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Despite widespread acceptance of the idea that those who are sexually abused as children are more likely to abuse others, this is a notion that has been challenged more and more through research. There is also convincing evidence that a) disconnect through empathy, not mediated through social differences, abd b) genuine attraction to children, which emerges from pedophiles no notion to outside influences. Confounding factors also skew statistical analysis. Reporting bias is one factor which also causes problems in both measurement of the rates of abuse and analysing this with instruments. It is well documented that heterosexual (straight) men are significantly less likely to say if they experienced childhood sexual abuse. This can also express that those who were sexually abused are more likely to identify openly about their sexuality than others, and thus create a false causality model.

Another hypothesis is that first experiences or openness to it. "Boys in late childhood and early adolescence are often capable of sexual feelings. Some of those with homosexual feelings are surely motivated to act on them. And some get the opportunity to do so. Most gay men had their first homosexual feelings long before they ever acted on them."

Some good evidence against sexual conditioning, imprinting or learning in this regard comes from tribes in New Guinea who engage in homosexual acts as a ritual. The Sambia tribe in New Guinea take boys from their mothers between ages 7 and 10, and segregate them in all-male houses in their village for the next 10 years. Here they learn the Sambian ideology which stresses the importance of semen, which is considered important for male growth and health. They believe males are born without semen, and they can only "get" it by sucking it from older males so they can produce it in future. Older males teach them how to do this, and it is considered to be a favour by donating semen to younger boys. The younger boys who fell ate the older boys don't usually say that they're sexually aroused. The younger boys suck semen from the older boys a few times a week for several years until they undergo puberty. Then, the mature boys will begin supervising the initiation ritual of a new crop of 7-10 year old boys, now as the older fellated partner. In their late teens or early twenties, these young men are ready to be married to women. The men stop having homosexual encounters with the younger boys and begin having vaginal intercourse with their wives. Most men make this transition. Only a small minority of men remain bachelors and seem to prefer the males. According to J. Michael Bailey, "the other Sambia ridicule these men and think they are odd. We might call them gay". In Hjernevask, Danish anthropologist Arve Sørum says that despite long-term engagement in oral fellatio, "they have the same sexual orientation as they would have had otherwise", and said a similar proportion of gay men exist in their society as in Denmark. Anthropological research published in 1981 indicates that an estimated 5% of males engage in long-term same sex behavior (including bisexual behavior), and that a smaller portion are exclusively homosexual.

In women, sexual abuse may mediate some desire to avoid relationships with men although inferring plausibility is hard to gather. First a 'corrective rape', in which a homosexual person, often a woman, is targeted for rape by a man in the effort to 'turn' her straight, or targeted for her perceived disinterest in men, although this can occur within the confounds of a relationship (i.e. a girl resists, is assaulted, and thus does not partake). In addition, a variety of traits, for example trait agreeableness which has been associated in lesbian girls may make them more manipulatable by male approachers. A 2016 review authored by six experts in the fields of sexology, psychology, biology, neuroscience and endocrinology concludes that "it would also be less surprising to us (and to others) to discover that social environment affects female sexual orientation and related behavior" but "that possibility must be scientifically supported rather than assumed".

Research has not reliably demonstrated that childhood sexual abuse is a causal factor of sexual orientation, instead, sexual minorities are more likely to be targets of abuse due to childhood gender nonconformity (femininity in gay men, masculinity in lesbian women). Additionally, research has shown that many homosexual teenagers may get into risky situations, due to limited options for teenage sexual experiences whereas their heterosexual peers have many, resulting in higher rates of sexual abuse (which is generally defined as any unwanted sexual experience before age 18). In a meta-analysis of 17 different population samples, 21% of gay men reported experiencing childhood sexual abuse, in contrast, a British charity reports that 1 in 6 men in the general population (about 17%) experience childhood sexual abuse. However, this childhood sexual abuse also includes teenage experiences and such experiences are vastly different.

Overview
Scientific research does not support the hypothesis that childhood sexual abuse, molestation or early sexual experiences "cause" homosexuality, although some experts think it may have some effect in females. Instead, research has demonstrated that gay men are more likely to be targeted in childhood due to their visible gender nonconformity (femininity), they are more likely to engage in risky age discrepant relationships in their teenage years due to hiding their orientation, and the vast majority of homosexual males report experiencing this attraction before any sexual contact. The single study (Roberts et al. 2013) has been strongly criticised on methodological grounds. Proponents of the idea that early sexual experiences can cause homosexuality have failed to take into account tribes in New Guinea which enforce homosexual sex acts on male children and adolescents (as a ritual, they believe semen helps male growth), yet only a very small proportion of men in their society are homosexual at similar levels as is found at a low consistent rate across other countries. Longitudinal studies of students who attended single-sex boarding schools where homosexual behaviour and sexual abuse is common are no more likely to be homosexual than men and women who did not attend such schools. Additionally, the brain evolved to be resilient to such effects, and become attracted to women under masculinization effects in utero rather than postnatal learning theories. The hypothesis for women is that sexual abuse at the hands of a male would make them averse to relationships with men, which researchers have not ruled out but which requires scientific evidence rather than assumption. However, this explanation would seemingly be contradictory for males. Scientists in sexual orientation research favour biological theories for which evidence has slowly been building since the 1990's after a long term failure to demonstrate the influence of the postnatal social environment on male sexual orientation, although sexual abuse may have some effect on female sexuality who have less specific, and more fluid arousal and attraction patterns.

Research
One of the first landmark studies Bell are still important in this area today. This involved some 500 questions and a path analysis, finding that gender nonconformity accounted for almost all of the maltreatment. In this area, a more narrow depiction of childhood sexual abuse which involved comparing an adult showed no significant link, and researchers have credited the book with seemingly disproving the hypothesis that sexual abuse causes male homosexuality.

