Sexual orientation describes an enduring pattern of attraction—emotional, romantic, sexual, or some combination of these—to the opposite sex, the same sex, both, or neither, and the genders that accompany them. These attractions are generally subsumed under heterosexuality, homosexuality, bisexuality, and asexuality. According to the American Psychological Association, sexual orientation also refers to a person's sense of "personal and social identity based on those attractions, behaviors expressing them, and membership in a community of others who share them."
The term sexual preference largely overlaps with sexual orientation, but is distinguished in psychological research. A person who identifies as bisexual, for example, may sexually prefer one sex over the other. "Sexual preference" may also suggest a degree of voluntary choice. This is disputed in terms of sexual formation, as current consensus among scholars is that sexual orientation is not a choice.
No simple, single cause for sexual orientation has been conclusively demonstrated, but research suggests that it is by a combination of genetic, hormonal, and environmental influences, with biological factors involving a complex interplay of genetic factors and the early uterine environment. Research over several decades has demonstrated that sexual orientation ranges along a continuum, from exclusive attraction to the opposite sex to exclusive attraction to the same sex. It is usually discussed in terms of heterosexuality, bisexuality and homosexuality, though asexuality is increasingly recognized as a fourth category. These categories are aspects of the more nuanced nature of sexual identity. For example, people may use other labels or none at all.
While sexual orientation is reported in this article primarily within biology and psychology, including sexology, for reports within anthropology and history, including social constructionism, see the section on other explanations.
Sexual orientation distinguished from sexual identity and behavior
Most definitions of sexual orientation include a psychological component, such as the direction of an individual's erotic desire, or a behavioral component, which focuses on the sex of the individual's sexual partner/s. Some people prefer simply to follow an individual's self-definition or identity.
The American Psychological Association states that "[s]exual orientation is an enduring emotional, romantic, sexual, or affectional attraction toward others. It is easily distinguished from other components of sexuality including biological sex, gender identity (the psychological sense of being male or female), and the social gender role (adherence to cultural norms for feminine and masculine behavior). Sexual orientation exists along a continuum that ranges from exclusive heterosexuality to exclusive homosexuality and includes various forms of bisexuality. Bisexual persons can experience sexual, emotional, and affectional attraction to both their own sex and the opposite sex. Persons with a homosexual orientation are sometimes referred to as gay (both men and women) or as lesbian (women only). Sexual orientation is different from sexual behavior because it refers to feelings and self-concept. Individuals may or may not express their sexual orientation in their behaviors."
Sexual identity and sexual behavior are closely related to sexual orientation, but they are distinguished, with identity referring to an individual's conception of themselves, behavior referring to actual sexual acts performed by the individual, and orientation referring to "fantasies, attachments and longings." Individuals may or may not express their sexual orientation in their behaviors. People who have a homosexual sexual orientation that does not align with their sexual identity are sometimes referred to as 'closeted'. The term may, however, reflect a certain cultural context and particular stage of transition in societies which are gradually dealing with integrating sexual minorities. In studies related to sexual orientation, when dealing with the degree to which a person's sexual attractions, behaviors and identity match, scientists usually use the terms concordance or discordance. Thus, a woman who is attracted to other women, but calls herself heterosexual and only has sexual relations with men, can be said to experience discordance between her sexual orientation (homosexual or lesbian) and her sexual identity and behaviors (heterosexual).
Sexual identity may also be used to describe a person's perception of his or her own sex, rather than sexual orientation. The term sexual preference has a similar meaning to sexual orientation, but it suggest a degree of voluntary choice. The term has been a listed by the American Psychological Association's Committee on Gay and Lesbian Concerns as a wording that advances a "heterosexual bias".
Sexual orientations included
Sexual orientation traditionally was defined as including heterosexuality, bisexuality, and homosexuality. Asexuality is now considered a sexual orientation by some researchers and has been defined as the absence of a traditional sexual orientation. An asexual has little to no sexual attraction to males or females.
Androphilia and gynephilia
Androphilia and gynephilia (or gynecophilia) are terms used in behavioral science to describe sexual attraction, as an alternative to a homosexual and heterosexual conceptualization. They are used for identifying a subject's object of attraction without attributing a sex assignment or gender identity to the subject. Related terms like bisexual and pansexual do not make any assignations to the subject. Using androphilia and gynephilia can avoid confusion and offense when describing people in non-western cultures, as well as when describing intersex and transgender people.
Psychiatrist Anil Aggrawal explains that androphilia, along with gynephilia, "is needed to overcome immense difficulties in characterizing the sexual orientation of transmen and transwomen. For instance, it is difficult to decide whether a transman erotically attracted to males is a heterosexual female or a homosexual male; or a transwoman erotically attracted to females is a heterosexual male or a lesbian female. Any attempt to classify them may not only cause confusion but arouse offense among the affected subjects. In such cases, while defining sexual attraction, it is best to focus on the object of their attraction rather than on the sex or gender of the subject." Sexologist Milton Diamond writes, "The terms heterosexual, homosexual, and bisexual are better used as adjectives, not nouns, and are better applied to behaviors, not people. This usage is particularly advantageous when discussing the partners of transsexual or intersexed individuals. These newer terms also do not carry the social weight of the former ones."
