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Homosexual orientation is a sexual orientation. The term is used to refer to "an enduring pattern of or disposition to experience sexual, affectional, or romantic attractions primarily to" people of the same sex; "it also refers to an individual’s sense of personal and social identity based on those attractions, behaviors expressing them, and membership in a community of others who share them."[1][2] The exact proportion of the population that is homosexual is difficult to estimate reliably,[3] but studies place it at 2–7%.[4][5][6][7][8][9][10][11][12]

Sexual orientation is also distinguished from other aspects of sexuality, "including biological sex (the anatomical, physiological, and genetic characteristics associated with being male or female), gender identity (the psychological sense of being male, female or other), and social gender role (adherence to cultural norms defining feminine and masculine behavior)."[2] Etymologically, the word homosexual is a Greek and Latin hybrid with homos (often confused with the later Latin meaning of "man", as in Homo sapiens) deriving from the Greek word for same, thus connoting affections between members of the same sex, including lesbianism.[13]


Main article: Homosexuality

A person's sexual orientation is closely related to other aspects of sexuality, such as sexual identity and sexual behavior. Homosexuality refers to sexual behavior with or attraction to people of the same sex, in addition to a homosexual orientation.

Sexual identity

Main article: Sexual identity

A person's sexual identity, as it relates to sexual orientation, is how one identifies their own sexuality. A person with a homosexual identity may identify themselves as lesbian (for women) or gay (for men and women). If a person with a homosexual orientation does not identify as gay or lesbian, they are often referred to as closeted. Since sexual orientation is difficult to measure, and one's sexual identity does not necessarily match their true sexual orientation, professional literature will often use the term same-sex attraction to refer to anyone who is attracted to the same sex, includes both homosexuals and bisexuals, regardless of sexual identity.

Sexual behavior

Further information: Same-sex relationship

Many people with a homosexual orientation enter into same-sex relationships. Studies have found same-sex and opposite-sex couples to be equivalent to each other on measures of relationship satisfaction and commitment. Many lesbians and gay men form durable relationships. For example, survey data indicate that between 18% and 28% of gay couples and between 8% and 21% of lesbian couples in the U.S. have lived together 10 or more years.[14] The types of relationships vary by region and what is permitted by law. The level of acceptance ranges from making same-sex relationships illegal, to having no official stance, to officially recognizing same-sex marriages.

People may not express their sexual orientations in their sexual behavior.[15] People with a homosexual orientation may be celibate, have sexual relationships with the same sex, the opposite sex, or both. A heterosexual marriage where one of the spouses has a homosexual orientation is referred to as a mixed-orientation marriage. Likewise, people of different sexual orientations may have same-sex relationships. In professional literature, people who have sexual relationships with their own gender may be referred to as men who have sex with men or women who have sex with women.

Gender identity

The earliest writers on a homosexual orientation usually understood it to be intrinsically linked to the subject's own sex. For example, it was thought that a typical female-bodied person who is attracted to female-bodied persons would have masculine attributes, and vice versa.[16] This understanding was shared by most of the significant theorists of homosexuality from the mid 19th to early 20th Centuries, such as Karl Heinrich Ulrichs, Richard von Krafft-Ebing, Magnus Hirschfeld, Havelock Ellis, Carl Jung and Sigmund Freud, as well as many gender variant homosexual people themselves. However, this understanding of homosexuality as sexual inversion was disputed at the time, and through the second half of the 20th Century, gender identity came to be increasingly seen as a phenomenon distinct from sexual orientation.

Transgender and cisgender people may be attracted to men, women or both, although the prevalence of different sexual orientations is quite different in these two populations (see sexual orientation of transwomen). An individual homosexual, heterosexual or bisexual person may be masculine, feminine, or androgynous, and in addition, many members and supporters of lesbian and gay communities now see the "gender-conforming heterosexual" and the "gender-nonconforming homosexual" as negative stereotypes. However, studies by J Michael Bailey and KJ Zucker have found that a majority of gay men and lesbians report being gender-nonconforming during their childhood years.[17]


Main article: Demographics of sexual orientation

Measuring the prevalence of homosexuality presents a number of difficulties:

