The study, published in the new issue of the American Sociological Review, found that couples who adopt spend more money on their children and invest more time on such activities as reading to them, eating together and talking with them about their problems.
"One of the reasons adoptive parents invest more is that they really want children, and they go to extraordinary means to have them," Indiana University sociologist Brian Powell, one of the study's three co-authors, said in a telephone interview Monday.
"Adoptive parents face a culture where, to many other people, adoption is not real parenthood," Powell said. "What they're trying to do is compensate. ... They recognize the barriers they face, and it sets the stage for them to be better parents."
Powell and his colleagues examined data from 13,000 households with first-graders in the family. The data was part of a detailed survey called the Early Childhood Longitudinal Study, sponsored by the U.S. Department of Education and other agencies.
The researchers said 161 families in the survey were headed by two adoptive parents, and they rated better overall than families with biological parents on an array of criteria — including helping with homework, parental involvement in school, exposure to cultural activities and family attendance at religious services. The only category in which adoptive parents fared worse was the frequency of talking with parents of other children.
The researchers noted that adoptive couples, in general, were older and wealthier than biological parents, but said the adoptive parents still had an advantage — albeit smaller — when the data was reanalyzed to account for income inequality.
In particular, the researchers said, adoptive parents had a pronounced edge over single-parent and stepparent families.
The researchers said their findings call into question the long-standing argument that children are best off with their biological parents. Such arguments were included in state Supreme Court rulings last year in New York and Washington that upheld laws against same-sex marriage.
The researchers said gay and lesbian parents may react to discrimination by taking extra, compensatory steps to promote their children's welfare.
"Ironically, the same social context that creates struggles for these alternative families may also set the stage for them to excel in some measures of parenting," the study concluded.
An opponent of same-sex marriage, Peter Sprigg of the conservative Family Research Council, noted that the study focused on male/female adoptive couples, not on same-sex couples, and he questioned whether it shed any new light on adoptive parenting by gays.
Sprigg, the research council's vice president for policy, said he warmly supports adoption, but believes it is best undertaken by married, heterosexual couples.
Another conservative analyst, psychologist Bill Maier of Focus on the Family, said the authors of the new study seemed to be pursuing a political agenda in support of gay marriage.
"Put simply, gay adoption creates families that are motherless or fatherless by design, permanently depriving children of either a mother or a father," Maier said.
Adam Pertman, executive director of the Evan B. Donaldson Adoption Institute, welcomed the study's findings, but cautioned against possibly exaggerated interpretations of it.
"It's an affirmation that there are all sorts of families that are good for kids," he said. "Adoptive parents aren't less good or better. They just bring different benefits to the table. In terms of how families are formed, it should be a level playing field."
The study was funded by the National Science Foundation, the Spencer Foundation and the American Educational Research Association. Powell's co-authors were Laura Hamilton, a doctoral student at Indiana University, and Simon Cheng, a sociology professor at the University of Connecticut.