[3] Medical institutions warn that conversion therapy is ineffective and may be harmful, and that there is no evidence that sexual orientation can be changed by such treatments.

Nicolosi proposed that homosexuality is often the product of a condition he described as gender-identity deficit caused by an alienation from, and perceived rejection by, formative individuals of the subject's gender which interrupts normal masculine or feminine identification process.

[11] Scientific research, summarized in a 2016 academic review, has not supported Nicolosi's hypothesis that parents play a role in the development of sexual orientation, and social theories are particularly weak for males.

[12]: 87 In 2009, Royal College of Psychiatrists criticized Nicolosi's appearance at a conference in London, saying that: "there is no sound scientific evidence that sexual orientation can be changed" and "furthermore, so-called treatments of homosexuality create a setting in which prejudice and discrimination can flourish.

[15] At the conference, Nicolosi performed "therapy" on a man live in front of the audience, a sight Patrick Strudwick described as "like I was watching a blood sport".

[22][23] Major Spanish department store El Corte Inglés was threatened with a boycott by the United Left coalition over its stocking of the works, but continued to market them in 2014.

Bailey informed Nicolosi that he could bring his patients to his lab at Northwestern University to test their automatic responses to erotic cues, i.e. men versus women.

Additionally Bailey notes that Conrad and Wincze found that physiological arousal measurements did not support the positive reports of men who had participated in sexual-reorientation therapy.