Two-adult families are on the rise in low-income neighborhoods, but that may not be the best news for the children in some of those families, according to researchers at Johns Hopkins.

The proportion of children living with two adults increased in poor areas of Boston, Chicago and San Antonio from 1999 to 2001, researchers Andrew Cherlin and Paula Fomby found. But most of the increase involved adding a man who wasn't the biological father. The proportion of children living with two biological parents didn't increase at all. And most of the increase in two-adult families involved cohabiting relationships rather than marriage.

Two-adult households that involve a mother and a stepfather aren't any better for children on average than single-mother households, the researchers say. And two-adult households built upon unmarried romantic relationships are not nearly as stable as families with married parents; many such households break apart in less than a year, disrupting children's living arrangements, Cherlin and Fomby say. Research shows that such family transitions may lead to academic and social problems for children in the long run.

The conclusion is the result of the latest research conducted in Boston, Chicago and San Antonio through Welfare, Children & Families: A Three-City Study. Started in 1998 by researchers at five universities, its purpose is to examine the consequences of welfare reform for the well-being of children and families. The results of the three-city study are more relevant than ever as Congress debates the reauthorization of the 1996 welfare reform law, one aim of which was to reduce the number of low-income single mothers raising children alone.

In the study's latest policy brief, "A Closer Look at Changes in Children's Living Arrangements in Low-Income Families," Cherlin and Fomby present the results of two sets of caregiver interviews, conducted 16 months apart in 1999 and 2000. Between the first and second interviews:

* The percentage of children living with two adults--including biological fathers, stepfathers and adoptive parents--increased from 34 percent to 38 percent.

* Virtually all the increase involved a mother and a man who was not the child's biological father.

* Forty-two percent of the mothers who were cohabiting at the time of the first interview had ended their relationships by the second interview; only 16 percent of the couples had married.

* Eighteen percent of the mothers who were married at the first interview had separated from their husbands by the second interview.

* Overall, 22 percent of the children had experienced a change in their mother's living arrangement between the two interviews.

The more family flux a child experiences, the lower that child's well-being, according to the researchers. For instance, they say, the greater the number of family transitions experienced by an adolescent girl, the greater her chances of becoming a teenage mother. Behavioral problems in boys and poorer school adjustment among sixth-graders have also been linked to a high rate of family transition, they say.

"We have reason to question the extent to which the kinds of two-adult families mothers formed in our sample will benefit the children involved," says Cherlin, the study's lead investigator and chairman of the Krieger School of Arts and Sciences' Sociology Department. "It still may be true that children will benefit from targeted policies that provide services to biological parents who wish to marry. But the modest benefits of the kinds of families that are forming in Boston, Chicago and San Antonio suggest a cautionary note. Policies that broadly encourage the formation of two-parent families may have effects on the well-being of poor children that are more limited than their advocates expect."

The findings were first presented as a working paper at the National Press Club in Washington, D.C., on Feb. 20.