Children raised by same-sex couples are 35pc less likely to make normal progress through school according to a new study.

The study, published in the journal Demography, is based on data from 1.6 million children in the 2000 U.S. Census, which included 8,632 children who lived in same-sex households.

It re-examined research published in 2010 by Stanford sociologist Michael J Rosenfeld, which suggested that there was virtually no difference between the educational progress attained by children raised by same-sex parents compared to those raised in traditional families.

The Rosenfeld study was different from previous studies on children raised by same-sex parents since it used a large sample from the Census rather than a small self-selected one which is more typical of this kind of research.

However, since the Census provides only a single-year picture of children’s living arrangements, its data do not reflect their full family history.

For example, married households include both first-marriage and remarried couples. The 2010 study tried to address this issue by limiting its sample in two significant ways that may not represent the full family experiences of all children.

The 2010 study excluded children who were not biologically related to the head of household and who were not in the same home for at least five years.

This reduced “the sample size by more than one-half,” according to the authors of the latest study.

The 2012 study explains that putting the children who had been in unstable households (lived at the same address less than five years) back into the sample increases the sample “by more than 80 percent.”

The new study re-examines the data without these two limitations.

The new study finds that, when the sample consisted of only biological children, regardless of residential stability, children in married heterosexual households were 25.8 percent more likely to make typical school progress than peers raised in same-sex households.

It further found that, when the sample consisted of all children, regardless of their biological status or residential stability, children in married heterosexual households were 35.4 percent more likely to make normal progress in school than peers in same-sex households.

In his response to the study, Mr Rosenfeld said that including these extra children in the study distorted the findings.

He said that the study’s findings were due to its conflation of “the initial disadvantage of children who come into same-sex couple families (a disadvantage that appears to be substantial) with the progress children experience during the time when they are actually being raised by same-sex couples (progress that is excellent)”.

Meanwhile, the Supreme Court of Mexico has ruled that a Mexican province in which same-sex marriage is illegal must recognise a same-sex marriage contracted in Mexico City.

Legal experts suggest that the ruling, which was unanimous, could pave the way to legal same-sex marriage throughout Mexico.

The court ruled that a law defining marriage as a legal union between a man and a woman in the state southern Oaxaca was unconstitutional, the Washington Post reports.

The decision came in favor of three gay couples who sued Oaxaca state on grounds the law violated the principle of equal treatment for all citizens.

Currently, the only Mexican province in which same-sex marriage is legal is Mexico City.

Under Mexican law, two more cases from Oaxaca must be decided in the same way before a precedent is established allowing gay marriage — and only in that state. For each of Mexico’s 30 other states, five separate cases would have to be decided that way to establish precedent for gay marriage.

Experts say the Oaxaca ruling puts gay couples in the state a step closer to being able to get legally married.