Some researchers advocate use of the terminology to avoid bias inherent in Western conceptualizations of human sexuality. Writing about the Samoan fa'afafine demographic, sociologist Johanna Schmidt writes that in cultures where a third gender is recognized, a term like "homosexual transsexual" does not align with cultural categories.
Other researchers, such as Bruce Bagemihl, have criticized the labels "androphilia" and "gynephilia/gynecophilia" as confusing and degrading. Bagemihl writes, "...the point of reference for 'heterosexual' or 'homosexual' orientation in this nomenclature is solely the individual's genetic sex prior to reassignment (see for example, Blanchard et al. 1987, Coleman and Bockting, 1988, Blanchard, 1989). These labels thereby ignore the individual’s personal sense of gender identity taking precedence over biological sex, rather than the other way around." Bagemihl goes on to take issue with the way this terminology makes it easy to claim transsexuals are really homosexual males seeking to escape from stigma.
Gender, transgender, cisgender, and conformance
The earliest writers on sexual 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. This understanding was shared by most of the significant theorists of sexual orientation from the mid nineteenth to early twentieth century, 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 twentieth 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.
The majority of transgender people today identify with the sexual orientation that corresponds with their gender; meaning that a transwoman who is solely attracted to women would often identify as a lesbian. Female-attracted transmen often consider themselves straight men, yet some participate in the lesbian community.
Sexual orientation sees greater intricacy when non-binary understandings of both sex (male, female, or intersex) and gender (man, woman, transgender, third gender, or gender variant) are considered. Sociologist Paula Rodriguez Rust (2000) argues for a more multifaceted definition of sexual orientation:
...Most alternative models of sexuality... define sexual orientation in terms of dichotomous biological sex or gender... Most theorists would not eliminate the reference to sex or gender, but instead advocate incorporating more complex nonbinary concepts of sex or gender, more complex relationships between sex, gender, and sexuality, and/or additional nongendered dimensions into models of sexuality.
— Paula C. Rodriguez Rust
Relationships outside of orientation
Gay and lesbian people can have sexual relationships with someone of the opposite sex for a variety of reasons, including the desire for a perceived traditional family and concerns of discrimination and religious ostracism.
While some GLBT people hide their respective orientations from their spouses, others develop positive gay and lesbian identities while maintaining successful marriages. Coming out of the closet to oneself, a spouse of the opposite sex, and children can present challenges that are not faced by gay and lesbian people who are not married to people of the opposite sex or do not have children.
Fluidity of sexuality
The American Psychiatric Association (APA) has stated, "some people believe that sexual orientation is innate and fixed; however, sexual orientation develops across a person's lifetime". The APA also says that "most people experience little or no sense of choice about their sexual orientation". "[F]or some ["people"] the focus of sexual interest will shift at various points through the life span..." A community may change over time. In a joint statement with other major American medical, psychology, educator, and religious organizations, the APA says that "different people realize at different points in their lives that they are heterosexual, gay, lesbian, or bisexual". 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". "There . . . [was, as of 1995,] essentially no research on the longitudinal stability of sexual orientation over the adult life span. . . . [I]t [was] . . . still an unanswered question whether . . . [the] measure [of "the complex components of sexual orientation as differentiated from other aspects of sexual identity at one point in time"] will predict future behavior or orientation. Certainly, it [was] . . . not a good predictor of past behavior and self-identity, given the developmental process common to most gay men and lesbians (i.e., denial of homosexual interests and heterosexual experimentation prior to the coming-out process)."
"[A number of] lesbian women, and some heterosexual women as well, perceive choice as an important element in their sexual orientations."
Born bisexual, then monosexualizing
Innate bisexuality, or predisposition to bisexuality, is an idea introduced by Sigmund Freud, based on work by his associate Wilhelm Fliess. According to this theory, 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.
Efforts to change sexuality
No major mental health professional organization has sanctioned efforts to change sexual orientation and virtually all of them have adopted policy statements cautioning the profession and the public about treatments that purport to change sexual orientation. These include the American Psychiatric Association, American Psychological Association, American Counseling Association, National Association of Social Workers in the USA, the Royal College of Psychiatrists, and the Australian Psychological Society. According to the American Psychological Association and the Royal College of Psychiatrists' Gay and Lesbian Mental Health Special Interest Group, there is no sound scientific evidence that sexual orientation can be changed.