Definitions and age discrepancy
In recent research, the definition of childhood sexual abuse has shifted to include any unwanted prior to age 18, or anything age discrepant, even if same-sex orientation is already known, which some have used to assume to be a 'cause' of a homosexual orientation. To demonstrate this problem, J. Michael Bailey gives an example of two male teenagers, one heterosexual and one homosexual: The heterosexual male is openly heterosexual and interacts with many female adolescents who may be sexually attracted to him. However, women who are five years older than a heterosexual male tend not to be interested in 16 year old partners, even in states where this is legal. Thus, he will likely have sex with adolescent females around his own age. In contrast, the homosexual teenager has much lower odds of knowing many other gay males especially because most of them are likely to be hiding their sexual orientation. Hence, they are far more likely to have sexual contact with somebody outside of their social circle. Through mediums where he can meet gay men, the men will likely be older than him. Assuming he is sexually mature, his body may be attractive to some homosexual men in the same manner that 16 year old girls in the final stages of puberty are attractive to heterosexual men. Whether or not such a male is above the age of consent, there are some men who will take such a risk, and the likelihood of this occurring is far higher than the likelihood of an older woman having sex with a younger heterosexual male. Given simultaneous interest in sexual activity, it is to be expected that some gay males will have sex earlier than straight males, because their desired partners (males) have higher sex drives and a greater desire for casual sex than females. Bailey concludes that "the hypothesis of homosexual recruitment is not needed to explain the fact that homosexual males tend to have earlier, and more age-discrepant, sexual interactions compared with heterosexual males". "I have never understood how this is plausible. A boy with no interest in homosexual activity wouldn't find it pleasant even if he agreed to it. How is this supposed to turn him gay?"

Gender nonconforming children are targeted for abuse
The childhood gender nonconformity of homosexuals has been linked with them being more likely to be targeted, or singled out for abuse, especially in males. Boys who veer from typical male behavior are more likely to be targeted for abuse, including parental maltreatment, physical assault and so forth, they are more likely to be for sexual abuse by adults who dislike gender nonconformity, or see them as particularly vulnerable and manipulatable, or "read" their future orientation. Some research has indicated that heterosexual, gay and bisexual men are more likely to experience childhood sexual abuse than gay, bisexual and straight men who were typically masculine in childhood.

Cross cultural evidence
Homosexuality exists at a low rate across populations, yet when observing cultures in which homosexual ritual behavior is enforced on young boys in the Sambia tribes of New Guinea. These boys are segregated from women for 10 years between ages 7 and 10, and are made to perform oral fellatio on older males to receive semen for "growth potential". In Hjernevask, Danish anthropologist Arve Sørum says that despite long-term engagement in oral fellatio, "they have the same sexual orientation as they would have had otherwise", and said a similar proportion of gay men exist in their society as in Denmark.

Studies and criticisms
One of the first landmark studies Bell are still important in this area today. This involved some 500 questions and a path analysis, finding that gender nonconformity accounted for almost all of the maltreatment. In this area, a more narrow depiction of childhood sexual abuse which involved comparing an adult showed no significant link, and researchers have credited the book with seemingly disproving the hypothesis that sexual abuse causes male homosexuality.Not only do Roberts et al.’s results fail to provide support for the idea that childhood maltreatment causes adult homosexuality, the pattern of differences between males and females is opposite what should be expected based on better evidence This area is ultimately poor methods to control for, and has resulted in controversy and criticism. A 2013 study titled "Does Childhood Maltreatment Effect Sexual Orientation?" by Roberts et al (2013) caused controversy when it only accounted for gender nonconformity and failed to account for all other confounders. According to Simon LeVay, a controversy arose over whether or not she had applied these statistical instruments correctly. These results have been strongly criticised for making unjustified assumptions in their instrumental regression, a criticism by J. Michael Bailey and Drew H. Bailey says “Not only do Roberts et al.’s results fail to provide support for the idea that childhood maltreatment causes adult homosexuality, the pattern of differences between males and females is opposite what should be expected based on better evidence.” There is general agreement that women have a component of sexual fluidity, and the likelihood that sexual abuse would somehow effect male sexuality and not female sexuality is unlikely, instead reflects a lack of controlling for other unidentified variables, such as teenage risk tasking, genetic factors (which predispose people to risk taking), reporting bias and statistical models. Additionally, Roberts et al. relied on instruments typically only reserved for economics modelling, and thus are a wider part o of causality that cannot be established. Bailey concludes that this conclusion poorly fits experience of gay men and concludes that there is "compelling evidence that male sexual orientation is fixed early in development, probably before birth and certainly before childhood adversity could plausibly affect it" and that "previous research is inconsistent with the hypothesis that childhood experiences play a significant causal role in adult sexual orientation, especially in men". Thus, it could be that sexual abuse results in women to veer from it, but this theory seemingly makes little sense in men.

In 2016 neuroscientist Simon LeVay points to another research group which 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. LeVay concludes that "the evidence does not support the notion that childhood abuse is a causal factor in the development of homosexuality", and is an especially dubious link since most homosexuals were not sexually abused, and one would also have to explain why even in countries with widespread sexaul abuse, they do not grow up to be homosexual.

In a 30-year longitudinal study published in the Archives of Sexual Behavior, authors 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 said that "sexual abuse may result in uncertainty regarding sexual orientation and greater experimentation with both same- and opposite-sex relationships", but not affect ultimate sexual orientation. They conclude that "these findings do not suggest that same-sex sexual orientation is caused by child abuse".

Fringe studies
A 2001 paper titled "Comparative Data of Childhood and Adolescence Molestation in Heterosexual and Homosexual Persons" by Marie Tomeo and Donald Templer, a prominent white supremacist, has been criticized for methodology, data errors and misrepresentation. The study did not include "molestation" in the form and then changed it in publication. Thus, most of the effect measured were age discrepant teenage relationships. Additionally, the paper contains several contradictions in the data meaning it's reporting is questionable, it did not include bisexuals which has been indicated that the authors sought out particular people at the event (rather than selecting people at random) and it used a completely different sample of heterosexuals (rather than people from the event).

Cross cultural research
Additionally, the hypothesis also lead to the creation of the "recruitment hypothesis" that people who are abused are more likely to abuse others, and go on to create a "cycle of abuse". Early psychologists typically based this assumption on the reports of some child abusers who reported experiencing childhood sexual abuse. However, abuse is common in the general population and research has not empirically found that those are more likely to abuse others. Recent research has suggested that abusers may have shared genetic effects, thus those who were abused by those in their family also share genes with these family members which contribute to the desire to abuse others, which created the image of a 'cycle of abuse'.

