The idea that children raised in same-sex households fare as well as children of married opposite-sex couples may not withstand scrutiny, according to a recent collection of studies.

Princeton University professor John B. Londregan said that the studies collected in a new book show "that the jury is still very much out on this question."

"The American Psychological Association has declared that there are no differences in the parenting capacity of same-sex couples as compared with heterosexual married couples," he told CNA March 3.

However, he explained, "This declaration is based largely on evidence from studies using small 'convenience samples,' and it has had a chilling effect on research, while it conveys a misleading message for the public debate."

"The likely enactment of same-sex marriage by judicial fiat will enlist a segment of the next generation of American children as human guinea pigs in a giant social experiment to see how much same-sex marriage improves parenting outcomes of same sex couples," he said.

Londregan, who teaches politics and international affairs at Princeton, authored the introduction to the new collection of scholarly papers titled "No Differences? How Children in Same-Sex Households Fare."

Published by the Princeton, N.J.-based Witherspoon Institute, the studies in the "No Differences?" book indicate some significant statistical differences between children raised by same-sex couples and children raised by married parents.

The children raised in same-sex households resemble those raised by never-married single women, facing "relatively troubled outcomes" like higher rates of drug abuse, unemployment and dropping out of school.

Londregan said that the "relative instability" of same-sex couples compared to married heterosexual parents could be the primary factor in the different outcomes for children.

At the same time, he acknowledged, research into children's outcomes faces obstacles in that that there are "relatively few" households led by same-sex couples that are raising children. A researcher therefore must survey "a huge number of people" to find enough households to make inferences.

Londregan summarized the "No Differences?" papers in a Feb. 24 essay at the Public Discourse website.

Some of the papers represent original analysis, while others examine the reliability of different studies on family structure with a special focus on same-sex parenting.

One of the papers, by Loren Marks, a family studies professor at Louisiana State University, focused on the American Psychological Association's justifications for its position, adopted in 2005, that same-sex couples raise children at least as effectively as heterosexual couples.

Marks found that most of the studies on childrearing by same-sex couples rely on "convenience samples" of "very small" sample sizes with one or two dozen same-sex couples raising children and a similar number of heterosexual couples, at most. The sample sizes do not constitute a representative sample and introduce statistical bias into the analysis.

Many of the small studies fail to use a sufficient comparison group of heterosexual parents or compare "educated and affluent lesbian couples to single heterosexual parents," Londregan said.

Several of the papers examine Stanford sociologist Mark Rosenfeld's analysis of the 2000 census data which appeared to indicate that children raised by same-sex couples showed no difference in outcomes than those raised by heterosexual married couples.

Rosenfeld had argued that data for children who are adopted or geographically mobile should be excluded on two grounds: because this indicates family instability and because he contends that same-sex couples are more likely to adopt troubled children.

However, this approach neglects the possibility that family instability may be "an important mechanism by which the children of parents in same-sex relationships fare worse than those with married heterosexual parents," Londregan said.

Another study in the collection finds that same-sex female parents are about twice as likely to break up as heterosexual married parents.

Researcher Douglas Allen, examining data from the 2006 Canadian Census, found that late-adolescent children raised by two women finish high school at about the same rates as those raised by single women, while late-adolescent children raised by two men finish high school at about the same rates as those raised by single men. These are "significant deficits" compared to children raised by a married man and woman.

Allen also found that daughters raised in same-sex male households fare worse than sons – a unique disparity not evident in any other family structure he has studied.

While data sampled from the U.S. Census provides the largest statistical sample, it is limited by access to "relatively few variables" about the well-being of children, Londregan said.

Another paper in the "No Differences?" collection defends the research of University of Texas at Austin sociologist Mark Regnerus as being within accepted standards. A 2012 study by Regnerus contacted over 15,000 young and early-middle-aged adults to ask about their childhoods, including whether at least one of their parents had been involved in a same-sex relationship.

He found "statistically significant" differences in 25 of 40 outcomes between adult children of married opposite-sex parents and adult children of mothers who had a same-sex relationship. The study drew much media attention and criticism from LGBT activists, one of whom filed a misconduct complaint against Regnerus, which his university rejected upon investigation.

Regnerus' study is included in the Witherspoon Institute book.

Londregan told CNA that the differences between children of same-sex couples and those raised by a mother and a father are "still an open question" depending on the sample size and other conditions a study controls for.