Environment and sexual orientation

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. Sexual orientation is stable and unlikely to change for the vast majority of people, but some research indicates that some people may experience change in their sexual orientation, and this is more likely for women than for men. The American Psychological Association distinguishes between sexual orientation (an enduring 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. == Prenatal environment ==

The influence of hormones on the developing fetus has been the most influential causal hypothesis of the development of sexual orientation. In simple terms, the developing fetal brain begins in a "female" typical state. The presence of the Y-chromosome in males prompts the development of testes, which release testosterone, the primary androgen receptor-activating hormone, to masculinize the fetal brain. This masculinizing effect pushes males towards male typical brain structures, and most of the time, attraction to females. It has been hypothesized that gay men may have been exposed to lower levels of testosterone in key regions of the brain, or had different levels of receptivity to its masculinizing effects, or experienced fluctuations at critical times. In women, it is hypothesized that high levels of exposure to testosterone in key regions may increase likelihood of same sex attraction.). Lesbians on average were found to have significantly more masculine digit ratios, a finding which has been replicated numerous times in studies cross-culturally, but sometimes contradicted, such as when ethnicity is taken into account. While direct effects are hard to measure for ethical reasons, animal experiments where scientists manipulate exposure to sex hormones during gestation can also induce life long male-typical behavior and mounting in female animals, and female-typical behavior in male animals. Maternal immune responses during fetal development are strongly demonstrated as causing male homosexuality and bisexuality. Research since the 1990s has demonstrated that the more male sons a woman has, there is a higher chance of later born sons being gay. During pregnancy, male cells enter a mother's bloodstream, which are foreign to her immune system. In response, she develops antibodies to neutralize them. These antibodies are then released on future male fetuses and may neutralize Y-linked antigens, which play a role in brain masculinization, leaving areas of the brain responsible for sexual attraction in the female-typical position, or attracted to men. The more sons a mother has will increase the levels of these antibodies, thus creating the observed fraternal birth order effect. Biochemical evidence to support this effect was confirmed in a lab study in 2017, finding that mothers with a gay son, particularly those with older brothers, had heightened levels of antibodies to the NLGN4Y Y-protein than mothers with heterosexual sons. J. Michael Bailey has described maternal immune responses as "causal" of male homosexuality. This effect is estimated to account for between 15-29% of gay men, while other gay and bisexual men are thought to owe sexual orientation to genetic and hormonal interactions. The size of the sheep oSDN has also been demonstrated to be formed in utero, rather than postnatally, underscoring the role of prenatal hormones in masculinization of the brain for sexual attraction. Rice et al. propose that these epimarks normally canalize sexual development, preventing intersex conditions in most of the population, but sometimes failing to erase across generations and causing reversed sexual preference. Testing this hypothesis is possible with current stem cell technology. There is evidence that mutations in NLGN4X and NLGN4Y are linked to autism spectrum conditions and such conditions may be elevated in asexual people. Thus, NLGN4X/Y may affect neurological functioning associated with, broadly, the forming of social connections to others, including sexual/romantic ones. == Childhood gender nonconformity ==

Researchers have found childhood gender nonconformity (CGN) to be the largest predictor of homosexuality in adulthood. Gay men often report being feminine boys, and lesbian women often report being masculine girls. In men, CGN is a strong predictor of sexual orientation in adulthood, but this relationship is not as well understood in women. Women with congenital adrenal hyperplasia (CAH), which effects production of sex steroids, report more male typical play behaviors and show less heterosexual interest. Bailey believes childhood gender nonconformity to be a clear indicator that male homosexuality is an inborn trait – the result of hormones, genes and other prenatal developmental factors. Bailey says boys are "punished much more than rewarded" for their gender nonconformity, and that such behavior "emerges with no encouragement, and despite opposition", making it "the sine qua non of innateness". Bailey describes gender nonconforming boys as the "poster children for biological influences on gender and sexuality, and this is true whether or not we measure a single biological marker". Daryl Bem proposed the "exotic becomes erotic" theory (EBE) in 1996. Bem argues that biological factors, such as prenatal hormones, genes and neuroanatomy, predispose children to behave in ways that do not conform to their sex assigned at birth. Gender nonconforming children will often prefer opposite-sex playmates and activities. These become alienated from their same-sex peer group. As children enter adolescence "the exotic becomes erotic" where dissimilar and unfamiliar same-sex peers produces arousal, and the general arousal become eroticized over time. Wetherell et al. state that Bem "does not intend his model as an absolute prescription for all individuals, but rather as a modal or average explanation." Bem was criticized for relying on a non-random sample of gay men from the 1970s and for drawing conclusions that appear to contradict the original data. An "examination of the original data showed virtually all respondents were familiar with children of both sexes", and that only 9% of gay men said that "none or only a few" of their friends were male, and most gay men (74%) reported having "an especially close friend of the same sex" during grade school. the theory "lacks empirical support". In 2003, Lorene Gottschalk, a self-described radical feminist suggested there may be a reporting bias within the links between gender nonconformity to homosexuality in literature. Researchers have explored the possibility of bias by comparing childhood home videos with self-reports of gender nonconformity, finding that the presence of gender nonconformity was highly consistent with self-reporting, emerged early and carried into adulthood. == Family and upbringing ==

