Children usually benefit from having two parents instead of one.
The gender of each parent, however, does not have a significant impact on a child's success.
This finding strikes at the heart of one of the major arguments of gay marriage and adoption opponents.
In a finding that confronts deeply rooted beliefs about parenting, a new study concludes that parents' genders have little impact on children -- suggesting that same-sex couples are as effective at raising children as heterosexual couples.
On average, children succeed most when raised by two parents rather than one. The parents' genders, however, make little difference in terms of a child's development, according to a landmark study published in the Journal of Marriage and Family.
The analysis of 81 parenting studies by sociologists Judith Stacey of New York University and Tim Biblarz of the University of Southern California challenges the widely held notion that children need both a mother and a father in their household in order to thrive.
"What we're saying is there is no best (household) structure," said Stacey, a professor of social and cultural analysis at NYU. "There are better parenting practices, and certainly better relationships and worse relationships, but they don't come in one particular structure."
Many of the studies that highlight gender-specific parenting skills, such as a father's masculine interactions with sons and a mother's nurturing care, only compared heterosexual married couples with divorced or single-parent families. Lesbian- and gay-parented households as well as single adoptive parents were usually left out.
By not controlling for the number of parents, sexual identity, marital status and biogenetic relationship to the children, the research often failed to isolate the real impact of gender on effective parenting, according to Biblarz and Stacey's study.
Recent research on lesbian-parented households seems to support the study's gender-neutral thesis. Overall, studies indicate that children raised with lesbian co-parents do just as well as children raised by heterosexual married couples. The children of lesbian co-parents may even have fewer behavioral problems and higher self-esteem.
In addition, single-mother households correlate to lower child delinquency rates, greater parental control and higher educational performance than single-father families.
The study therefore raises the controversial question of whether men are necessary at all for successful parenting.
"The one thing that I worry about in this discussion about gender is that fatherhood will be devalued," said Jeanne Hilton, a professor of social work at the University of Nevada at Reno who has studied parent and child behaviors in various household structures. "Fathers do matter to children."
Hilton has worked extensively with custodial fathers and hasn't observed noticeable differences from single-mother caretaking.
"I think that men and women are equally capable of parenting and that they rise to occasion as needed," Hilton said. "But adapting and doing a good job doesn't mean that children don't qualitatively benefit in different ways from parenting of mothers and fathers."
Stacey was also quick to point out that while statistics may favor women as parents, it doesn't imply that men are inherently less capable of raising children.
"On average, women tend to be more motivated and more invested in good parenting than men, but plenty of men are at least as good if not better than lots of women," Stacey told Discovery News.
"It's like saying that men are taller than women," she added. "These are average differences, but they don't divide the two groups."
Regardless of gender, Hilton and Stacey both note that parental resource availability and investment is a key predictor of positive child development.
In fact, Stacey expects that as more research emerges on childhood outcomes of gay-parented families, gay men may turn out as the best parents because of the deep commitment required for them to legally father children.
"So it isn't about being gay or being male or being female," Stacey said. "It's about how important this (child rearing) is, how big a part of your life it is and how much you want to give to it."
Cristen Conger is a writer for HowStuffWorks.com.