Other confounding and distortionary factors include: heterosexuals underreporting abuse, which is a particularly common issue among heterosexual men; non-heterosexuals may be more likely to be honest about having experienced abuse in coming to terms with their same-sex attraction, common psychological differences among them may make them more likely to experience, and genetic influences also contribute to risk taking behavior. Many of these confounders will be difficult to control for and thus causality is hard to establish. Bailey instead believes the 'better evidence', comparing research from other areas, showing that male sexuality doesn't seem to respond to the social environment while it may sometimes for women.

Another part of the discrepancy is that LGB adults are more likely than heterosexual adults to report abuse, although this would be a small factor.

Female sexuality
Lesbians are much more likely to experience childhood sexual abuse. Research has been inconsistent in it's findings about age discrepant relationships in women. First a 'corrective rape', in which a homosexual person, is targeted for rape in the effort to 'turn' her straight, or targeted for her perceived disinterest in men, although this can occur within the confounds of a relationship. In addition, a variety of traits, for example trait agreeableness which has been associated in lesbian girls may make them more manipulatable by male approachers. Lisa M. Diamond has suggested that this research has been inconclusive, although she suggests that focus should not matter how a woman comes to be in a same sex relationship. A 2016 review authored by six experts in the fields of sexology, psychology, biology, neuroscience and endocrinology concludes that "it would also be less surprising to us (and to others) to discover that social environment affects female sexual orientation and related behavior" but "that possibility must be scientifically supported rather than assumed".

Causes
Scientists favor biological models for the cause of sexual orientation. Contrary to popular belief, environmental factors but a large volume of biological processes such as maternal immune responses (shown through the fraternal birth order on male sexual orientation), hormonal responses, foods, chemicals and how these interplay with underlying biology. They generally believe that it is determined by a complex interplay of biological and environmental factors, and is shaped at an early age. There is considerably more evidence supporting nonsocial, biological causes of sexual orientation than social ones, especially for males.

In addition, experiments which saw boys surgically reassigned into girls, raised as girls, indicates that males do not have a learning concept for sexual orientation. The gender nonconformity typically emerges extremely early despite common discouragement of the behavior, indicating some sex differentiation. Some scientists have suggested that homosexuals may have a "mosaic" brain behavior, which also leads them to make sex atypical decisions in later life. J. Michael Bailey has suggested that all male sexual orientation is more or less inborn.

Generally the most common theory is that fetuses begin in female form, and are masculinized around 12 weeks leading most brains to be pushed in the heterosexual direction, and the female brain left unmasculinized. A variety of theories relate to this, genes which control and moderate it, and hormones interacting, maternal immune responses which interfere with other sex differentiation in men (which has been well established and biochemical evidence was found in 2017), and a variety of other factors There is no substantive evidence which suggests parenting or early childhood experiences play a role with regard to sexual orientation. Scientists do not believe that sexual orientation is a choice.

The American Academy of Pediatrics stated in Pediatrics in 2004:

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.

The American Psychological Association, American Psychiatric Association, and National Association of Social Workers stated in 2006:

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.

No psychological trait is determined by a single gene, instead, numerous genes together play a small role. There is substantial evidence for a genetic influence for homosexuality, especially in males. Based on twin studies; some association with regions of Chromosome 8, the Xq28 locus on the X chromosome, and other sites across many chromosomes.

Starting in the 2010s, potential epigenetic factors have become a topic of increased attention in genetic research on sexual orientation. A study presented at the ASHG 2015 Annual Meeting found that the methylation pattern in nine regions of the genome appeared very closely linked to sexual orientation, with a resulting algorithm using the methylation pattern to predict the sexual orientation of a control group with almost 70% accuracy.

Research into the causes of homosexuality plays a role in political and social debates and also raises concerns about genetic profiling and prenatal testing.

The idea that gay men are more likely to be pedophiles has been described as a harmful myth. There is no scientific evidence to support this idea. To test this idea, psychiatrist Kurt Freund carried out studies in which adult straight and gay men were exposed to sexually suggestive photos of males and females at various ages (children, teenagers and adults). The men were linked to penile strain gauges to measure their sexual arousal. Results The idea that gay men are promiscuous has been described as a stereotype. First, research indicates gay and straight men have equally high sex drives. On average, gay men are reported as having more sexual partners, however average differences are skewed by a minority of gay men having a large number of sexual partners. Use of the median figure, which is not subject to distortion is more useful. Additionally,

Summary
Prominent scientists, and evidence for biological non-genetic influences, such as maternal immune responses which interfere with brain masculinization. Two general views of sexual orientation generally exist, one which believes that it is. Writing in the Journal of epidemiologist, researchers conclude that "we believe there is substantially more evidence for the former than the latter".

A variety of biological markers point to prenatal influences on brain arrangement, including masculinized finger ratios in lesbians, hyper-masculinized brain regions in gay men. In addition, animal models of sheep who express lifelong homosexuality, have the same oDSN brain arrangement as homosexual men in humans.

Neuroscientific findings which point that brain arrangement. In addition, the experiments which surgically reassigned as boys, reared them as girls, prescribed estrogen pills, grew up to be attracted to women, is indicative of the lack of parental effect on male sexual orientation. In addition, a variety of research indicates a cross-sex interest in typical behavior, which come from within the individual, rather than encouragement or discouragement, have led some scientists to suggest a mosaic of female-male typical brains within non-heterosexuals.

A variety of models, including prenatal hormone theory (in which excess testosterone is converted to estrogen), maternal immune response hypothesis (in which maternal immune responses interfere with brain masculinization), epigenetic model (in which epigenetic leakage preventing intersex conditions in offspring, occasionally creating homosexual offspring). Most prominent sex researchers do not think rearing has any evidence, and have suggested that 100 years have not produced much convincing evidence that upbringing effects sexual orientation.

