Often, sexual orientation and sexual orientation identity are not distinguished, which can impact accurately assessing sexual identity and whether or not sexual orientation is able to change; sexual orientation identity can change throughout an individual's life, and may or may not align with biological sex, sexual behavior or actual sexual orientation. While the Centre for Addiction and Mental Health and American Psychiatric Association state that sexual orientation is innate, continuous or fixed throughout their lives for some people, but is fluid or changes over time for others, the American Psychological Association distinguishes between sexual orientation (an innate attraction) and sexual orientation identity (which may change at any point in a person's life). Scientists and mental health professionals generally do not believe that sexual orientation is a choice.
The American Psychological Association states that "sexual orientation is not a choice that can be changed at will, and that sexual orientation is most likely the result of a complex interaction of environmental, cognitive and biological factors...is shaped at an early age...[and evidence suggests] biological, including genetic or inborn hormonal factors, play a significant role in a person's sexuality." They say that "sexual orientation identity—not sexual orientation—appears to change via psychotherapy, support groups, and life events." The American Psychiatric Association says "individuals maybe become aware at different points in their lives that they are heterosexual, gay, lesbian, or bisexual" and "opposes any psychiatric treatment, such as 'reparative' or 'conversion' therapy, which is based upon the assumption that homosexuality per se is a mental disorder, or based upon a prior assumption that the patient should change his/her homosexual orientation". They do, however, encourage gay affirmative psychotherapy.
Scholar Lisa Diamond, when reviewing research on lesbian and bisexual women's sexual identities, stated that studies find "change and fluidity in same-sex sexuality that contradict conventional models of sexual orientation as a fixed and uniformly early-developing trait."
Researchers have found childhood gender nonconformity to be the largest predictor of homosexuality in adulthood. Daryl Bem suggests that some children will prefer activities that are typical of the other sex. Choice of activity consistent with societally defined gender roles will make a gender-conforming child feel different from opposite-sex children. Gender-nonconforming children, on the other hand, will feel different from children of their own sex. In either case, this feeling of difference 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. Researchers have suggested that this nonconformity may be a result of genetics, prenatal hormones, personality, parental care or other environmental factors.
Bearman and Bruckner showed that males with a female twin are twice as likely to report same-sex attractions, unless there was an older brother. They say that their findings can be explained by the hypothesis that less gendered socialization in early childhood and preadolescence shapes subsequent same-sex romantic preferences. They suggest that parents of opposite-sex twins are more likely to give them unisex treatment, but that an older brother establishes gender-socializing mechanisms for the younger brother to follow. However, Bearman and Bruckner found no direct evidence for the effect of gender socialization on sexual orientation. In fact, no evidence has ever been found linking the gender socialization of parents to the sexual orientation of their children while several twin studies have suggested that almost all of the familial resemblance that is observed for sexual orientation is attributable to genes, not family environment. The proportion of adolescents reporting same-sex attraction is significantly higher than the proportion reporting same-sex sexual experience. In addition to attraction, opportunity has to present itself. Since opportunity is clearly socially structured, the expectation is that social influences should be stronger for behavior than attraction.
Researchers have provided evidence that gay men report having had less loving and more rejecting fathers, and closer relationships with their mothers, than non-gay men. Some researchers think this may indicate that childhood family experiences are important determinants to homosexuality, or that parents behave this way in response to gender-variant traits in a child. Michael Ruse suggests that both possibilities might be true in different cases.
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." One study reported that homosexual males reported more positive early relationships with mothers than did homosexual females. A 2000 American twin study showed that familial factors, which may be at least partly genetic, influence (but do not determine) sexual orientation.
Research also indicates that homosexual men have significantly more older male siblings than the homosexual women, who, in turn, have significantly more siblings than heterosexual men. A 2006 Danish study compared people who had a heterosexual marriage versus people who had a same-sex marriage. Heterosexual marriage was significantly linked to having young parents, small age differences between parents, stable parental relationships, large numbers of siblings, and late birth order. Children who experience parental divorce are less likely to marry heterosexually than those growing up in intact families. For men, same-sex marriage was associated with having older mothers, divorced parents, absent fathers, and being the youngest child. For women, maternal death during adolescence and being the only or youngest child or the only girl in the family increased the likelihood of same-sex marriage.
Results from a 2008 twin study were consistent with moderate, primarily genetic, familial effects, and moderate to large effects of the nonshared environment (social and biological) on same-sex sexual behavior; the study concluded that, for same-sex sexual behavior, shared or familial environment plays no role for men and minor role for women. By contrast, in a study doing genetic analysis of 409 pairs of homosexual brothers, including twins, strong evidence was found that some homosexual men are born homosexual. The study, including approximately three times as many people as the previous largest study on this subject, indicates that it is significantly more statistically reliable. It links sexual orientation in men with two regions of the human genome that have been implicated before. Lead author of the study, Alan Sanders, however, states that "complex traits such as sexual orientation depend on multiple factors, both environmental and genetic." A region on the X chromosome called Xq28, was originally identified in 1993 by Dean Hamer of the US National Institutes of Health in Bethesda, Maryland. Another region in the twist of chromosome 8, known as 8q12, was first identified in 2005.
