Riding the momentum of the Irish referendum and United States supreme court ruling legalising same-sex marriage, there are expectations Australian legislators will change the Marriage Act.

A cross-party bill to legalise same-sex marriage is expected to be introduced in the Federal Parliament as early as August 2015, when politicians return from their winter break.

But passage of the bill is not guaranteed, and many MPs are against the change.

An argument often used against legalising same-sex marriage by groups like the Australian Christian Lobby (ACL), and even a group of 40 religious leaders from multiple faiths, is that it will have a negative impact on children's wellbeing.

Opponents of same-sex marriage, including groups like Family Voice Australia claim that studies show a man and a woman are best placed to raise children.

ABC Fact Check takes a look at the research on the effects of gender on parenting.

Gender of parents doesn't matter

More than 100 studies have compared the outcomes for children of same-sex and heterosexual parents.

A US study of 44 randomly selected teenagers raised by same-sex couples showed that they had low levels of depression and anxiety and high self-esteem.

It found that they were no different from the children of opposite-sex parents from a national population sample.

Of the 10 outcomes studied, including indicators like the warmth they felt from their parents and being part of their neighbourhood, the only difference between the groups was that the children of same-sex parents felt more connected at school.

A 2010 United States review by two sociologists from the University of Southern California and New York University looked at 33 studies published since 1990 comparing same-sex and heterosexual parents.

It concluded that "at this point no research supports the widely held conviction that the gender of parents matters for child wellbeing".

The findings were echoed in a 2007 literature review by the Australian Psychological Society, which found that children's wellbeing was affected by the quality of parenting and family relationships, rather than parental number, gender or sexuality.

In 2013, the Australian Institute of Family Studies published a report entitled "Same-sex parented families in Australia".

It also concluded that same-sex parented children did as well emotionally, socially and educationally as the children of heterosexual couples.

Deborah Dempsey, who wrote the report, told Fact Check the research didn't show that children needed a parent of each gender.

But she cautioned that most of the research had been done in lesbian-parented families and not much was known about families parented by two gay male parents.

Potential bias in same-sex studies

Dr Dempsey's report noted that many studies were based on small and homogenous samples of highly educated and middle-class participants, using volunteers rather than randomly selected samples of adults raised by same-sex parents.

A report from Princeton University and the Brookings Institution in the US also noted that many same-sex parenting studies focused on small samples of white, middle-class, well-educated parents.

But the authors said four "state of the art" rigorous studies did show children raised by same-sex parents had the same outcomes to those raised by heterosexual parents, a similar conclusion reached by the two Australian reviews.

One way of overcoming the problem of small numbers of same-sex parents is to pool the data from several studies to get enough statistical power to find any differences.

One of these "meta-analyses" combined data from 19 studies of more than 500 same-sex families.

Of the six broad outcomes they looked at, including intelligence, gender identity, and whether they had psychological problems like depression and anxiety, the only difference was that same-sex parented children had a better relationship with their parents.

Jennifer Power, a research fellow at the Australian Research Centre in Sex, Health and Society at La Trobe University, told Fact Check that repetition of results was important.

"Obviously its important to understand the limitations, I don't argue that, method and rigour are important. But I also think there's a reasonably strong argument that those [volunteer] samples are... fairly reflective of the population as same-sex couples," she said.

Dr Dempsey said that she believed concerns over selection bias were overplayed.

"If there were harms accruing to children that were by virtue of children being raised by same-sex parents, those would show up in samples of highly privileged parents because it would be a harm that was by virtue of the gender and sexuality of the parents," she said.

Australian research shows no difference

More recently, the Australian Study of Child Health in Same-Sex Families (ACHESS) of 390 same-sex parents showed that of the 17 outcomes studied, the only differences were that the children of same-sex parents scored higher for children's behaviour, health, and how well families got along than those from the general population.

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But two-thirds of the children from same-sex parented households experienced at least some stigma related to their parents and this was linked to worse mental health and emotional symptoms, though not enough to see differences in mental and emotional health between the two groups of children.

Lead investigator Simon Crouch, an honorary research fellow at the Jack Brockhoff Child Health and Wellbeing Program at the University of Melbourne, told Fact Check the ACHESS study factored in family income and education, to eliminate some of the selection bias that came from using same-sex parents who volunteered for the study.

"I argue that our research is more representative, we tried to capture a broader range [of couples]," he said.

"When we compared it to the average population, in fact some of the same-sex female couples, their income distribution was fairly similar to the general population."

Adverse findings

In 2012, a study from the University of Texas, which did have a large sample size and a randomly selected population, reached some different conclusions on same-sex parenting.

The survey of young adults showed those with parents in a same-sex relationship were more likely to be unemployed, depressed, earn less, and be arrested more often compared with those from stable heterosexual parent families.

Publication of the findings was met with a storm of controversy.

An independent review of the data by sociologists from the University of Connecticut and Indiana University showed that only a quarter of the respondents could plausibly be considered to have been raised by same-sex parents and that there were only four significant differences between the groups, which were unreliable because of the smaller sample size.

A misleading comparison

In their submission to a 2012 parliamentary inquiry into marriage equality, the ACL referenced a review paper, "Marriage from a Child's Perspective: How Does Family Structure Affect Children, and What Can We Do about It?".

They said the paper, authored by two US academics, supported their contention that "an extensive body of research tells us that children do best when they grow up with both biological parents".

The paper was also referenced by Tasmanian Liberal Senator Eric Abetz in a 2012 speech to the Young Liberals, in support of his claim that two men or two women could not provide the diversity and vital experience of a home with a mother and father.

The 2002 paper found that children who are born to unmarried mothers, growing up with step-parents, in homes with high-conflict, or with unmarried parents, all do worse than those who grow up with two biological married parents.

But an author's note published with the paper says: "No conclusions can be drawn from the research about the wellbeing of children raised by same-sex parents or adoptive parents."

Dr Power said those studies showed children raised in stable homes do better than children who are raised in homes where there is instability, conflict or violence.

"There are studies that show kids do better with more resources and more resources come from two parents of either gender," Dr Power said.

Similarly, an Australian review cited by the ACL and South Australian Liberal senator Cory Bernardi as evidence that children do best with married biological parents, also did not look at same-sex parented families.

The report, For kid's sake - repairing the social environment for Australian children and young people, was commissioned by the ACL.

The report's author Patrick Parkinson, a family law expert at the University of Sydney law school, told Fact Check his report focused on heterosexual parenting and contrasted two married biological parents with single parents, blended families and step-parent families.

"[The report] said nothing whatsoever about same-sex parenting," he told Fact Check. "It wasn't about that."

Professor Parkinson said 40 years of studies showed that children benefited from having a mother and a father but said it wasn't clear what could be concluded about same-sex parents.

"We know that kids do ok in same-sex couples but we don't know to what extent some of those have a role model of the other gender."

Fact or fiction?

Some opponents of same-sex marriage say it will be detrimental to children because they do better with a father and a mother.

While studies do show children benefit from a mother and a father, they compare with single parents and don't show whether it is parental gender, or having two parents, that confers the benefit.

The experts Fact Check spoke to said there was a dearth of definitive credible data demonstrating that children of same-sex parents experienced worse outcomes than children of heterosexual parents.

Sources