Most research into sexual orientation occurs at a slow pace since funding for such research faces criticism from both social conservatives and those opposed to preventing homosexuality. In addition, concerns have been raised about the misuse of research in countries where homosexuality is legally punishable with prison and death. A variety of treatments including electroshock therapy, deep brain surgeries, forcible lobotomies in which parts of the brains have been scraped off, and the removal of factors have not eliminated homosexual orientation. The available tests of claims of sexual orientation change, have not been supported by penile pletholographs, which found they were still attracted to men. In addition. J. Michael Bailey had offered to do more research of men claiming to have changed, however received no responses. Despite scientific evidence, the claim that it can be changed is commonplace and persists in many countries today.

Not just in neural structure, but also in how the emotion is processed. In addition, the cross-sex interest in career is indicative of a biological, not social difference in homosexuals. Most people agree that things come innately.

The claim that non-heterosexuals orientations are the result of rearing is generally not supported, since it typically diverging from most people. Often gender nonconformity emerges early despite protestation from the parents, who typically may even feel discomfort about it. J. Michael Bailey calls this evidence of it, and has even suggested that many gay men deny being feminine in childhood due to their femiphobia and attraction to masculine men.

Scientific research into the etiology of sexual orientation has been slow mostly due to the difficulty for researchers to get funding, in the U.S. researchers have often faced criticism from social conservatives who believe it to be a waste of taxpayers money. Others raise concerns that biological evidence could be used to prevent non-heteroseuxality in future. Recent biochemical evidence to support the mechanism for a maternal immune response which causes male homosexuality has raised concerns that drugs could soon be developed to prevent male homosexuality in offspring, with Marc Breedlove saying this may move us towards a world "in which we all look more alike".

Summary
The evidence for biological implications on homosexuality is stronger for men than women, indicating a prenatal and unalterable orientation. However, in women this may be for evolutionary reasons which is less object specific and fixed.

In the absence of testosterone, this generally is, however this may be different mechanism for men and women. It is generally agreed that male sexuality persists at a low rate irrelevant to cultural variables, however in women, the expression of homosexual behaviour appears to vary on cultural levels.

A variety of studies have compared these differneces, including a childhood gender nonconformity resistent to outside influence, the failure to sex reassign infant boys and rear them as girls (this did not induce gender nonconformity or make them same sex attracted), maternal immune responses to developing male foetuses which may interfere with brain masculinisation, and clear evidence of some role for hormonal and genetic traits. Some evolutionary postits back to epigenetic unerased epi-marks, although this requires extensive dissections and research is hard to achieve. There is no evidence for sexual learning, using evidence from cross-cultural patterns which would prove a useless evolutionary strategy. Most gay men have zero response, resistant to change, and they show zero arousal pattern response to female in lab tests. A variety of experiments showing interesting cross-sex brain structures, although this is difficult to interpret given the current understanding of biological mechanisms. There is no evidence for sexual "learning" or abuse causes, since evolution would otherwise select against such a strategy. There is no evidence for social environment or parenting on men, since millions of years of evolution would have insulated against such effects. Many researchers believe there is enough evidence to conclude that exclusive male homosexual orientation is very likely inborn and resistant to change, although the exact biological prenatal mechanism for each gay man may be different, but may lead to one "common pathway" in brain arrangement underlying homosexual orientation.

In women, it is generally agreed that many evolved to have bisexual attraction. This is due to one egg per month, similar to bonobo sexuality. However, there are lesbians who report, and evidence supports masculinisation of the fetal carrier. This may too link back to the epigenetic inheritance of unerased male epimarks, although it is not yet clear. A variety of other outside influences, estrogenics, could also affect how masculinised the female fetus is to this, and result in exclusive homosexual orientation. Suggesting for this are masculinised digit ratios in masculine lesbians, and inner ear structures which resemble that of heterosexual males (measured through auto-acoustic clicks) which is a variable highly set from birth.

Hormones and fetal brain organisation
Boys surgically reassigned as girls in infancy and reared as girls including use of hormones did not support the "blank slate" theory of human development, that we are "socialised" into sexuality or that we are born without them. Instead, animal models show that under the influence of testosterone in fetal development pushes male sexual behaviour, and in females, this exhibits female typical behaviro. Hormones are the largest predictor of gender nonconformity, and this is also related in the section below. The importance of prenatal brain differentiation on sexual attraction and masculinity in men is underscored by experiments in which infant boys were sex reassigned into girls (their genitalia was removed), socialised as girls, but still had typical male behaviour (despite parents trying to encourage female behavior, and all cases grew up to be strongly attracted to females.

Animal models were also applied. In rams, exclusive homsoexual orientations are found in 5-8% of rams, measured over periods of years who show no desire to mount female rams. Charles Roselli then dissected brains and found that they had the same feminized sexually dimorphic nucleus of the brain, which ergulates sexual behavior, and thus provides evidence for a similar biological mechanism between rams and gay men.

Evidence supporting this idea are from animal models in which altering the amount of hormones males were exposed to changes them to female typical behavior. Additionally, this area where fetal brain development has been confirmed in sheep, where some 6-8% of rams are exclusively homosexual. Recent understandings have moved mostly to 'receptivity' to hormones, that may alter how much regions of brains absorb through the androgen receptor. Thus, the incomplete masculinisation of the fetal brain, and in lesbians more masculinisation of the fetal brain. During this process are non hormonal effects, such as Y-antigens which masculine male brains and have been implicated in male homosexuality.

Early hormones acting out on the fetal development, and have been highly linked in women. This corresponds with their gender nonconformist behavior, and are generally agreed through the Y-chromosome to masculinize. A variety of debate, although this is unknown, but clues suggest to some hormonal influence although this is not clear cut nad has been offered. In sheep, injecting masculine hormones can set up a sheep to have 'masculinized' behavior rather than masculinized genialia. Thus, hormones in the second trimester are thought to be omre imporatnt in this regard. These do not induce homosexuality in these animals, however in rams there are 8% who are exlcusively homosexual (covered under brain studies).

