One in 10 children in the United States lives in a family of "mixed immigration status" in which at least one parent is a foreign national and one child is a U.S. citizen, according to a new study by the Urban Institute.

The large number of these "mixed-status families," which surprised researchers of the Washington-based think tank, reflects high immigration levels in recent years, the U.S. tradition of birthright citizenship and the tendency of immigrants to have more children than the native-born. "The prevalence of so many mixed-status families suggests that recent welfare and illegal-immigration reforms limiting the rights and benefits of noncitizen adults are affecting a large number of the nation's U.S. citizen children who live in the same houses and eat at the same tables," said Wendy Zimmerman, co-author of the Urban Institute study.

According to the study, entitled "All Under One Roof: Mixed-Status Families in an Era of Reform," 75 percent of children in immigrant families nationwide are U.S. citizens.

Mixed-status families are especially common in California and New York, two states with high concentrations of immigrants, the study found. In Los Angeles, 47 percent of all children live in mixed-status families, it concluded. In New York City, 27 percent live in such families.

According to Michael Fix, the study's other author, use of welfare benefits has dropped much more sharply among mixed families than among U.S. citizen families with comparable incomes. He attributed this in part to "confusion" about welfare reform among noncitizen parents and fear that use of welfare by their citizen children would adversely affect their own immigration status.

Critics of the current high immigration levels said the report ignores a fundamental policy issue -- the admission by the United States of large numbers of poor, unskilled immigrants who are ill-equipped to compete in a modern economy and thus likely to resort to welfare.

"The important question is not how these immigrant families are faring in the welfare system, but rather why our immigration policy is admitting so many poor people in the first place," said Mark Krikorian, director of the Center for Immigration Studies here. He said he views the study as a "tool in the lobbying campaign to reinstate welfare eligibility for noncitizens."