The field of psychology has extensively studied homosexuality as a human sexual orientation. The American Psychiatric Association listed homosexuality in the DSM-I in 1952 as a "sociopathic personality disturbance," but that classification came under scrutiny in research funded by the National Institute of Mental Health. That research and subsequent studies consistently failed to produce any empirical or scientific basis for regarding homosexuality as anything other than a natural and normal sexual orientation that is a healthy and positive expression of human sexuality.

As a result of this scientific research, the American Psychiatric Association removed homosexuality from the DSM-II in 1973. Upon a thorough review of the scientific data, the American Psychological Association followed in 1975 and also called on all mental health professionals to take the lead in "removing the stigma of mental illness that has long been associated" with homosexuality. In 1993, the National Association of Social Workers adopted the same position as the American Psychiatric Association and the American Psychological Association, in recognition of scientific evidence. The World Health Organization, which listed homosexuality in the ICD-9 in 1977, removed homosexuality from the ICD-10 which was endorsed by the 43rd World Health Assembly on 17 May 1990.

The consensus of scientific research and clinical literature demonstrate that same-sex attractions, feelings, and behaviors are normal and positive variations of human sexuality. There is now a large body of scientific evidence that indicates that being gay, lesbian, or bisexual is compatible with normal mental health and social adjustment.

Historical background The view of homosexuality as a psychological disorder has been seen in literature since research on homosexuality first began; however, psychology as a discipline has evolved over the years in its position on homosexuality. Current attitudes have their roots in religious, legal, and cultural underpinnings. Some Ancient Near Eastern communities, such as the Israelites, had strict codes forbidding homosexual activity, and when Christianity began, it adopted their Jewish predecessors attitudes surrounding homosexual activities. Among the New Testament authors Paul in particular is notable for his affirmation and reinforcement of such texts in his letters to nascent churches. Later, the Apostolic Fathers and their successors continued to speak against homosexual activity whenever they mentioned it in their writings. In the early Middle Ages the Christian Church ignored homosexuality in secular society; however, by the end of the 12th century, hostility towards homosexuality began to emerge and spread through Europe's secular and religious institutions. There were official expressions condemning the "unnatural" nature of homosexual behavior in the works of Thomas Aquinas and others. Until the 19th century, homosexual activity was referred to as "unnatural, crimes against nature", sodomy or buggery and was punishable by law, sometimes by death.

As people became more interested in discovering the causes of homosexuality, medicine and psychiatry began competing with the law and religion for jurisdiction. In the beginning of the 19th century, people began studying homosexuality scientifically. At this time, most theories regarded homosexuality as a disease, which had a great influence on how it was viewed culturally. There was a paradigm shift in the mid 20th century in psychiatric science in regards to theories of homosexuality. Psychiatrists began to believe homosexuality could be cured through therapy and freedom of self, and other theories about the genetic and hormonal origin of homosexuality were becoming accepted. There were variations of how homosexuality was viewed as pathological. Some early psychiatrists such as Sigmund Freud and Havelock Ellis adopted more tolerant stances on homosexuality. Freud and Ellis believed that homosexuality was not normal, but was "unavoidable" for some people. Alfred Kinsey's research and publications about homosexuality began the social and cultural shift away from viewing homosexuality as an abnormal condition. These shifting viewpoints in the psychological studies of homosexuality are evident in its placement in the first version of the Diagnostic Statistical Manual (DSM) in 1952, and subsequent change in 1973, in which the diagnosis of ego-dystonic homosexuality replaced the DSM-II category of "sexual orientation disturbance". However, it was not until 1987 in DSM-III-R that it was entirely dropped as a mental disorder.

A 2016 survey of the European Union Agency for Fundamental Rights found that many medical professionals in countries such as Bulgaria, Hungary, Italy, Latvia, Poland, Romania and Slovakia believe that homosexuality is a disease and that such interpretations continue to exist in professional materials. This goes against Council of Europe Recommendation 2010(5) which recommends that homosexuality not be treated as a disease. As of 2018, homosexuality was popularly considered a disease in Lebanon.