  • The research must measure some characteristic that may or may not define sexual orientation. The class of people with same-sex desires may be larger than the class of people who act on those desires, which in turn may be larger than the class of people who self-identify as gay/lesbian/bisexual.[18]

Reliable data as to the size of the gay and lesbian population is of value in informing public policy.[18] For example, demographics would help in calculating the costs and benefits of domestic partnership benefits, of the impact of legalizing gay adoption, and of the impact of the U.S. military's Don't Ask Don't Tell policy.[18] Further, knowledge of the size of the "gay and lesbian population holds promise for helping social scientists understand a wide array of important questions—questions about the general nature of labor market choices, accumulation of human capital, specialization within households, discrimination, and decisions about geographic location."[18]

Estimates of the incidence of an exclusive homosexual orientation range from >1% to 10% of the population, usually finding there are slightly more gay men than lesbians.[19][20][21]

Sexual orientation and the law

Further information: Homosexuality laws of the world, LGBT rights by country
  • Employment discrimination refers to discriminatory employment practices such as bias in hiring, promotion, job assignment, termination, and compensation, and various types of harassment. In the United States there is "very little statutory, common law, and case law establishing employment discrimination based upon sexual orientation as a legal wrong."[22] Some exceptions and alternative legal strategies are available. President Bill Clinton's Executive Order 13087 (1998) prohibits discrimination based on sexual orientation in the competitive service of the federal civilian workforce,[23] and federal non-civil service employees may have recourse under the due process clause of the U.S. Constitution.[24] Private sector workers may have a Title VII action under a quid pro quo sexual harassment theory,[25] a "hostile work environment" theory,[26] a sexual stereotyping theory,[27] or others.[22]
  • Housing discrimination refers to discrimination against potential or current tenants by landlords. In the United States, there is no federal law against such discrimination on the basis of sexual orientation or gender identity, but at least thirteen states and many major cities have enacted laws prohibiting it.[28]
  • Hate crimes (also known as bias crimes) are crimes motivated by bias against an identifiable social group, usually groups defined by race, religion, sexual orientation, disability, ethnicity, nationality, age, gender, gender identity, or political affiliation. In the United States, 45 states and the District of Columbia have statutes criminalizing various types of bias-motivated violence or intimidation (the exceptions are AZ, GA, IN, SC, and WY). Each of these statutes covers bias on the basis of race, religion, and ethnicity; 32 of them cover sexual orientation, 28 cover gender, and 11 cover transgender/gender-identity.[29]


Main article: Homosexuality and psychology

Most lesbian, gay, and bisexual people who seek psychotherapy do so for the same reasons as heterosexuals (stress, relationship difficulties, difficulty adjusting to social or work situations, etc.); their sexual orientation may be of primary, incidental, or no importance to their issues and treatment. Whatever the issue, there is a high risk for anti-gay bias in psychotherapy with lesbian, gay, and bisexual clients.[30] Psychological research in this areas has been relevant to counteracting prejudicial ("homophobic") attitudes and actions, and to the LGBT rights movement generally.[31]


Psychology was one of the first disciplines to study a homosexual orientation as a discrete phenomenon. The first attempts to classify homosexuality as a disease were made by the fledgling European sexologist movement in the late nineteenth century. In 1886 noted sexologist Richard von Krafft-Ebing listed homosexuality along with 200 other case studies of deviant sexual practices in his definitive work, Psychopathia Sexualis. Krafft-Ebing proposed that homosexuality was caused by either "congenital [during birth] inversion" or an "acquired inversion". In the last two decades of the nineteenth century, a different view began to predominate in medical and psychiatric circles, judging such behavior as indicative of a type of person with a defined and relatively stable sexual orientation. In the late 19th and early 20th centuries, pathological models of homosexuality were standard.

Today, the consensus of medical and scientific professional organizations is that homosexuality is not a mental illness. In 1973 the American Psychiatric Association (APA) removed homosexuality as a disorder from the Sexual Deviancy section of the Diagnostic and Statistical Manual of Mental Disorders, the DSM-II.[32] The World Health Organization's ICD-9 (1977) listed homosexuality as a mental illness, and in 1990, a resolution was adopted to remove it in the ICD-10 (1993).[33] The ICD-10 added ego-dystonic sexual orientation to the list, which refers to people who want to change their gender identities or sexual orientation because of a psychological or behavioral disorder (F661). Groups that advocate reparative therapy, including both secular organizations such as NARTH and religious organizations such as Exodus International, do not accept this position.

