The best study, Siegel said, is the National Longitudinal Lesbian Family Study, which began in 1986 with 154 lesbian mothers who conceived children through artificial insemination. A recent look at 78 offspring found that the children did fine — better, even, than children in a similar study involving more diverse families.
Many opponents of same-sex marriage argue that the academy's conclusions are premature. They point to some recent studies, including one from Mark Regnerus, a sociology professor from the University of Texas at Austin. Regnerus, who could not be reached for this article, found that adults who reported being raised by a person who had a homosexual experience were more likely to be on welfare or experience sexual abuse.
Regnerus has been the subject of intense criticism from mainstream researchers and pro-gay marriage activists. But opponents of same-sex marriage say his work should provide a note of caution on an issue that has yet to be studied in adequate depth.
"What the social science makes clear, and it has for several decades, is that children tend to do best when they're raised by their married biological parents," said Jennifer Marshall, director of domestic policy studies for the conservative Heritage Institute. "In the case of same-sex households, there is not yet evidence that [children] are going to be the same. There's every reason to believe that different family structures will have different outcomes."
Susan Brown, a professor of sociology at Bowling Green State University in Ohio who studies family structures, said it is true that decades of research show that children turn out slightly better when they are raised by their biological parents compared with those reared by single parents or in "step" households.
But children raised in committed same-sex couple-led households do not appear to do statistically worse, she said.
"One thing we're finding that's very important for children is stability in their family life," Brown said. "To the extent that marriage is a vehicle through which children can achieve stability, it only follows that marriage is something that would be beneficial to children."