Freud and psychoanalysis Sigmund Freud's views on homosexuality were complex. In his attempts to understand the causes and development of homosexuality, he first explained bisexuality as an "original libido endowment", by which he meant that all humans are born bisexual. He believed that the libido has a homosexual portion and a heterosexual portion, and through the course of development one wins out over the other.

Some other causes of homosexuality for which he advocated included an inverted Oedipus complex where individuals begin to identify with their mother and take themselves as a love object. This love of one's self is defined as narcissism, and Freud thought that people who were high in the trait of narcissism would be more likely to develop homosexuality because loving the same sex is like an extension of loving oneself.

Freud believed treatment of homosexuality was not successful because the individual does not want to give up their homosexual identity because it brings them pleasure. He used psychoanalysis and hypnotic suggestion as treatments, but showed little success. It was through this that Freud arrived at the conclusion that homosexuality was "nothing to be ashamed of, no vice, no degradation, it cannot be classified as an illness, but a variation of sexual function". He further stated that psychoanalysts "should not promise to abolish homosexuality and make normal heterosexuality take its place", as he had concluded in his own practice that attempts to change homosexual orientations were likely to be unsuccessful. While Freud himself may have come to a more accepting view of homosexuality, his legacy in the field of psychoanalysis, especially in the United States, viewed homosexuality as negative, abnormal and caused by family and developmental issues. It was these views that significantly impacted the rationale for putting homosexuality in the first and second publications of the American Psychiatric Association's DSM, conceptualizing it as a mental disorder and further stigmatizing homosexuality in society.

Havelock Ellis

Havelock Ellis (1859–1939) was working as a teacher in Australia, when he had a revelation that he wanted to dedicate his life to exploring the issue of sexuality. He returned to London in 1879 and enrolled in St. Thomas's Hospital Medical School. He began to write, and in 1896 he co-authored Sexual Inversion with John Addington Symonds. The book was first published in German, and a year later it was translated into English. Their book explored homosexual relationships, and in a progressive approach for their time they refused to criminalize or pathologize the acts and emotions that were present in homosexual relationships.

Ellis disagreed with Freud on a few points regarding homosexuality, especially regarding its development. He argued that homosexuals do not have a clear cut Oedipus complex but they do have strong feelings of inadequacy, born of fears of failure, and may also be afraid of relations with women. Ellis argued that the restrictions of society contributed to the development of same-sex love. He believed that homosexuality is not something people are born with, but that at some point humans are all sexually indiscriminate, and then narrow down and choose which sex acts to stick with. According to Ellis, some people choose to engage in homosexuality, while others will choose heterosexuality. He proposed that being "exclusively homosexual" is to be deviant because the person is a member of a minority and therefore statistically unusual, but that society should accept that deviations from the "normal" were harmless, and maybe even valuable. Ellis believed that psychological problems arose not from homosexual acts alone, but when someone "psychologically harms himself by fearfully limiting his own sex behavior".

Ellis is often credited with coining the term homosexuality but in reality he despised the word because it conflated Latin and Greek roots and instead used the term invert in his published works. Soon after Sexual Inversion was published in England, it was banned as lewd and scandalous. Ellis argued that homosexuality was a characteristic of a minority, and was not acquired or a vice and was not curable. He advocated changing the laws to leave those who chose to practice homosexuality at peace, because at the time it was a punishable crime. He believed societal reform could occur, but only after the public was educated. His book became a landmark in the understanding of homosexuality.

Alfred Kinsey

Alfred Charles Kinsey (1894–1956) was a sexologist who founded the Institute for Sex Research, which is now known as the Kinsey Institute for Research in Sex, Gender and Reproduction. His explorations into different sexual practices originated from his study of the variations in mating practices among wasps. He developed the Kinsey Scale, which measures sexual orientation in ranges from 0 to 6 with 0 being exclusively heterosexual and 6 being exclusively homosexual. His findings indicated that there was great variability in sexual orientations. Kinsey published the books Sexual Behavior in the Human Male and Sexual Behavior in the Human Female, which brought him both fame and controversy. The prevailing approach to homosexuality at the time was to pathologize and attempt to change homosexuals. Kinsey's book demonstrated that homosexuality was more common than was assumed, suggesting that these behaviors are normal and part of a continuum of sexual behaviors.

The Diagnostic and Statistical Manual