In 2009 the American Psychological Association Task Force on Appropriate Therapeutic Responses to Sexual Orientation conducted a systematic review of the peer-reviewed journal literature on sexual orientation change efforts (SOCE) and concluded:
Efforts to change sexual orientation are unlikely to be successful and involve some risk of harm, contrary to the claims of SOCE practitioners and advocates. Even though the research and clinical literature demonstrate that same-sex sexual and romantic attractions, feelings, and behaviors are normal and positive variations of human sexuality, regardless of sexual orientation identity, the task force concluded that the population that undergoes SOCE tends to have strongly conservative religious views that lead them to seek to change their sexual orientation. Thus, the appropriate application of affirmative therapeutic interventions for those who seek SOCE involves therapist acceptance, support, and understanding of clients and the facilitation of clients' active coping, social support, and identity exploration and development, without imposing a specific sexual orientation identity outcome.
The National Association for Research & Therapy of Homosexuality, which describes itself as a "professional, scientific organization that offers hope to those who struggle with unwanted homosexuality," disagrees with the mainstream mental health community's position on conversion therapy. The American Psychological Association and the Royal College of Psychiatrists expressed concerns that the positions espoused by NARTH are not supported by the science and create an environment in which prejudice and discrimination can flourish.
Measuring sexual orientation
Varying definitions and strong social norms about sexuality can make sexual orientation difficult to quantify.
Scales for assessments
From at least the late nineteenth century in Europe, there was speculation that the range of human sexual response looked more like a continuum than two or three discrete categories. Berlin sexologist Magnus Hirschfeld published a scheme in 1896 that measured the strength of an individual's sexual desire on two independent 10-point scales, A (homosexual) and B (heterosexual). A heterosexual individual may be A0, B5; a homosexual individual may be A5, B0; an asexual would be A0, B0; and someone with an intense attraction to both sexes would be A9, B9.
Males do not represent two discrete populations, heterosexual and homosexual. The world is not to be divided into sheep and goats. It is a fundamental of taxonomy that nature rarely deals with discrete categories... The living world is a continuum in each and every one of its aspects. While emphasizing the continuity of the gradations between exclusively heterosexual and exclusively homosexual histories, it has seemed desirable to develop some sort of classification which could be based on the relative amounts of heterosexual and homosexual experience or response in each history... An individual may be assigned a position on this scale, for each period in his life.... A seven-point scale comes nearer to showing the many gradations that actually exist.
The Kinsey scale measures sexual orientation from 0 (exclusively heterosexual) to 6 (exclusively homosexual), with an additional category, X, for those with no sexual attraction to either women or men. Unlike Hirschfeld's scale, the Kinsey scale is one-dimensional. Simon LeVay wrote, "it suggests (although Kinsey did not actually believe this) that every person has the same fixed endowment of sexual energy, which he or she then divides up between same-sex and opposite-sex attraction in a ratio indicative of his or her own sexual orientation."
Means of assessment
Means typically used include surveys, interviews, cross-cultural studies, physical arousal measurements sexual behavior, sexual fantasy, or a pattern of erotic arousal. The most common is verbal self-reporting or self-labeling, which depend on respondents being accurate about themselves.
Research has identified several biological factors which may be related to the development of sexual orientation, including genes, prenatal hormones, and brain structure. No single controlling cause has been identified, and research is continuing in this area.
The prevailing view is that sexual orientation is biological in nature, determined by a complex interplay of genetic factors and the early uterine environment. Sexual orientation is therefore not a choice. That is, individuals do not choose to be homosexual, heterosexual, bisexual, or asexual. There is no substantive evidence to support the suggestion that early childhood experiences, parenting, sexual abuse, or other adverse life events influence sexual orientation. However, studies do find that as aspects of sexuality expression have an experiential basis, parental attitudes towards a particular sexual orientation may affect how their children experiment with behaviours related to a certain orientation. 
Homosexuality was once thought to be the result of troubled family dynamics or faulty psychological development. Those assumptions are now understood to have been based on misinformation and prejudice . Current scientific investigation seeks to find biological explanations for the adoption of a particular sexual orientation. As yet there are no replicated scientific studies supporting any specific biological etiology for human sexual orientation. although scientific studies have found a number of statistical biological differences between gay people and heterosexuals, which may result from the same underlying cause as sexual orientation itself.
Genes may be related to the development of sexual orientation. At one time, studies of twins appeared to point to a major genetic component, but problems in experimental design of the available studies have made their interpretation difficult, and one recent study appears to exclude genes as a major factor.
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.
Fetal hormones may be seen as either the primary influence upon adult sexual orientation or as a co-factor interacting with genes or environmental and social conditions.
A 2010 endocrinology study by Garcia-Falgueras and Swaab states that intrauterine exposure to hormones is largely the determinative criteria.
Henley et al. examined the possibility of homosexuality due to prenatal androgen excess in rats by pre and postnatally exposing male rats to testosterone and observing their later sexual behavior and partner preference. Overall, they found that male rats having been exposed to postnatal exogenous testosterone had reduced sexual behavior in adulthood, as well as altered partner preference. However, the data was not supportive of the hypothesis that excessive androgen and reduced male sexual behavior is a result of reduced motivation to approach females. Postnatal testosterone was shown to increase the time spent with a stimulus male and reduction in sexual behavior with a female, although there was no difference in the amount of time spent with the female, making the rat likely to seek partnership in both males and females. Having said this, the data seems to suggest that high levels of androgen during early postnatal development are likely to result in later bi-sexual social preference more than a homosexual preference. The same results are likely to occur in humans, but one should also examine how environmental and cultural factors would influence sexual orientation as well. 