Childhood gender nonconformity
Beginning in the 1960's, research focused on predictors of adult homosexuality. This trait is childhood gender nonconformity, and is a result of female behavior in boys, and male typical behavior in girls. In men, this is strongly associated with an adult homosexual orientation, although this is not the rule. Some of Green's research indicated that the average gay man was more feminine than 90% of heteroseuxal men. However, the evidence first focused on the idea that it was encouraged by the family, but this has not been supproted in long term evidence. First, the gender nonconformity emerges early, and despite protestation from the social environment, including discomfort. On these grounds of "gender learning", other doctors advised the sex reassignment of boys into girls, which did not result in these boys becoming female typical behavior (they were typically masculine) and they grew up to be strongly attracted to women, which was contradictory of the expected result. Large experiments ran at UCLA in the 1900's also did not support this finding, and Green came to favour biological explanations for male homosexuality. J. Michael Bailey has suggested that gay men often deny the fact that they were gender nonconforming in childhood because they were shamed for it growing up, and because they are often not attracted to this femininity themselves. He suggests that acknowledgement of this is to realize that gender nonconformity is the largest predictor of an innate trait. In women, the gender nonconformity is present too, although this is of a predictor of adult homosexuality. Most masculine girls grow up to be heterosexual women. It is generally agreed that the mechanisms for homosexuality in women may be quite different for those relating to men. Not all gay men or women were gender nonconforming in childhood, and objection to this idea has been raised by social constructionists and feminist scholars who react it as stereotyping, or that gays are "overreporting" their childhood gender nonconformity. However, experts including J. Michael Bailey consider this unlikely, and contend it's likely the opposite. Bailey contends that gay men underreport their gender nonconformity in an effort to distance themselves from this image because they dislike the stereotype, were bullied for their gender nonconformity, and often find it unattractive in other males. The general finding has been described as robust and valid. This gender nonconformity is important in recent twin studies. A variety of experiments relating to hormonal exposure, although it is unclear of their relevance for humans. It is generally agreed that given our long gestation, hormone levels tend to adjust, thus it is likely some rare experiment, but that the receptivity or 'uptake' of testosterone and sex hormones may influence brain masculinisation or findings. Some neuroscience findings support this idea (discussed later). The idea that gender nonconformity is a predictor of male homosexuality has been criticised by others who call it forecasting orientation in children, that it assumes gendered stereotypes, although scientists have countered that monkey's also express sex differences in behavior, and thus the idea is good evidence for a biological influence on brain arrangement.

Overall, the results indicated 89 percent of the gay men in the studies had been more feminine as boys than the typical heterosexual men had been. Similarly, 81 percent of the lesbians had been more masculine as girls than the typical heterosexual women had been

Gay men have reported more cross-sex interest in female typical behavior, while females show more masculinzied play forms on average. There are exceptions, but Richard Green said that even masculine gay men often recall above average female interest than their heterosexual male peers did. In addition this also supported by a variety of biological constraints. J. Michael Bailey, a prominent researcher in the sexual orientation etiology, has said that this is a clear indicator of innateness, which emerges despite it. Bailey has criticized other biological markers, since they would appear to support this hypothesis, including later behavior. Even in environments that are encouraging, gay men show more average interest in female-typical professions than straight men do, while lesbians are typically overrepresented in areas of masculinity. One such comment includes the observation. Many have reacted negatively this as a "sterotype", typically from sociology areas. However, Bailey says that many gay men may even deny their femininity for two reasons, the first being that they were bullied for it in early childhood and thus feel ashamed of it, and the second, that they are often more attracted to masculinity, so to concede that they themselves are feminine would mean conceding that something is unattractive about themselves.

A large volume of research, beginning in the 1900's started this. Figures included Berger, who challenged this view since it emerged early, and later Bell & Weinberg (1981) which reported that "homosexuals show an early predisposition". More recent research has shown this is somewhat heritable, indicating a similar genetic influence. In twin studies, the identical twin who was gay shows an early feminine proclivity noticeable to the parents (discussed below), and is indicative of the idea of homosexual orientation.

Twin studies (genes vs. environment)
Twin studies have been used by scientists to determine the importance of genes and the environmental factors which influence traits. Large twin studies unrelated to sexual orientation have shown that identical twins reared apart from birth were just as similar as those reared together, indicating that the shared environment of parents, culture and rearing have little correlation with all psychological traits. In the study of sexual orientation, early twin studies relied on convenience samples and indicated a strong genetic effect, which was about 52% concordance, or how often one twin shared the same sexual orientation. Later, more accurate research relied on representative samples which have found about 24% concordance for sexual orientation. This indicates a moderate genetic effect, although many thousands of genes together. Why two twins have differing orientations is a key part in determining what differences occurred in utero, in addition, twins had the same upbringing and thus the parenting effect has no correlation with this. Additionally, the twin method allows researchers to calculate out which environments play a role through mathematical probabilities.

In men, the correlation with the shared environment is 0. This means that all things that make siblings similar such as parenting, rearing, culture and media do not influence male sexual behavior, and consequently, male sexual orientation. These are also cross cultural, since twins in Sweden show no correlation, indicating that a more accepting environment does not cause homosexual orientation in men or influence them to engage in same-sex behaviour. In women, there is some evidence of the shared environment playing a role. It is unclear which environments play a role, but scientists have generally found that female sexual behaviour is more dependent on cultural variables, for example, women may be more likely to engage in bisexual relationships under certain cultural environments, although this is not clear evidence that the shared environment could cause an exclusive homosexual orientation in women, however.

Twin studies allow scientists to determine how much a role genetics play in a trait, and using a statistical model, they can find out which environmental factors influence a trait. Beginning in the 1990's much interest was found when J. Michael Bailey began twin studies, showing a high level of concordance which also prompted the idea of there being a "single gay gene". The idea of single genes for traits began to diminish when scientists found that things were highly genetic, but that they are influenced by thousands of genes together, along with random developmental processes in brain formation. A 2016 meta analysis by J. Michael Bailey reports that the concordance among identical twins is 24%, and which is roughly 10 times higher than the general population rate, and a 15% rate of concordance for fraternal twins (who are as similar genetically as regular siblings). Both of these findings are consistent with a small genetic influence, and calculate out to genes explaining 33% of the variance in men, and 18% of the variance in women. The concept of 'variance' is commonly misunderstood by the general public, thus does not mean that 33% of gays owe their sexual orientation to genes, but rather, 33% of the differences between people can be prescribed to genetics.

