Documents recently published concerning the “Regnerus study”, widely used by conservatives as evidence against same-sex marriage, have shown it was organised by a religious think tank to sway the Supreme Court’s deliberations on marriage equality.

The American Independent (TAI) news network obtained records, published with the Huffington Post, which suggest that religious right think tank The Witherspoon Institute recruited Professor Mark Regnerus of the University of Texas to carry out a study on LGBT parents, which they designed to influence public policy against marriage equality.

The Regnerus study, published in July 2012, was titled “How Different Are The Adult Children of Parents Who Have Same-sex Relationships? Findings from the New Family Structures Study”.

Mr Regnerus compared the lives of children raised by parents gay relationship, of any length, with those who had grown up with straight, married parents. The first group appeared to be more likely to be in therapy, have extra marital affairs and experience other negative outcomes in later life.

The study has already been criticised for failing to receive “ethical and appropriate professional peer review”, and for focussing on children whose gay or bisexual parents had separated, but surveying children whose straight parents had stayed together.

The documents obtained by TAI show that the Witherspoon Institute, which paid $700,000 (£470,460) for the study, were involved in developing the research – a claim which they had previously denied. They also show that data analysis for the study was carried out by a Witherspoon Institute fellow.

There are also letters from Witherspoon Institute president Luis Tellez, in which he urges Mr Regnerus to finish the study before the Supreme Court could make a decision on same-sex marriage.

One of Mr Tellez’ letters from 2011 said: “The future of the institution of marriage at this moment is very uncertain. It is essential that the necessary data be gathered to settle the question in the forum of public debate about what kinds of family arrangement are best for society.”

Since its publication it has been widely used by anti-marriage equality groups such as the National Organisation for Marriage. It has also been cited by groups lobbying the Supreme Court to uphold the anti-equality law Proposition 8.

After the documents were released Mr Tellez denied that he had known what the study would show, and said he believed the quality of the research should come before anything else.

James Wright, editor of Social Science Research, which published the study, claimed that he was unaware of the political motive behind it.

“So far as the Supreme Court is concerned, I consider marriage and adoption rights for GLBT people to be a matter of civil rights… not something to be ‘resolved’ by empirical research,” he said. “I resent having social science data and research drawn into such debates.”

The senior counsel and director of Lambda Legal, Jennifer C. Pizer, said: “Even if it were true – which it isn’t – that straight people make better parents than gay people, it would not make any sense to say, therefore, gay people can’t get married, because gay people do have children, so whose children can possibly be helped by denying some parents the right to marry?”

In February the director of the Illinois Family Institute cited the Regnerus study as justification for why he claimed people who accepted families with same-sex parents should be “cast into the sea”.

Professor Regnerus came under heavy criticism for the report, and was accused of “falsifying” the results, and of deviating from “ethical standards”. He was also criticised because the report was funded by the Witherspoon Institute and the Bradley Foundation, both conservative groups with links to the National Organisation for Marriage.