Malleability of sexual orientation

The American Psychiatric Association has stated "some people believe that sexual orientation is innate and fixed; however, sexual orientation develops across a person’s lifetime."[34] In combination with other major American medical organizations, they have put out a statement which said: "Sexual orientation develops across a person's lifetime—different people realize at different points in their lives that they are heterosexual, gay, lesbian, or bisexual."[35] A report from the Centre for Addiction and Mental Health states: "For some people, sexual orientation is continuous and fixed throughout their lives. For others, sexual orientation may be fluid and change over time."[36] One study has suggested "considerable fluidity in bisexual, unlabeled, and lesbian women's attractions, behaviors, and identities."[37][38]

However, they have said "most people experience little or no sense of choice about their sexual orientation."[39] American medical organization have further stated therapy cannot change sexual orientation, and have expressed concerns over potential harms.[35] The director of the APA's LGBT Concerns Office explained: "I don't think that anyone disagrees with the idea that people can change because we know that straight people become gays and lesbians.... the issue is whether therapy changes sexual orientation, which is what many of these people claim."[40] The American Psychiatric Association has stated "To date, there are no scientifically rigorous outcome studies to determine either the actual efficacy or harm of "reparative" treatments," and supports research to further determines risks versus its benefits.[41] Similarly, United States Surgeon General David Satcher issued a report stating that "there is no valid scientific evidence that sexual orientation can be changed".[42]

Partially because of religious reasons, some gay men and lesbians seek to change their sexual orientations through religious faith and practice. In a survey of 882 people who were undergoing conversion therapy, attending ex-gay groups or ex-gay conferences, 22. 9% reported they had not undergone any changes, 42.7% reported some changes, and 34.3% reported much change in sexual orientation.[43] Exodus International is the largest ostensibly ex-gay group. A major ally of Exodus International is Focus on the Family, who works with Exodus International in their Love Won Out ministry.


Further information: Societal attitudes towards homosexuality

Coming out

Main article: Coming out

Many people with a homosexual orientation have a so-called "coming out" at some point in their lives. Generally, coming out is described in three phases. The first phase is the phase of "knowing oneself," and the realization and acceptance that one has a homosexual orientation. This is often described as an internal coming out. The second phase involves one's decision to come out to others, e.g. family, friends, and/or colleagues. This occurs with many people as early as age 11, but others do not clarify their sexual orientation until age 40 or older. The third phase more generally involves living openly embracing a homosexual orientation.[44] In the United States today, people often come out during high school or college age. At this age, they may not trust or ask for help from others, especially when their orientation is not accepted in society. Sometimes their own parents are not even informed.

Outing is the practice of publicly revealing the sexual orientation of a closeted person.[45] Notable politicians, celebrities, military service people, and clergy members have been outed, with motives ranging from malice to political or moral beliefs. Many commentators oppose the practice altogether,[46] while some encourage outing public figures who use their positions of influence to harm other gay people.[47]


Main article: Homophobia

Homophobia is a fear of, aversion to, or discrimination against homosexuals.[48][49][50][51] It manifests in different forms, and a number of different types have been postulated, among which are internalized homophobia, social homophobia, emotional homophobia, rationalized homophobia, and others.[52]

In the United States, the FBI reported that 15.6% of hate crimes reported to police in 2004 were based on perceived sexual orientation. Sixty-one percent of these attacks were against gay men.[53] The 1998 murder of Matthew Shepard, a gay student, is one of the most notorious incidents in the U.S.

Social construct

Further information: Queer theory

Because a homosexual orientation is complex and multi-dimensional, some academics and researchers, especially in Queer studies, have argued that it is a historical and social construction. In 1976 the historian Michel Foucault argued that homosexuality as an identity did not exist in the eighteenth century; that people instead spoke of "sodomy", which referred to sexual acts. Sodomy was a crime that was often ignored but sometimes punished severely (see sodomy law).