Interestingly when they are discordant for orientation, the gay one was noticeably more feminine (or masculine, in the case of females). Additionally, when twins were concordance for orientation, the gender nonconformity was not particular differential. This means when both twins were gay, they were either both gender nonconforming or both gender conforming. Bailey gives the example of two gay identical twins who were both sports players and insisted they had always been masculine. Bailey says this suggests the non shared influences on sexual orientation begin early, either prenatally in the womb, or within the first two years of infancy (and not within childhood).

What environmental influences can attribute it, shared environment and individual environment. Contrary to popular understanding, this is not the "nurture" that was once thought to be playing a role. This also relates to psychological traits on a whole. Large scale twin studies of twins reared apart showed they were just as similar as those raised together. This has lead to the "nurture question", that these may be random developmental processes, dependent on unknown factors, and some role for environments created by the child themselves rather than something that happens to us by parental, media or societal influence. These findings have been replicated cross-culturally and have disputed the scientific belief in "nurture" as the deciding factor. In women, however there is a small contribution for the shared environment on same sex behaviour. This suggests first that cultural values may influence whether women act on bisexual urges, or that parental effects could also mediate psychosocial effects on them. Interestingly, Swedish men also show a 0 correlation, indicating that a socially liberal culture does not influence homosexuality for men. This is generally in line with other findings suggesting a strong biological yet unidentified effects in the womb relating to men.

Combining a variety of twin studies, however this is still at 10 times the rate of exclusive homosexual orientations, which is significant. Contrary to common belief, no trait is determined by a single gene (except a few rare mutations), thus in genome wide association studies there is no single gay gene, however there are many genes together which increase the probability of homosexual orientations. This is thought to be established very early, before childhood and potentially before birth.

Available research has also found that the homosexual twin is more gender nonconforming on average, than their heterosexual twin, which is attributable to a biological cause. The epigenetic theory suggests that perhaps one twin is left with an androgen receptor, resulting in less testosterone masculinzing the brain in some areas, leaving them on the 'default' neural circuitry which would be oriented towards men. Supporting this, are neural studies (discussed below), showing gay men have female typical neural arrangements, a finding which has also been replicated in rams in the equivalent region of the brain who are exclusively homosexual through their life.

The apparent lack of social environment on male sexual orientation is in line with research findings suggesting some prenatal event. For example, if the social environment were important one would likely expect higher rates of concordance for sexual orientation.

Given the differences in orientation, this idea is being explored. It is found that 30% of identical twins have a separate placenta, and some may have a seperate amniotic sac. Some research has also focused on hormone exposure of the 2D:4D digit ratio, a marker of how much testosterone a developing fetus was exposed to (it is present in fetal tissue, and is not dependent on later hormones in teenage years). Other scientists have proposed epigentic models which are sometimes "unerased" but are prevented through genes to prevent intersex condiitons in the vast majority of the population, but which are not erased and sometimes lead to homosexual orientation in men, or maternal immune responses, which have been linked with male homosexuality, and potentially female homosexuality, may effect one fetus and not the other.

Genome wide associated studies
After twin studies in the 1990's showed a moderate genetic influence, this lead to the search for a "gay gene" or "gay genes". There have been numerous implicated in a variety of studies, however no single determinant, however this was to be expected since evolution would have selected against it. Instead, there are thousands of genes together, a finding which has become increasingly noticeable in the 2010's with other GWAS studies showing no single genes for traits, despite being strongly genetic, and are instead informed by thousands of genes. This does not mean it is not under genetic influence, for example, there is no height gene, however it indicates that many thousands of unknown genes, each with a tiny effect size, together may influence the probability of someone developing a heterosexual, bisexual or homosexual orientation.

This does not mean that only 25% of gay men owe their orientation to genes, but rather, the variance between all populations can be attributed to genes. For example, intelligence is 50–70% genetic, but the environmental factor shows no relation with the shared environment of parenting, schools, learning and otherwise. The "nurture" assumption has been challenged more and more in recent years, leading to a rethink in much of psychology about what nurture actually consists of.

Birth order effect and maternal immune responses
Maternal immune responses during pregnancy have been strongly associated with male homosexuality, and are an example of a biological (but non-genetic) environmental influence on sexual orientation. Beginning in the 1990's, it was shown that each additional older brother a man has from the same mother, the likelihood of homosexual orientation increased by 33%. This was only shown for those with older biological brothers, not older adoptive or step brothers, indicating it was not a social effect. Blanchard and Bogaert theorised this may be the mothers immune system remembering male cells, which are foreign to the female immune system, and responding with an immune response to neutralise them. Thus, as a mother has more sons, the immune response becomes stronger and increases the likelihood of same-sex attraction in later born sons. After twenty years of suggestive studies, the first lab study found biochemical evidence to support this hypothesis in 2017. This has been linked to the NGYL4 protein, which is thought to play a role in brain masculinization. Some critics have asked why it would not occur in testes, however animal models show that the testes form in the first trimester, but sexual differentiation of the brain occurs at a later stage in fetal development so such an immune response may only alter sexual attraction.

Evidence suggests that because upwards of 50 percent of all pregnancies end in miscarriage, this may too leave male fetal tisuse behind. synapse change in the brain. Blanchard has argued that fetal attacks may also explain first born gay sons, who have a lighter birthweight, but that is may not be the same immune response being implicated. Given why this might survive evolution are a) preventing too much masculinization of the brain which could cause autism, and b) women with male cells in their system may have additional health benefits. In a 2018 paper, Blanchard concludes that "It is apparent that we have come a long way from the tabula rasa theorizing of the last century, but we still have a long way to go. The current consensus of active researchers may be summarized as Born This Way—But How?". Neuroscientist Marc Breedlove raised the ethical dilemma that this may lead to creation of drugs which could shield the mothers immune system from the antigen in an attempt to prevent homosexuality in men, while Blanchard has said such a possibility is true, it is a long way off and research in this area helps raise acceptance for gay people.

