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, or simply a complex combination of nature and nurture. The American Psychological Association, for example, states, "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. [..] Most people experience little or no sense of choice about their sexual orientation." Scientific consensus is that sexual orientation, unlike sexual orientation identity, is not a choice, as there has been no strong evidence to validate it as a lifestyle choice.
Although there is "no substantive evidence" which suggests parenting or early childhood experiences play a role in sexual orientation, some studies have linked parenting or familial environment to non-heterosexual identities, as well as childhood gender nonconformity and homosexuality.
Childhood gender nonconformity
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, 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 determinate) 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.
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 the Royal College of Psychiatrists states that there is "no substantive 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.
Fraternal birth order
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.
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.
- Biology and sexual orientation
- Demographics of sexual orientation
- Homosexuality and psychology
- Nature versus nurture
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