General Hypotheses for the impact of the post-natal social environment on sexual orientation are weak, especially for males. There is no substantial evidence which suggests parenting or early childhood experiences influence sexual orientation. Research has linked early childhood gender nonconformity and homosexuality; gay men, on average, have been observed to be significantly more feminine from early childhood while lesbians are significantly more masculine. Bisexuals also report childhood gender nonconformity, but the difference is not as large as it is for gay men and lesbians. Early gender nonconformity is suggestive evidence that non-heterosexual orientations are dependent on early biological factors (genetic influence, prenatal hormones, or other factors during fetal development), since this sex atypical behavior emerges despite no encouragement from the social environment or parents. Parents and adults may react negatively to this gender nonconformity in their children, resulting in higher rates of maltreatment. Early hypotheses presumed that childhood maltreatment experienced by some non-heterosexuals was the cause of non-heterosexual orientations, a theory which has not been supported by more thorough examination. Fraternal birth order Since the 1990s, a large volume of research has demonstrated that each younger biological brother from the same mother has a 28–48% greater chance of being gay. This phenomenon is known as the fraternal birth order effect. The correlation is not found in those with older adoptive or step brothers, leading scientists to attribute this to a maternal immune response to developing male fetuses, rather than a social effect. It has been estimated that between 15% and 29% of gay men owe their orientation to this effect, although this may be higher when factoring in miscarriages of male fetuses (which are unable to be factored into calculations). In 2017, biochemical evidence for the effect was found which demonstrated that mothers of sons, particularly those with gay sons, had significantly higher levels of antibodies to male NLGN4Y proteins than mothers with no sons. Biologist Jacques Balthazart said that the finding "adds a significant chapter to growing evidence indicating that sexual orientation is heavily influenced by prenatal biological mechanisms rather than by unidentified factors in socialization". The effect is an example of a non-genetic influence on male sexual orientation occurring in the prenatal environment. The effect does not mean that all or most sons will be gay after several male pregnancies, but rather, the odds of having a gay son increase from approximately 2% for the first born son, to 3% for the second, 5% for the third (and becomes stronger with each successive male fetus). == Boys who were surgically reassigned and raised as females ==

Between the 1960s and 2000, many newborn and infant boys were surgically reassigned as females if they were born with malformed penises, or if they lost their penises in accidents. Many surgeons believed such males would be happier being socially and surgically reassigned female. In all seven published cases that have provided sexual orientation information, the subjects grew up to be strongly attracted to females. In Psychological Science in the Public Interest, six scientists including J. Michael Bailey state this establishes a strong case that male sexual orientation is partly established before birth: They further argue that this raises questions about the significance of the social environment on sexual orientation, stating, "If one cannot reliably make a male human become attracted to other males by cutting off his penis in infancy and rearing him as a girl, then what other psychosocial intervention could plausibly have that effect?" It is further stated that neither cloacal exstrophy (resulting in a malformed penis), nor surgical accidents, are associated with abnormalities of prenatal androgens, thus, the brains of these individuals were male-organized at birth. Six of the seven identified as heterosexual males at follow up, despite being surgically altered and reared as females, with researchers adding: "available evidence indicates that in such instances, parents are deeply committed to raising these children as girls and in as gender-typical a manner as possible." Bailey et al. describe the occurrence of these sex reassignments as "the near-perfect quasi-experiment" in measuring the impact of 'nature' versus 'nurture' with regards to male homosexuality. == Childhood sexual abuse, molestation or early experiences ==

The hypothesis that sexual abuse, molestation or early sexual experience, causes homosexuality has been a subject of speculation but does not have scientific support. while only a small number of males have homosexual orientations, at a similar level as found in Western cultures. Bailey states that Robert's instrumental regression and analysis were "almost certainly violated" by the confounding factor of genes shared between parents and children. In 2016, LeVay reported that another research group found evidence "supporting the original idea" that an increased rate of childhood sexual abuse among gay men is entirely due to being targeted for their childhood gender-nonconformist behaviour. This research found that gay, bisexual, and straight men who were gender nonconforming in childhood were equally as likely to report experiencing childhood sexual abuse, while gay, bi and straight men who were typically masculine in childhood were significantly less likely to report experiencing sexual abuse. Additionally, significant numbers of heterosexuals experience childhood sexual abuse, and yet grew up to be heterosexual. LeVay concludes that "the weight of the evidence does not support the notion that childhood abuse is a causal factor in the development of homosexuality". Additionally, studies relying on convenience samples can result in higher rates of abuse, which have limited validity describing rates of abuse in the wider population. Childhood sexual abuse, when defined as "sexual experiences with an adult or any other person younger than 18 years when the individual did not want the sexual experience or was too young to know what was happening", is combining a number of different experiences, likely to have different causes and effects. This may include sexual experiences of children too young to have understood what was happening, and the sexual experiences of late adolescents who understood those experiences but did not want them, as well as abusive experiences with the same sex and with the other sex. The American Psychiatric Association states: "...no specific psychosocial or family dynamic cause for homosexuality has been identified, including histories of childhood sexual abuse". Scientists in sexual orientation research favour biological theories, for which evidence has been building after a long-term failure to demonstrate the influence of the postnatal social environment on sexual orientation, and this is especially so for males. == Chemical disruptors ==

Environmental compounds such as plastic softeners (phthalate esters), which are pervasive environmental chemicals with anti-androgenic effects may interfere with sexual differentiation of the human brain during prenatal development. Researchers are investigating exposure to these endocrine disruptors during pregnancy and later sexual orientation of offspring, although scientists caution that no conclusions can be made yet. Historical records indicate homosexual people were present and recognized in many times, cultures and places prior to industrialisation. == Other ==

Data scientist Seth Stephens-Davidowitz reported that the actual prevalence of gay men does not appear to vary between states in the U.S. because the percentage of Internet porn searches that are for gay male porn are nearly the same in all states, about 5%. On this basis he believes migration of gay men to cities is overstated, and says that in states where there is a social stigma against homosexuality so that "many more gay men are in the closet than are out". == See also ==