Brain studies
A variety of brain studies, first gay men had feminized INAH-3 found by Simon LeVay which was replicated by William "Bill" Byne and later by Dick Swaab in male to female transsexuals. In addition, a variety of neuroscience using FMRi scans. Bailey has said that it is unlikely, since these correspond with feminized behaviour which shows itself early in life. These areas are called sexually dimorphic regions of the brain, meaning they show clear average differences between men and women. The interesting finding suggests that homosexuals may have more, clearly demonstarted in men than in women.

Ivanka Savic at the Karolinska Institute, but not just in attraction, but also in emotion processing. These variables are generally thought to be under the organizational influence of sexual orientation. Neuroscienist Dick Swaab showed gay men had feminized INAH-2 regions, and later Simon LeVay controleld in the INAH-3. This was a blind study, and used dead AIDS patients, although the difference was still not seen in heteroseuxal men who died of AIDS so the likelihood that is would have been AIDS is implausible given the sex differences. This findings by LeVay was later replicated by William Byne, who found they had INAH-3 in the similar fashion to women but described it as a trend. LeVay has said that the two-tailed statistical effect lessoned this and it was uneccessary. Dick Swaab later replicated this finding in male-to-female transsexual women.

Since the 1990's, professor Charles Roselli has studied long term homosexuality in rams. This is studied by performing studies over several years, involving the tying up of female ewes or male rams, and seeing their selection of partners. In addition, ram industry had become interested in why several were not reproducing. Rams do not have theories like absent fathers or sexual abuse (theories which are no-longer supported in humans), yet eight percent show exclusive homosexual orientation. Rams sharing the same sexual orientation as men, and the same neural correlate. This may tie in with androgen receptivity.

Behavioral traits
Under the gender nonconformity, however these are generally thought to be suggestive of some traits whihc are evidential of biological, nto social differrences. For exmaple, gay men are underrepresented in fields such as engineering, and overrepresented. While these may erst on 'stereotypes', they do indicate some creative differences in brain structure. Lippa has suggested this is evidence of difference, not socialisation. Others have suggested gay men are sometimes hyper-masculine on other traits, however this can be explained purely through access to sexual partners by male-male sex drive. In another case, lesbians may be highly overrepresented in military careers while gay men are comparatively rare. Simon LeVay has suggested this results in a range of 'gendered differences', not just sexual attraction. Of course, not all gay men are feminine but Bailey generally agrees that stereotypes are more useful for biological innateness than they are of offence.

Random developmental chance
Others have suggested that given the lack of genetic variance, but a lack of shared environment on male sexual behaivor, that homosexual males could be the result of 'chance' brain formation in thew womb that pushes random variable chance. This idea has become more common for all psychological traits given that twin studies show zero influence of the shared environment. This is, environmental mutations that are not part of the normal range either. Given that regular family influences do not correlate with any trait. These results have been similar around the world. This has moved away from the idea of "nature versus nurture" because nurture is not the nurture we thought it was.

Men vs. women
It is generally thought that male and female homosexuality come about through different biological mechanisms. In women, this is highly linked with testosterone exposure, although this shows fluidity component not seen in men, and thus may be under the influence of social infleunce. It is disputed as to whether or not this same fluidity actually exists in men. for example, men who come out of the closet typically say they "knew they were gay the whole time", however in women there are cases of women who find they are attracted to women and engage in a same sex relationship. In addition, this research is complicated by the finding that many gay men may falsely come out as "bisexual" at first because they feel it grants them more social acceptance and later state that they are really exclusively gay and had no attraction to women. This distorts research in two ways: it creates the illusion of fluidity in these men, or "orientation change", and it also furthers the widespread assumption that male bisexuals are simply gay men who are "lying" about their bisexual orientation. There has been more research showing male bisexual arousal to both men and women does indeed occur. In general, there is no evidence that societal acceptance encourages men. Thus, evolutionary specific patterns that females respond to social environment, whereas men respond to their underlying attractions (for men, their arousal is their sexual orientation).

Discredited or unsupported theories
The idea that this stemmed from early "psychologists" from Freud onwards. These have largely been discredited, since no variable effect. Sexual abuse is another common, yet the rate before 16. Inflated rates of early childhood sexual abuse since the definition in much research includes teenage experiences which were age discrepent or illegal, or forceful (even after sexual orientaiton was known) and thus is not causality. In addition, there is no reason for male sexuality to be malleable in this regard since it would have been selected against through millions of years of evolution.

Biological differences in gay men and lesbian women
Some studies have found correlations between physiology of people and their sexuality; these studies provide evidence which suggests that:


 * Gay men and straight women have, on average, equally proportioned brain hemispheres. Lesbian women and straight men have, on average, slightly larger right brain hemispheres.
 * The suprachiasmatic nucleus of the hypothalamus was found by Swaab and Hopffman to be larger in gay men than in non-gay men, the suprachiasmatic nucleus is also known to be larger in men than in women.
 * Gay men report, on average, slightly longer and thicker penises than non-gay men.
 * The average size of the INAH 3 in the brains of gay men is approximately the same size as INAH 3 in women, which is significantly smaller, and the cells more densely packed, than in heterosexual men's brains.
 * The anterior commissure is larger in women than men and was reported to be larger in gay men than in non-gay men, but a subsequent study found no such difference.
 * The functioning of the inner ear and the central auditory system in lesbians and bisexual women are more like the functional properties found in men than in non-gay women (the researchers argued this finding was consistent with the prenatal hormonal theory of sexual orientation).
 * The startle response (eyeblink following a loud sound) is similarly masculinized in lesbians and bisexual women.
 * Gay and non-gay people's brains respond differently to two putative sex pheromones (AND, found in male armpit secretions, and EST, found in female urine).
 * The amygdala, a region of the brain, is more active in gay men than non-gay men when exposed to sexually arousing material.
 * Finger length ratios between the index and ring fingers have been reported to differ, on average, between non-gay and lesbian women.
 * Gay men and lesbians are significantly more likely to be left-handed or ambidextrous than non-gay men and women;  Simon LeVay argues that because "[h]and preference is observable before birth... [t]he observation of increased non-right-handness in gay people is therefore consistent with the idea that sexual orientation is influenced by prenatal processes," perhaps heredity.
 * A study of over 50 gay men found that about 23% had counterclockwise hair whorl, as opposed to 8% in the general population. This may correlate with left-handedness.
 * Gay men have increased ridge density in the fingerprints on their left thumbs and little fingers.
 * Length of limbs and hands of gay men is smaller compared to height than the general population, but only among white men.

