Though the early consensus is that they do not, even the investigators acknowledge the field is too young, the numbers too few, the variables too many and the research too values-laden to qualify as definitive.
In 2001, Judith Stacey and Timothy J. Biblarz, then sociologists at the University of Southern California, published a review of 21 previous studies of the children of homosexual parents (most of them lesbians). Almost uniformly, they wrote, the research found no systematic differences between children reared by a mother and father and those raised by same-sex parents.
But Stacey and Biblarz also observed that researchers who found no differences sometimes skewed their interpretation of results to suit their own leanings. "Ideological pressures," they concluded, "constrain intellectual development in this field . Because anti-gay scholars seek evidence of harm, sympathetic researchers defensively stress its absence."
Some studies, said Stacey, have ignored or downplayed early indications there may, in fact, be differences in the development of character and gender roles, among them that children of same-sex parents may be more open to homosexual experimentation.
"I think they'll be more tolerant, more flexible in terms of gender conformity," said Stacey, who now teaches at New York University. "The boys may be less aggressive. There's some indication the girls will have a wider array of career aspirations."
Charlotte J. Patterson, a professor of psychology at the University of Virginia and a prominent researcher in the field, has found that the purposefulness inherent in same-sex parenting tends to counter any societal disadvantages. "I think what we're seeing overall is pretty positive adjustment on the part of these kids," she said. "What that suggests, I think, is that we may have overrated the role of gender in parenting in our theoretical notions about these matters."
Over the last decade, that general proposition has been embraced, to varying degrees, by the American Academy of Pediatrics, the American Medical Assn., the American Psychological Assn., the American Psychiatric Assn. and a variety of child welfare groups.
The psychological association holds that "the research has been remarkably consistent in showing that lesbian and gay parents are every bit as fit and capable as heterosexual parents, and their children are as psychologically healthy and well-adjusted as children reared by heterosexual parents."
The American Academy of Pediatrics, in a 2002 review of the literature, also found no negative effects. "Compared with heterosexual fathers, gay fathers have been described to adhere to stricter disciplinary guidelines, to place greater emphasis on guidance and the development of cognitive skills, and to be more involved in their children's activities," the group wrote.
Even social science articles that oppose same-sex parenting typically do not claim significant evidence of dire consequences for children.
Instead, opponents have argued that parenting by a mother and father is optimal, and that much of the existing research has been "compromised by methodological flaws and driven by political agendas," in the words of a 2005 Family Research Council report.
The report's author, Timothy J. Dailey, also said that "openly lesbian researchers" — he named Patterson specifically — "sometimes conduct research with an interest in portraying homosexual parenting in a positive light." To do so, Dailey wrote, ignores "the accumulated wisdom of cultures and societies from time immemorial, which testifies that the best way for children to be raised is by a mother and a father who are married to each other."
Both sides agree that large numbers of cases will need to be studied.
Those cases could become available in a generation or two. The 2000 Census found that 34% of female same-sex households included children under 18, an increase of 72% since 1990, and that 22% of male same-sex households included children, a quadrupling since 1990.