The organisers of a Christian advertising campaign that had been expected to start next week claim they have scientific support for their belief in the power of "therapy" to change the sexual orientation of gay people. In support of their campaign, Core Issues Trust and Anglican Mainstream cite a study carried out by Stanton L Jones of Wheaton College in Illinois and Mark Yarhouse of the School of Psychology and Counselling at Regent University in Virginia. The study, published last year in the journal Sex and Marital Therapy, is titled "A Longitudinal Study of Attempted Religiously Mediated Sexual Orientation Change" and anti-gay campaigners claim it shows how spiritual "therapy" can be used to change sexual orientation.

The authors followed 98 people (72 men and 26 women)who were undergoing religion-based "therapy" for homosexuality and tracked their progress for up to seven years. The participants were rated on a seven-point scale on attraction, infatuation and fantasy with people of the same or different sex. A measure of zero meant they were exclusively heterosexual, while a rating of 6 meant they were exclusively homosexual.

Over the course of the study the participants went from an average of 4.08 to 3.3 on the attraction scale, a difference that could be within random error, according to Prof Michael King, director of mental health sciences at University College London. On infatuation the average went from 3.4 to 2.8 and on fantasy the average went from 4.6 to 3.8.

"They showed that 15% of [the participants] had some significant change, they're claiming, in moving in a heterosexual direction. Another 10%-15% became celibate. Even [the researchers] say the changes are very very small – they might be statistically significant but they're not clinically significant," said Professor Michael King.

"Nobody would accept this as high-grade evidence," he added. "There's no comparison group, there's no randomisation, we don't know what the therapy was, 40% of them dropped out and about whom we know nothing."

There were several problems with the design of the study, said King, starting with the selection of the participants by Exodus, a group composed mainly of formerly-gay Christians. "The Exodus ministries wrote to the patients and said will you take part in the study - we don't know anything about the selection. You could easily say the ministries chose the people they thought might give the best results."

The details of the therapy undertaken by participants was also not clear. In their paper, Jones and Yarhouse wrote that the "change process" referred broadly to general involvement in an Exodus-affiliated ministry group, "which typically incorporates worship, prayer, education, and discussion."

King said the omission of detail was serious, given the claims being made on behalf of the research, because it would prevent others from being able replicate the study.

He said the study was part of a long line of attempts to "cure" homosexuality. "Most of the 20th century people have been trying to treat homosexuals - from psychoanalysis, oestrogen therapy, electric shock and aversion therapy. This is the latest round is much more spiritually-based and has been going for at least 25 years in the US and, increasingly, here. It seems to be a mixture of a feeling that the person is "wounded" - they talk about emotional woundedness in their stuff. That seems to be old psychoanalytical theory about a distant father and an overbearing mother, stuff that's been already shown to be not associated with sexuality, but they [campaigners] keep on this line."

In 2009 the American Psychological Association put together a taskforce to review scientific literature on efforts to change sexual orientation. It looked at 83 studies in English published from 1960 to 2007. "We found serious methodological problems in this area of research, such that only a few studies met the minimal standards for evaluating whether psychological treatments, such as efforts to change sexual orientation, are effective," the authors of the report wrote. "Few studies—all conducted in the period from 1969 to 1978—could be considered true experiments or quasi-experiments that would isolate and control the factors that might effect change."

They recommended that APA should, "take a leadership role in opposing the distortion and selective use of scientific data about homosexuality by individuals and organisations and in supporting the dissemination of accurate scientific and professional information about sexual orientation in order to counteract bias."

King said that promoting so-called treatments for homosexuality on the sides of buses can be "tremendously harmful within society because it underscores the stigma against gay and lesbian people that, somehow, they're ill and need to be healed. The NHS would never pay for a treatment with this grade of evidence. It's just poor."

Alok Jha