Environment and sexual orientation

The relationship between environment and sexual orientation is a subject of research into possible environmental influences on the development of human sexual orientation. Some researchers distinguish environmental influences from hormonal influences, while others include biological influences such as prenatal hormones as part of environmental influences.

Sexual orientation is theorized as possibly being a combination of genetic, hormonal, and environmental influences. The American Psychological Association stated, "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 is determined by any particular factor or factors. Many think that nature and nurture both play complex roles." However, according to Royal College of Psychiatrists, there is "no substantive evidence" which suggests parenting or early childhood experiences play a role.

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.

Childhood gender non-conformity
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. 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. 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, our expectation is that social influences should be stronger for behavior than attraction. He suggests possible 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.

Family influences
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 sexual orientation.

Research also indicates that homosexual men have significantly more 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.

Fraternal birth order
According to several studies, each 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.

City of origin
In the United States, the Social Organization of Sexuality 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 hypothesize that "Large cities may provide a congenial environment for the development and expression of same-gender interest."

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.

Cultural influences
Anthropologists had observed that relatively uncompetitive primitive cultures such as those that do not distinguish or reward the best hunters in distinction to the other men in the tribe have virtually no homosexuality. Miron Baron commented, "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 increase 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."

History of sexual abuse
The American Psychiatric Association stated: "...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 found 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." 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."

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.