The term homosexual is often used in European and American cultures to encompass a person’s entire social identity, which includes self and personality. In Western cultures some people speak meaningfully of gay, lesbian, and bisexual identities and communities. In other cultures, homosexuality and heterosexual labels don’t emphasize an entire social identity or indicate community affiliation based on sexual orientation.[54]

Theories of causality

The American Academy of Pediatrics has stated that "sexual orientation probably is not determined by any one factor but by a combination of genetic, hormonal, and environmental influences."[55] The amount that each influence plays is highly debated. One study on Swedish twins suggested that there was a moderate, primarily genetic, familial effects, and moderate to large effects of the nonshared environment (social and biological) on same-sex sexual behavior.[56]

The American Psychological Association has stated that "there are probably many reasons for a person's sexual orientation and the reasons may be different for different people". It also stated that for most people, sexual orientation is determined at an early age.[57]

The American Psychiatric Association has stated that, "to date there are no replicated scientific studies supporting any specific biological etiology for homosexuality. Similarly, no specific psychosocial or family dynamic cause for homosexuality has been identified, including histories of childhood sexual abuse."[34]

Research into how sexual orientation may be determined by genetic or other prenatal factors plays a role in political and social debates about homosexuality, and also raises fears about genetic profiling and prenatal testing.[58]

Genetic explanations

Further information: Biology and sexual orientation

In 1993, Dean Hamer found the genetic marker Xq28 on the X chromosome. Hamer's study found a link between the Xq28 marker and male homosexuality,[59] but the original study's results have been disputed.[60] Several mutations have been identified in flies, such as changes in the fruitless gene, cause male flies to court and attempt to mate with other males; however, when a modified male fruit fly is isolated with only female fruit flies, then he will attempt to mate with them.

Twin studies give indications that genes may predispose some men to seek partners of the same sex. Hamer commented, "From twin studies, we already know that half or more of the variability in sexual orientation is not inherited. Our studies try to pinpoint the genetic factors, not to negate the psychosocial factors."[61] One common type of twin study compares the monozygotic (or identical) twins of people possessing a particular trait to the dizygotic (non-identical, or fraternal) twins of people possessing the trait. Bailey and Pillard (1991) in a study of gay twins found that 52% of monozygotic brothers and 22% of the dizygotic twins were concordant for homosexuality.[62] Bailey, Dunne and Martin (2000) used the Australian twin registry to obtain a sample of 4,901 twins.[63] Recently, Långström and colleagues reported on a study of 3,826 twin pairs, comprising all twins between the ages of 20 and 47 in Sweden.[64] Their results showed that genetic factors explained about 34-39% of the variation, whereas specific environmental effects explained the remaining 61-66% in men. In women, the genetic part of the variation was 18-19%, with 16-17% for shared environmental and 64-66% for unique environmental factors.

Prenatal hormonal theory

Main article: Prenatal hormones and sexual orientation

The hormonal theory of sexuality holds that, just as exposure to certain hormones plays a role in fetal sex differentiation, such exposure also influences the sexual orientation that emerges later in the adult.[65][66]

Prenatal stress

It has been hypothesized that prenatal stress (stress to the mother during pregnancy) can increase the probability of the child being homosexual.[67][68][69]

Physiological differences in gay men and lesbians

Recent studies have found notable differences between the physiology of gay people and non-gay people. There is evidence that:

  • The average size of the INAH-3 in the brains of gay men is approximately the same size as INAH 3 in women, which is significantly smaller, and the cells more densely packed, than in heterosexual men's brains.[3]
  • The suprachiasmatic nucleus was found by Swaab and Hopffman to be larger in gay men than in non-gay men,[70] the suprachiasmatic nucleus is also known to be larger in men than in women.[71]
  • The anterior commissure is larger in women than men and was reported to be larger in gay men than in non-gay men,[72] but a subsequent study found no such difference.[73]
  • Gay men report, on an average, slightly longer and thicker penises than non-gay men.[74]
  • Gay men's brains respond differently to fluoxetine, a selective serotonin reuptake inhibitor.[75]
  • The functioning of the inner ear and the central auditory system in lesbians and bisexual women are more like the functional properties found in men than in non-gay women (the researchers argued this finding was consistent with the prenatal hormonal theory of sexual orientation).[76]
  • The startle response (eyeblink following a loud sound) is similarly masculinized in lesbians and bisexual women.[77]
  • Three regions of the brain (medial prefrontal cortex, left hippocampus, and right amygdala) are more active in gay men than non-gay men when exposed to sexually arousing material.[78]
  • Gay and non-gay people emit different armpit odors.[79]
  • Gay men are more likely to have a counter-clockwise hair whorl pattern.[80]
  • Gay and non-gay people's brains respond differently to two human sex pheromones (AND, found in male armpit secretions, and EST, found in female urine).[81][82][83]
  • Finger length ratios between the index and ring fingers may be different between non-gay and lesbian women.[84][85][76][86][87][88]

