Over the past three decades, Congress has conducted a major experiment in anti-poverty policy. Legislators have restructured benefits and tax breaks intended for the poor so that they penalize unmarried, unemployed parents — the modern-day version of the “undeserving poor.” At the same time, working parents, the aged and the disabled are getting larger benefits.

Legislative changes in three major programs have driven these shifts.

Before 1996, Aid to Families With Dependent Children was the single most important program that provided direct cash payments to poor families, the overwhelming majority of which were headed by single women. Just under 60 percent of adult recipients were never-married mothers, and 24 percent were divorced or separated mothers.

Welfare reform enacted in 1996 by a Republican Congress and signed into law by President Clinton replaced A.F.D.C. with the far more restrictive Temporary Assistance for Needy Families, which limited the lifetime receipt of welfare benefits to five years, and required recipients to find a job within two years of entering the program. The number of welfare recipients fell from 12.3 million in 1996 to 4.4 million by 2010.

The second legislative change was the enactment, in 1975, of a modest version of the earned-income tax credit. In subsequent years, the number of people covered and the cost of the tax credit has been repeatedly enlarged — in 1986, 1990, 1993, and 2001. More than 40 percent of recipients are married couples, a far higher proportion than in the original A.F.D.C. or the current T.A.N.F. population. In fact, the earned-income tax credit favors married couples by granting them credits at higher levels of income than is the case for single parents.