Several studies have verified that prenatal androgens will permanently affect the neural circuits which underlie sexually dimorphic behaviors in rhesus monkeys, more specifically sexual and social behaviors that are influenced by social experience. In juveniles, play and mounting become masculinized, and approaches and sexual contacts in adults have also been shown to become masculinized. Prenatal exposure to testosterone results in the masculinization of the external genitalia of female monkeys, and in large amounts over long periods of time, female genitalia are reported to be indistinguishable from that of the males. Early androgen treatment has been shown to masculinize and defeminize such vocalizations by shifting their features towards male-typical patterns and away from female-typical patterns. Prenatal androgen exposure both masculinized and defeminized adult sexual behaviors and attractiveness. Having said this, it is clear that prenatal androgens can both masculinize and defeminize juvenile and adult behaviors of rhesus monkeys. Growing evidence suggests a significant role for organizational effects on both juvenile and adult behavior in humans, such as the numerous sexually dimorphic behaviors in humans children that are similar to those produced by androgens in rhesus monkeys—rough and tumble play, playmate choice, interest in infants, etc. Several of these behaviors have been reported to lean in the direction of male-typical pattern in human females exposed to prenatal androgens. 
Meyer-bahlburg et al. investigated the relationship between sexual orientation and the specific molecular genotypes of classical congenital adrenal hyperplasia (CAH), also including the investigation of non-classical (NC) CAH. CAH is caused by a deficiency of the enzyme 21-hydroxylase, and CAH women tend to have varying degrees of masculinized body and behavior, due to excess adrenal androgen production, as well as increased bisexual and homosexual tendencies. NC CAH becomes clinically symptomatic after birth in childhood or adolescence. Although most women in this study were heterosexual, rates of bisexual and homosexual orientation were increased above controls in both women with classical CAH as well as NC women, correlating with the degree of prenatal androgenization. This study revealed an increase in bisexuality/homosexuality in NC CAH women above controls, which particularly interesting because although these shifts were more towards masculinized behavior rather than sexual orientation, behavioral shifts in NC women were unexpected due to its classification as being symptomatic in later childhood. The fact that a small amount of androgen excess could slightly affect sexual differentiation of the brain suggests the need for further research on varying amounts of androgen exposure and the corresponding effects. 
Previous studies have shown that in animals, prenatal exposure to anticonvulsant drugs alter steroid hormone levels, thus leading to sexual differentiation. In order to find out if this is relevant in humans, Dessens et al. conducted a study with a focus on infants having been exposed to phenobarbital and phenytoin, anticonvulsant drugs commonly prescribed to pregnant women, and their effects on psychosexual development in comparison to non-exposed infants. Results of this experiment were not entirely significant, as exposed and control subjects did not show any difference regarding gender role behavior. However, reports of current or past cross-gender behavior and gender dysphoria were found to be more common in the prenatally anticonvulsant exposed individuals. Interestingly, this study did come across gender dysphoric and extreme cross-gendered individuals which may have been partly due to prenatal anticonvulsant exposure, correlating with similar results of animal research. Having said this, it may be useful for one to conduct a study of transsexual subjects investigating any prenatal exposure to such hormones. 
The auditory system
Sexual orientation depends on the ability of one person to identify the sex of another. In humans, this is a learned mental process. In 2008, it was suggested that innate root cues and mechanisms determine the sexual orientation of the adult.  These root cues and mechanisms process information that the child receives from the environment. They identify other sexual orientation cues that are embedded in the information flow, and internalize them in information structures. At puberty, sexual orientation emerges. Adults of the same sexual orientation share common root cues and mechanisms. Different root cues and mechanisms result in different orientations. Different life experiences cause personal variations within an orientation.
The innate cues satisfy universality conditions: they do not depend on race, ethnicity, culture, or historical period. Root cues that are transmitted by men are different from those transmitted by women. The brains of boys and girls process those cues using the same innate mechanisms. Boys and girls whose innate brains are tuned to root cues transmitted by men will be sexually attracted, at puberty, to men. Boys and girls whose innate brains are tuned to women’s root cues will be sexually attracted, at puberty, to women. The emerging sexual orientation at puberty depends on that actual innate tuning of the boy or the girl. That innate tuning may be genetic or due to sex hormones concentration in early developmental stages.
In humans, hearing is the only sense that satisfies the universality conditions. Both the vocal and auditory systems are genetically differentiated between the sexes. Voice is one of the most robust, reliable, universal cues which are used for identifying the sex of a person. In addition, the auditory system shows differentiation along sex and orientation lines.
Currently, most of our knowledge about the underlying causes of sexual orientations is in the form of correlations between physiological features and sexual orientation. The auditory system provides not only correlations, but also feasible explanations of cause-effect relationships between system and mechanisms on one hand, and observed behaviors on the other. More studies are needed, though, to resolve the issue.
