult to obtain, researchers said. The Trump administration has proposed adding similar work requirements for people receiving food and housing assistance and medical coverage.

“Many safety net programs have been eviscerated, and work requirements have increased,” said Carol Burnett, executive director of the Mississippi Low Income Child Care Initiative, a nonprofit that supports working mothers. If more single mothers are working, she said, it’s for a simple reason: “They need the money.”

At the same time, some states and cities have passed policies that help working families.

One example is minimum wage increases. Areas that raised it most saw the largest rise in the rate of single mothers who work, the Times analysis found. (This doesn’t prove that the higher wages caused the return to work, though — areas with strong economies might have felt more confident raising minimum wages, for example.)