Although there is no substantial evidence which suggests parenting or early childhood experiences play a role in sexual orientation, a Cameron 2006 study found that "parents' sexual inclinations influence their children's." A study published in 2010 confirmed this result and stated, "Despite numerous attempts to bias the results in favour of the null hypothesis and allowing for up to 20 (of 63, 32%) coding errors, Cameron's (2006) hypothesis that gay and lesbian parents would be more likely to have gay, lesbian, bisexual or unsure (of sexual orientation) sons and daughters was confirmed. ... social and parental influences may influence the expression of non-heterosexual identities and/or behaviour." Bearman, on the other hand, acknowledges a possibility that socialization experiences might shape desire, but not subsequent adult sexual orientation. It is possible that genetic influence could operate on the pathway from attraction to behavior.
According to several studies, each additional older brother increases a man's odds of developing a homosexual orientation by 28%–48%. Most researchers attribute this to prenatal environmental factors, such as prenatal hormones. McConaghy (2006) found no relationship between the strength of the effect and degree of homosexual feelings, suggesting the influence of fraternal birth order was not due to a biological, but a social process.
In their landmark study of sexual behavior in the United States—reported in the Social Organization of Sexuality—the University of Chicago sociologist Edward Laumann and his colleagues found that homosexuality was positively correlated with urbanization of the place of residence at age 14. The correlation was more substantial among men than women. The authors hypothesized that "Large cities may provide a congenial environment for the development and expression of same-gender interest." This idea was further elaborated in Laumann's later book, The Sexual Organization of the City, which showed that expression of sexual orientation is contingent on the existence of "sex marketplaces," or venues where people with specific sexual orientations can congregate and meet.
In Denmark, people born in the capital area were significantly less likely to marry heterosexually, and more likely to marry homosexually, than their rural-born peers.
Miron Baron stated, "Some cultures – for example, the Assyrian and Graeco-Roman – were more tolerant of homosexuality. The behavior was practiced openly and was highly prevalent. Sexual patterns are to some extent a product of society's expectations, but it would be difficult to envisage a change in the prevalence of the genetic trait merely in response to changing cultural norms." This hypothesis had previously been enunciated by Richard Burton as the Sotadic zone.
In the US, there has been an increasing number of women developing an attraction for other women. Susan Bordo has stated that when a taboo is lifted or diminished, it gives individuals the space to explore and express their sexual orientation. Binnie Klein has stated that "It's clear that a change in sexual orientation is imaginable to more people than ever before, and there's more opportunity – and acceptance – to cross over the line."
The American Psychiatric Association states: "...no specific psychosocial or family dynamic cause for homosexuality has been identified, including histories of childhood sexual abuse. Sexual abuse does not appear to be more prevalent in children who grow up to identify as gay, lesbian, or bisexual, than in children who identify as heterosexual."
One study in the Journal of the American Medical Association reported that "Abused adolescents, particularly those victimized by males, were up to 7 times more likely to self-identify as gay or bisexual than peers who had not been abused". However, the study went on to state that victims may self-identify as gay or bisexual before the abuse occurs, implying that their non-heterosexual orientation and identity is not caused by the abuse, and reported that no longitudinal study had determined a causal relationship between sexual abuse and sexual orientation.
Another study found that "Forty-six percent of the homosexual men in contrast to 7% of the heterosexual men reported homosexual molestation. Twenty-two percent of lesbian women in contrast to 1% of heterosexual women reported homosexual molestation". However, the study has been criticized for being non-clinical, for using un-representative samples of homosexuals and heterosexuals, for not reporting how homosexual participants were recruited, for contradictions between the results and their interpretation in the dissertation, for contradictions in the original study and the conclusions drawn in the dissertation and journal, and for implying that the frequency of homosexual self-identification was a consequence of the abuse even though it reports that 68% of men and 62% of women identified as homosexual before their molestation experience. The study is also criticized as some people are aware of their same-sex attraction at very early ages and people usually become aware of their same-sex attractions before they start self-identifying as homosexual; however, the study asked participants about the age at which they regarded themselves as homosexual, not the age at which they became aware of their same-sex attraction, thus rendering it impossible "to be certain about when the abuse occurred – before or after awareness of same-sex attraction." Causation between sexual abuse and homosexuality cannot be inferred by this study. The study itself states that its "homosexual participants may not be representative of [all] homosexual persons" and that molestation "may not, however, be a casual factor [of non-heterosexual orientation] in either gender." The study has been further criticized for not mentioning the word "molestation" in the questionnaires distributed to participants, using the subjective term "sexual contact" instead, and then switching to using the word "molestation" in the analysis of the data." The study has also come under criticism for relying on the work of Paul Cameron, who was expelled by the American Psychological Association and has been condemned by the American Sociological Association, Canadian Psychological Association and the Nebraska Psychological Association for consistently misinterpreting and misrepresenting scientific research on sexuality, male homosexuality and lesbianism.
In a 30-year longitudinal study published in the Archives of Sexual Behavior, although the authors found that men with histories of childhood sexual abuse were more likely to report ever having had same-sex sexual partners, they did not find any "significant relationships between childhood physical abuse or neglect and same-sex sexual orientation in adulthood"; neither men nor women with histories of childhood physical abuse, sexual abuse, or neglect reported more same-sex sexual partners in the previous year or same-sex romantic cohabitation compared to men and women without such histories. Authors of the study speculated that "sexual abuse may result in uncertainty regarding sexual orientation and greater experimentation with both same- and opposite-sex relationships," but may not affect ultimate sexual orientation.