McFadden et al. examined prenatal androgen exposure and its effects on sex and sexual orientation and how it may play a role in altering some aspects of the auditory structure. Previous animal research has shown that otoacoustic emissions (OAEs) become masculinized due to prenatal androgen exposure in late gestation, suggesting that such exposure can affect both sexual orientation and the auditory system. In newborn male infants, OAEs are lower than those of newborn females, and are weaker in nonheterosexual females than in heterosexual females. The masculinization of OAEs in homosexual females may be due to the fact that their cochlear amplifiers becoming weakened during prenatal development due to the exposure of androgens. Auditory evoked potentials (AEPs) also exhibit sex differences beginning early in life. Some AEPs are different for heterosexual and nonheterosexual females, and other AEPs are different for heterosexual and nonheterosexual males. Much evidence suggests that the cochlear amplifier system can be prenatally masculinized in mammals as well. If such effects are due to androgen exposure, then basic differences in OAEs and AEPs, at birth as well as young adults, may alter the gene which leads to the global difference in prenatal androgen exposure. This would also suggest that OAE differences in homosexual females are associated with the effects of atypical androgens exposure, which may be the effect of some system which produces either a global or local over-exposure of androgens during a crucial period in prenatal development. Lastly, it is possible that AEP differences seen in homosexual females and males are associated with prenatal androgen exposure. In order to fully develop this idea of prenatal effects of androgens on the auditory system, more research should be carried out among populations having disorders such as CAH, AIS, or ASD. 
Studying human sexual arousal has proved a fruitful way of understanding how men and women differ as genders and in terms of sexual orientation. A clinical measurement may use penile or vaginal photoplethysmography, where genital engorgement with blood is measured in response to exposure to different erotic material.
Some researchers who study sexual orientation argue that the concept may not apply similarly to men and women. A study of sexual arousal patterns found that women, when viewing erotic films which show female-female, male-male and male-female sexual activity (oral sex or penetration), have patterns of arousal which do not match their declared sexual orientations as well as men's. That is, heterosexual and lesbian women's sexual arousal to erotic films do not differ significantly by the genders of the participants (male or female) or by the type of sexual activity (heterosexual or homosexual). On the contrary, men's sexual arousal patterns tend to be more in line with their stated orientations, with heterosexual men showing more penis arousal to female-female sexual activity and less arousal to female-male and male-male sexual stimuli, and homosexual and bisexual men being more aroused by films depicting male-male intercourse and less aroused by other stimuli.
Another study on men and women's patterns of sexual arousal confirmed that men and women have different patterns of arousal, independent of their sexual orientations. The study found that women's genitals become aroused to both human and nonhuman stimuli from movies showing humans of both genders having sex (heterosexual and homosexual) and from videos showing non-human primates (bonobos) having sex. Men did not show any sexual arousal to non-human visual stimuli, their arousal patterns being in line with their specific sexual interest (women for heterosexual men and men for homosexual men).
These studies suggest that men and women are different in terms of sexual arousal patterns and that this is also reflected in how their genitals react to sexual stimuli of both genders or even to non-human stimuli. It must be stated that sexual orientation has many dimensions (attractions, behavior, identity), of which sexual arousal is the only product of sexual attractions which can be measured at present with some degree of physical precision. Thus, the fact that women are aroused by seeing non-human primates having sex does not mean that women's sexual orientation includes this type of sexual interest. Some researchers argue that women's sexual orientation depends less on their patterns of sexual arousal than men's and that other components of sexual orientation (like emotional attachment) must be taken into account when describing women's sexual orientations. In contrast, men's sexual orientations tend to be primarily focused on the physical component of attractions and, thus, their sexual feelings are more exclusively oriented according to sex.
More recently, scientists have started to focus on measuring changes in brain activity related to sexual arousal, by using brain-scanning techniques. Given that sexual feelings are the product of perceptions or fantasies which are processed in the brain, measuring brain regions' activity should further show how people of different sexual orientations differ in this respect.
A study on how heterosexual and homosexual men's brains react to seeing pictures of naked men and women has found that both hetero- and homosexual men react positively to seeing their preferred sex, using the same brain regions. The only significant group difference between these orientations was found in the amygdala, a brain region known to be involved in regulating fear.
Although these findings have contributed to understanding how sexual arousal can differentiate between genders and sexual orientations, it is still a matter of debate whether these results reflect differences which are the result of social learning or genetic or biological factors. Further studies are needed to clarify how much of people's reactions to sexual stimuli of their preferred gender are due to learned or innate factors.
Recent studies found an increased chance of homosexuality in men whose mothers previously carried to term many male children. This effect is nullified if the man is left-handed.