Cognitive differences in gay men and lesbians

Recent studies suggest the presence of subtle differences in the way gay people and non-gay people process certain kinds of information. Researchers have found that:

  • Gay men and lesbians are significantly more likely to be left-handed or ambidextrous than are non-gay men and women;[89][90][91] Simon LeVay argues that because "[h]and preference is observable before birth[92]... [t]he observation of increased non-right-handness in gay people is therefore consistent with the idea that sexual orientation is influenced by prenatal processes," perhaps heredity.[3]
  • Gay men[93] and lesbians are more verbally fluent than heterosexuals of the same sex[94][95][96] (but two studies did not find this result).[97][98]
  • Gay men may receive higher scores than non-gay men on tests of object location memory (no difference was found between lesbians and non-gay women).[99]

Fraternal birth order

Main article: Fraternal birth order and sexual orientation

There is evidence from numerous studies that homosexual men tend to have more older brothers than do heterosexual men, known as the "fraternal birth order effect."[100][101] One reported that each older brother increases the odds of being gay by 33%.[102] The fraternal birth order effect is the strongest known predictor of sexual orientation.[103] Interestingly, this relation seems to hold only for right-handed males.[104][105] There has been no observed equivalent for women.[106] Peter Bearman repeated the experiments, but found no correlation between older brothers and male homosexuality and questions the sampling methods used.[107]

The effect has been found even in males not raised with their biological brothers, suggesting an in-utero environmental causation.[103] To explain this finding, a maternal immune response has been hypothesized.[101] Male fetuses produce H-Y antigens which may be involved in the sexual differentiation of vertebrates.[101] Other studies have suggested the influence of birth order was not due to a biological, but a social process.[108]


Main article: Environment and sexual orientation

Researchers have found childhood gender nonconformity to be the largest predictor of homosexuality in adulthood.[109] Daryl Bem's Exotic Becomes Erotic theory suggests that some children will prefer activities that are typical of the other sex and that this will make a gender-conforming child feel different from opposite-sex children, while gender-nonconforming children will feel different from children of their own sex, which may evoke physiological arousal when the child is near members of the sex which it considers as being "different", which will later be transformed into sexual arousal. Peter Bearman showed that males with a female twin are twice as likely to report same-sex attractions, unless there was an older brother. He says that his findings support the hypothesis that less gendered socialization in early childhood and preadolescence shapes subsequent same-sex romantic preferences. He suggests that parents of opposite-sex twins are more likely to give them unisex treatment, but that an older brother establishes gendersocializing mechanisms for the younger brother to follow.[107]

From their research on 275 men in the Taiwanese military, Shu and Lung concluded that "paternal protection and maternal care were determined to be the main vulnerability factors in the development of homosexual males." Key factors in the development of homosexuals were "paternal attachment, introversion, and neurotic characteristics."[110] Other researchers have also provided evidence that gay men report having had less loving and more rejecting fathers, and closer relationships with their mothers, than non-gay men.[111] Whether this phenomenon is a cause of homosexuality, or whether parents behave this way in response to gender-variant traits in a child, is unclear.[112][113]

Innate bisexuality

Main article: Innate bisexuality
Further information: Kinsey Reports, Latent homosexuality

Innate bisexuality (or predisposition to bisexuality) is a term introduced by Sigmund Freud (based on work by his associate Wilhelm Fliess) that expounds that all humans are born bisexual but through psychological development (which includes both external and internal factors) become monosexual, while the bisexuality remains in a latent state.


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