Nicolosi has described his theories in Reparative Therapy of Male Homosexuality: A New Clinical Approach and three other books. Nicolosi proposes that homosexuality is often the product of a condition he describes as gender-identity deficit caused by an alienation from, and perceived rejection by, individuals of the subject's gender. He holds a Ph.D. from the California School of Professional Psychology. Nicolosi is a founding member of the National Association for Research & Therapy of Homosexuality (NARTH) and was its president for some time. NARTH is a professional association that promotes the acceptance of conversion therapy, whose adherents purport successfully changing homosexuals into heterosexuals. He is an advisor to, and officer of, NARTH.