Known as the ‘‘fraternal birth order’’ (FBO) effect, this theory has been backed up by strong evidence of its prenatal origin, although no evidence thus far has linked it to an exact prenatal mechanism. However, research suggests that this may be of immunological origin, caused by a maternal immune reaction against a substance crucial to male fetal development during pregnancy, which becomes increasingly likely after every male gestation. As a result of this immune effect, alterations in later-born males' prenatal development have been thought to occur. This process, known as the maternal immunization hypothesis (MIH), would begin when cells from a male fetus enter the mother's circulation during pregnancy or while giving birth. These Y-linked proteins would not be recognized in the mother's immune system due to the fact that she is female, causing her to develop antibodies which would travel through the placental barrier into the fetal compartment. From here, the anti-male bodies would then cross the blood/brain barrier (BBB) of the developing fetal brain, altering sex-dimorphic brain structures relative to sexual orientation, causing the exposed son to be more attracted to men over women. 
Influences on sexual orientation: professional organizations' statements
A legal brief dated September 26, 2007, and presented on behalf of the American Psychological Association, California Psychological Association, American Psychiatric Association, National Association of Social Workers, and National Association of Social Workers, California Chapter, stated:
Currently, there is no scientific consensus about the specific factors that cause an individual to become heterosexual, homosexual, or bisexual — including possible biological, psychological, or social effects of the parents' sexual orientation.
And added in a footnote:
Although much research has examined the possible genetic, hormonal, developmental, social, and cultural influences on sexual orientation, no findings have emerged that permit scientists to conclude that sexual orientation – heterosexuality, homosexuality, or bisexuality – is determined by any particular factor or factors. The evaluation of amici is that, although some of this research may be promising in facilitating greater understanding of the development of sexual orientation, it does not permit a conclusion based in sound science at the present time as to the cause or causes of sexual orientation, whether homosexual, bisexual, or heterosexual.
Currently, there is no scientific consensus about the specific factors that cause an individual to become heterosexual, homosexual, or bisexual – including possible biological, psychological, or social effects of the parents' sexual orientation. However, the available evidence indicates that the vast majority of lesbian and gay adults were raised by heterosexual parents and the vast majority of children raised by lesbian and gay parents eventually grow up to be heterosexual.
The mechanisms for the development of a particular sexual orientation remain unclear, but the current literature and most scholars in the field state that one's sexual orientation is not a choice; that is, individuals do not choose to be homosexual or heterosexual. A variety of theories about the influences on sexual orientation have been proposed. Sexual orientation probably is not determined by any one factor but by a combination of genetic, hormonal, and environmental influences. In recent decades, biologically based theories have been favored by experts. Although there continues to be controversy and uncertainty as to the genesis of the variety of human sexual orientations, there is no scientific evidence that abnormal parenting, sexual abuse, or other adverse life events influence sexual orientation. Current knowledge suggests that sexual orientation is usually established during early childhood.
Despite almost a century of psychoanalytic and psychological speculation, there is no substantive evidence to support the suggestion that the nature of parenting or early childhood experiences play any role in the formation of a person's fundamental heterosexual or homosexual orientation. It would appear that sexual orientation is biological in nature, determined by a complex interplay of genetic factors and the early uterine environment. Sexual orientation is therefore not a choice, though sexual behaviour clearly is.
The American Psychiatric Association stated:
No one knows what causes heterosexuality, homosexuality, or bisexuality. Homosexuality was once thought to be the result of troubled family dynamics or faulty psychological development. Those assumptions are now understood to have been based on misinformation and prejudice.
- American Academy of Pediatrics
- American Medical Association
- American Medical Student Association
- American Psychological Association: for public or educators
- Catholic Medical Association
- Christian Medical and Dental Association
Demographics of sexual orientation
The multiple aspects of sexual orientation and the boundary-drawing problems already described create methodological challenges for the study of the demographics of sexual orientation. Determining the frequency of various sexual orientations in real-world populations is difficult and controversial.
Most modern scientific surveys find that the majority of people report a mostly heterosexual orientation. However, the relative percentage of the population that reports a homosexual orientation varies with differing methodologies and selection criteria. Most of these statistical findings are in the range of 2.8 to 9% of males, and 1 to 5% of females for the United States – this figure can be as high as 12% for some large cities and as low as 1% for rural areas.
Estimates for the percentage of the population that are bisexual vary widely, at least in part due to differing definitions of bisexuality. Some studies only consider a person bisexual if they are nearly equally attracted to both sexes, and others consider a person bisexual if they are at all attracted to the same sex (for otherwise mostly heterosexual persons) or to the opposite sex (for otherwise mostly homosexual persons). A small percentage of people are not sexually attracted to anyone (asexuality).
In the oft-cited and oft-criticized Sexual Behavior in the Human Male (1948) and Sexual Behavior in the Human Female (1953), by Alfred C. Kinsey et al., people were asked to rate themselves on a scale from completely heterosexual to completely homosexual. Kinsey reported that when the individuals' behavior as well as their identity are analyzed, most people appeared to be at least somewhat bisexual — i.e., most people have some attraction to either sex, although usually one sex is preferred. According to Kinsey, only a minority (5–10%) can be considered fully heterosexual or homosexual. Conversely, only an even smaller minority can be considered fully bisexual (with an equal attraction to both sexes). Kinsey's methods have been criticized as flawed, particularly with regard to the randomness of his sample population, which included prison inmates, male prostitutes and those who willingly participated in discussion of previously taboo sexual topics. Nevertheless, Paul Gebhard, subsequent director of the Kinsey Institute for Sex Research, reexamined the data in the Kinsey Reports and concluded that removing the prison inmates and prostitutes barely affected the results.
