The evidence from natural science is clear: homosexuality in humans (and in animals) is entirely natural, non-pathological, and occurs consistently in all human societies and in all periods of history. The form that it takes, and social and legal responses, vary from one society to another, but its existence is consistent. Estimates of the extent of this occurrence vary widely, depending on precisely what is meant, how the estimates are derived, and the social conditions under which the information is gathered (there are substantial differences for instance, between the numbers that identify as gay and are willing to disclose that to researchers, those that see themselves as gay but will not disclose that to others, and those that do not see themselves as gay, but in practice engage primarily in sexual encounters with the same sex). 6% of men identify themselves as gay.
Although in the early days of the development of psychology, homosexuality was views as a form of mental illness, this is no longer the case. Since the 1970s, the consensus of the health and mental health professions globally is that homosexuality is a normal variation of human sexual orientation. In 1973, the American Psychiatric Association declassified homosexuality as a mental disorder. Other mental health professional bodies have since done the same, and it was finally declassified by the World Health Organization in 1990. Current research and clinical literature demonstrate that same-sex sexual and romantic attractions, feelings, and behaviors are normal and positive variations of human sexuality. While there are some mental health problems associated with homosexuality, it is generally agreed that these arise not from the condition itself, but from the difficulties gay and lesbian people have to cope with, arising from prejudice, discrimination and rejection in their social worlds, or from attempts to change one’s natural orientation. Mental health professional bodies agree that it is not advisable to attempt such change, and that so – called “reparative therapy” can be downright dangerous, especially in young people.
The “causes” of homosexuality are complex, probably multifactorial – and possibly irrelevant. It is likely that genetics plays a part, but is not simply a determining factor. Hormonal factors in the womb may also be a contributory, but not determining, factor. The theories that were once proposed by psychologists, relating to parental temperaments, childhood sexual abuse, or other conditioning factors of “nurture”, are now widely discredited.
While there is not yet any clear understanding of the causes (or if these even matter), it is now clear that sexual orientation is inherently fixed in our biology, possibly as early as in the womb. Numerous studies comparing the physiology of men and women, and of heterosexual and homosexual people, have shown instances where statistically speaking, gay men as a group, share some specific characteristics that are more like those of heterosexual women than those of other men – and the reverse pattern for lesbian women. These differences have been shown in brain studies, of the relative size of a particular part of the hypothalamus, in differential responses to straight or gay – oriented erotic images, and to male or female pheromones. Other physiological characteristics where studies have shown divergences between gay and straight men include “2D/4D finger studies” (of the ratio between the second and fourth fingers), fingerprint patterns, microsound emissions in the inner ear, and patterns of skeletal growth.