The presumption that children need both a mother and a father is widespread. It has been used by proponents of Proposition 8 to argue against same-sex marriage and to uphold a ban on same-sex adoption.

On the other end of the political spectrum, Barack Obama endorsed the vital role of fathers in a 2008 speech: "Of all the rocks upon which we build our lives, we are reminded today that family is the most important. And we are called to recognize and honor how critical every father is to that foundation."

The lead article in the February issue of Journal of Marriage and Family challenges the idea that "fatherless" children are necessarily at a disadvantage or that men provide a different, indispensable set of parenting skills than women.

"Significant policy decisions have been swayed by the misconception across party lines that children need both a mother and a father. Yet, there is almost no social science research to support this claim. One problem is that proponents of this view routinely ignore research on same-gender parents," said sociologist Timothy Biblarz of the USC College of Letters, Arts and Sciences.

Extending their prior work on gender and family, Biblarz and Judith Stacey of NYU analyzed relevant studies about parenting, including available research on single-mother and single-father households, gay male parents and lesbian parents. "That a child needs a male parent and a female parent is so taken for granted that people are uncritical," Stacey said.

In their analysis, the researchers found no evidence of gender-based parenting abilities, with the "partial exception of lactation," noting that very little about the gender of the parent has significance for children's psychological adjustment and social success.

As the researchers write: "The social science research that is routinely cited does not actually speak to the questions of whether or not children need both a mother and a father at home. Instead proponents generally cite research that compares [heterosexual two-parent] families with single parents, thus conflating the number with the gender of parents."

Indeed, there are far more similarities than differences among children of lesbian and heterosexual parents, according to the study. On average, two mothers tended to play with their children more, were less likely to use physical discipline, and were less likely to raise children with chauvinistic attitudes. Studies of gay male families are still limited.

However, like two heterosexual parents, new parenthood among lesbians increased stress and conflict, exacerbated by general lack of legal recognition of commitment. Also, lesbian biological mothers typically assumed greater caregiving responsibility than their partners, reflecting inequities among heterosexual couples.

"The bottom line is that the science shows that children raised by two same-gender parents do as well on average as children raised by two different-gender parents. This is obviously inconsistent with the widespread claim that children must be raised by a mother and a father to do well," Biblarz said.

Stacey concluded: "The family type that is best for children is one that has responsible, committed, stable parenting. Two parents are, on average, better than one, but one really good parent is better than two not-so-good ones. The gender of parents only matters in ways that don't matter."

This study is published in the February 2010 issue of the Journal of Marriage and Family.