Social constructionism and Western societies
Because sexual 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, philosopher and 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). He wrote, "'Sexuality' is an invention of the modern state, the industrial revolution, and capitalism."
Sexual orientation is a concept that evolved in the industrialized West and there is a controversy as to the universality of its application in other societies or cultures. Non-westernized concepts of male sexuality differ essentially from the way sexuality is seen and classified under the Western system of sexual orientation.[unreliable source?] The validity of the notion of sexual orientation as defined in the West, as a biological phenomenon rather than a social construction specific to a region and period, has also been questioned within the industrialized Western society.
Heterosexuality and homosexuality are terms 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 do not emphasize an entire social identity or indicate community affiliation based on sexual orientation.
Some historians and researchers argue that the emotional and affectional activities associated with sexual-orientation terms such as gay and heterosexual change significantly over time and across cultural boundaries. For example, in many English-speaking nations it is assumed that same-sex kissing, particularly between men, is a sign of homosexuality, whereas various types of same-sex kissing are common expressions of friendship in other nations. Also, many modern and historic cultures have formal ceremonies expressing long-term commitment between same-sex friends, even though homosexuality itself is taboo within the culture.
Anthropology, history, and sexology
Some scholars of anthropology, history, and sexology have argued that social categories such as heterosexual and homosexual are not universal. Different societies may consider other criteria to be more significant than sex, including the respective ages of the partners, whether partners assume active or passive sexual roles, and their social status.
Perception of sexual orientation and other people's reactions
One person may presume knowledge of another person's sexual orientation based upon perceived characteristics such as appearance, clothing, tone of voice, and accompaniment by and behavior with other people. Perceived sexual orientation may affect how a person is treated. For instance, in the United States, the FBI reported that 15.6% of hate crimes reported to police in 2004 were "because of a sexual-orientation bias." Under the UK Employment Equality (Sexual Orientation) Regulations 2003, as explained by Advisory, Conciliation and Arbitration Service, "workers or job applicants must not be treated less favourably because of their sexual orientation, their perceived sexual orientation or because they associate with someone of a particular sexual orientation."
Politics and theology
Two researchers, raising (1995) 'serious doubt whether sexual orientation is a valid concept at all,' warned against increasing politicization of this area.
Professor Michael King stated, "The conclusion reached by scientists who have investigated the origins and stability of sexual orientation is that it is a human characteristic that is formed early in life, and is resistant to change. Scientific evidence on the origins of homosexuality is considered relevant to theological and social debate because it undermines suggestions that sexual orientation is a choice."
- Affectional orientation
- Ascribed characteristics
- Hate crime
- LGBT (Lesbian, Gay, Bisexual, and Transgender)
- List of anti-discrimination acts
- LGBT rights by country or territory
- Human male sexuality, including non-western perspectives on sexual orientation
- Same-sex marriage
- Sexual orientation and military service
- Sexual Orientation Hypothesis
- Terminology of homosexuality
- Mental roots of sexual orientation
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- "Whereas there is no evidence from the present investigation to suggest that parents have a determining influence on the sexual orientation of their children, the findings do indicate that by creating a climate of acceptance or rejection of homosexuality within the family, parents may have some impact on their children's sexual experimentation as heterosexual, lesbian, or gay." Do Parents Influence the Sexual Orientation of Their Children?, ibid., in Speaking of Sexuality, id., p. 303 (adapted per id., p. 303).
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- Chinese Femininities, Chinese Masculinities: A Reader, by Susan Brownell & Jeffrey N. Wasserstrom (Univ. of Calif. Press, 2002 (ISBN 0520221168, 9780520221161)). Quote: "The problem with Sexuality Some scholars have argued that maleness and femaleness were not closely linked to sexuality in China. [[[Michel] Foucault]]'s "The History of Sexuality" (which dealt primarily with Western civilization and western Europe) began to influence some China scholars in the 1980s. Foucault's insight was to demonstrate that sexuality has a history; it is not fixed psychobiological drive that is the same for all humans according to their sex, but rather it is a cultural construct inseparable from gender constructs. After unmooring sexuality from biology, he anchored it in history, arguing that this thing we now call sexuality came into existencee in the eighteenth-century West and did not exist previously in this form. "Sexuality" is an invention of the modern state, the industrial revolution, and capitalism. Taking this insight as a starting point, scholars have slowly been compiling the history of sexuality in china. The works by Tani Barlow, discussed above, were also foundational in this trend. Barlow observes that, in the West, heterosexuality is the primary site for the production of gender: a woman truly becomes a woman only in relation to a man's heterosexual desire. By contrast, in China before the 1920s the "jia" (linage unit, family) was the priamary site for the production of gender: marriage and sexuality were to serve the lineage by producing the next generation of lineage members; personal love and pleasure were secondary to this goal. Barlow argues that this has two theoretical implications: (1) it is not possible to write a chinese history of heterosexuality, sexuality as an institution, and sexual identities in the European metaphysical sense, and (2) it is not appropriate to ground discussions of Chinese gender processes in the sexed body so central in "Western" gender processes. Here she echoes Furth's argument that, before the earlyu twentiethh century, sex-identity grounded on anatomical difference did not hold a central place in Chinese constructions of gender. And she ehoes the point illustrated in detail in Sommer's chapter on male homosexuality in the Qing legal code: a man could engage in homosexual behaviour without calling into question his manhood so long as his behaviour did not threaten the patriarchal Confucian family structure."
- The Psychology of Sexual Orientation, Behaviour, and identity, by Louis Diamant & Richard D. McAnulty (Greenwood Publishing Group, 1995 (ISBN 0313285012, 9780313285011) (522 pages). Quote from page 81: Although sexual orientation is a loaded Western concept, the term is still a useful one, if we avoid imposing Western thoughts and meanings associated with our language on non-Western, noncontemporary cultures.
- The Handbook of Social Work Direct Practice, by Paula Allen-Meares & Charles D. Garvin & Contributors Paula Allen-Meares & Charles D. Garvin (SAGE, 2001 (ISBN 0761914994, 9780761914990) (733 pages). Quote from page 478: The concept of sexual orientation is a product of contemporary Western thought.
- Sexual behavior and the non-construction of sexual identity: Implications for the analysis of men who have sex with men and women who have sex with women., [by?] Michael W. Ross & Ann K. Brooks. Quote from Page 9: Chou (2000) notes in his analysis of the lack of applicability of western concepts of sexual identity in China, just because a person has a particular taste for a specific food doesn't mean that we label them in terms of the food that they prefer. A similar approach to sexual appetite as not conferring identity may be operating in this sample. McIntosh (1968) has previously noted that people who do not identify with the classic western, white gay/lesbian role may not necessarily identify their behavior as homosexual;
- Transnational Transgender: Reading Sexual Diversity in Cross-Cultural Contexts Through Film and Video [by?] Ryan, Joelle Ruby (American Studies Association). Quote: Many of the projects which have historically investigated sex/gender variance in non-Western contexts have been ethnographies and anthropological studies. Due to strong and lingering problems with ethnocentrism, many of these research studies have attempted to transpose a Western understanding of sex, gender and sexuality onto cultures in Asia, Latin America and Africa. Terms such as "homosexual," "transvestite," and "transsexual" all arose out of Western concepts of identity based on science, sexology and medicine and often bear little resemblance to sex/gender/sexuality paradigms in the varied cultures of the developing world.
- Sexual Orientation, Human Rights and Global Politics, [by?] Matthew Waits (Matthew Waits of Dept. Sociology, Anthropology & Applied Social Sciences, Univ. of Glasgow, United Kingdom, <http://www.gla.ac.uk/departments/sociology/staff/waites.html>). Quote from the Abstract: The paper problematises utilisation of the concept of 'sexual orientation' in moves to revise human rights conventions and discourses in the light of social constructionist and queer theory addressing sexuality, which has convincingly suggested that 'sexual orientation' is a culturally specific concept, misrepresenting many diverse forms of sexuality apparent in comparative sociological and anthropological research conducted worldwide. I will argue in particular that 'orientation' is a concept incompatible with bisexuality when interpreted within the context of dominant dualistic assumptions about sex, gender and desire in western culture (suggested by Judith Butler's concept of the 'heterosexual matrix'). I will discuss the implications of the this for interpreting contemporary struggles among competing social movements, NGO and governmental actors involved in contesting the relationship of sexuality to human rights as defined by the United Nations.
-  McIntosh argues that the labelling process should be the focus of inquiry and that homosexuality should be seen as a social role rather than a condition. Role is more useful than condition, she argues, because roles (of heterosexual and homosexual) can be dichotomised in a way that behaviour cannot. She draws upon cross-cultural data to demonstrate that in many societies 'there may be much homosexual behaviour, but there are no "homosexuals"' (p71).
- Zachary Green & Michael J. Stiers, Multiculturalism and Group Therapy in the United States: A Social Constructionist Perspective (Springer Netherlands, 2002), pp. 233–246.
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- Church Times: How much is known about the origins of homosexuality?
- Anders Agmo, Functional and Dysfunctional Sexual Behavior (Elsevier, 2007).
- Brum, Gil, Larry McKane, & Gerry Karp, Biology – Exploring Life (John Wiley & Sons, Inc., 2d ed. 1994), p. 663. (About INAH-3.)
- De La Torre, Miguel A., Out of the Shadows, Into the Light: Christianity and Homosexuality (Chalice Press, 2009).
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