J. Michael Bailey has argued that the early childhood gender nonconforming behavior of homosexuals, as opposed to biological markers, are better evidence of homosexuality being an inborn trait. He argues that gay men are "punished much more than rewarded" for their childhood gender nonconformity, and that such behavior "emerges with no encouragement, and despite opposition", making it "the sine qua non of innateness".

Evolution
In women, this area may be considered a variable difference. Women have one egg per month, and thus, to avoid risk, could have evolved to first a) have sexual arousla with one another so they can seek pleasure in a way that does not encourage pregnancy, this is also seen in bonobo monkeys. Second, it could have been an evolutionary force that encourages some attraction of heterosexual males and thus would have done so. In men, this area will have evolved to eb targeted specific for reproduction. Thus, this is generally good evidence that male homosexuality is not the result of the social environment, but is a byproduct of some biological phenomena or whether it is selected for by evolutionary processes. Some indications suggest (see epigenetic theory).

Early theories focused on these, however this has been ruled out for mathematical implausibility. Later, the sexually antagonistic idea, gay men do indeed appear to have more sibships, although it is unclera how correct this is. Instead, this may have evolved for social reasons rather than biological implausibility.

One hypothesis it that homosexuality could be the result of a common pathogen in the environment, which leads to brain alteration in a few percent of the population. This would not be spread by gay men themselves, but could be the result of a common pathogen which causes a fetal response in a few percent of the population. In this regard, viruses can evolve faster than humans can, and may result in some areas. There is no evidence for the pathogenic hypothesis however.

Harvard's Peter Wringham proposed that male homosexuality may be the result of selection for reduced aggresivity, and thus genetic profiles are removing aggressino as an evolutionary strategy. He argues that evidence for this comes from us as an interlude between bonobos and chimps. First, bonobo males are more likely to engage in homosexual sex, and thus, may have evolved. In chimps, in contrast, males are aggressive and women routinely return from mating with scars and bruises and bleeding. Thus, Wringham argues, homosexuality in men could be the "overshoots" and produces a feminine phenotype that evolution is still adjusting too.

J. Michael Bailey maintains that male homosexuality is most likely inborn or innate, but has described it as "evolutionary maladaptive" and need of explanation.

Misuse and misrepresentation of research
A common misrepresentations of research, including in the media. Typically this involves misrepresenting, or "click bait' rather than actual representatino of science. This involves the plausibility of a "gay gene', yet it was known by the 1990's that there was no single gay gene. This led to the widespread headline of "no gay gene", yet the actual study examined same-sex behaviour, thus the likelihood of expressing ones same sex attraction, rather than social environment "causing" same sex attraction.

With the discovery that NGLY-4 is likely influenced in male homosexuality (se maternal immune responses), although it is unclear how much, it is possible to develop drugs to shield the infant fetus from surrounding maternal immune responses. In addition, Charles Roselli has suggested this difference.

The baptists said "if a maternal response is identified, then we will identify and use it. We see it as no different to eliminating blindness or deafness in a person, and thus it should be used". In addition, early areas in this field focused on "selective abortions". A DailyMail headline in 1993 suggests that homosexuality could be eliminated through genetic selection. In addition, through the epigenetic canalization you may be able to detect canalisation of the fetus and eliminate it. William R. Rice said that "in this regard, the cat's already out of the bag".

Stigmatization
In this area, although available evidence suggests that people are more comfortable with gay people if it focuses on biology rather than a "choice".

Notable researchers
Notable researchers in exploratory biological influences on sexual orientation (notably, non-heterosexuality)


 * J. Michael Bailey (psychologist) – early gay twin studies, gender nonconformity, which he says emerges from nature, not nurture.
 * Marc Breedlove (neuroscientist) – linked homosexuality with
 * Richard Green (sexologist) – studied gender nonconformity in boys, most of his control group grew up to be gay men. Removing toys did not adjust this outcome.
 * Charles Roselli (biologist) – studied gay sheep, found similar structure in the oSDN equivalent as was found in homosexual human males.
 * Ray Blanchard (sexologist) – notable for the fraternal birth order effect, described as the most important discovery in biology of sexual orientation.
 * Anthony Bogaert – notable for fraternal birth order effect, maternal immune response, asexuality
 * Simon LeVay (neuroscientist) – INAH-3 and authored books on the neurobiology of homosexuality.
 * William Byne (neuroscientist) – replication of the INAH-3 work by Simon LeVay.
 * Dick Swaab (neurobiologist) – found MtF transseuxals have an INAH-3 correlate.
 * Ivanka Savic (neuroscientist) – found gay men have feminized emotion processing, and studies linking gay men to cerebral midline structures.
 * Dennis McFadden (auditory scientist) – showed lesbians had hyper masculinised inner ear structures, and AEP's, two variables which are fixed from birth.
 * Qazi Rahman – twin studies, neuroscience, demonstrated the lack of shared environment on male sexual orientation.
 * Melissa Hines (neuroscientist) – research of neurobiology, suggests that prenatal influences relate to feminine and masculine behavior.
 * Eric Vilain (scientist) – expert on intersex conditions, epigenetics research.
 * Melissa Hines (neuroscientist) – influence of sex hormones on fetal development.
 * William R. Rice, Galverits and X (evolutionary biologists) – proposed the model for homosexuality through epigenetic canalization which protects against intersex conditions.

Twin studies
Twin studies compare the relative importance of genetics and environment in the determination of sexual orientation. Starting in the 1990's, a variety of studies have explored this relationship, most notably beginning with J. Michael Bailey, finding about 50%, however more recent meta analytic reviews of about 10 studies indicate that a contribution of .24 or about a 33% contribution of genes on sexual orientation. A variety of other physiological traits have similar heritability. For example, a 5% contribution is considered a significant genetic contribution. The gender nonconformity, which expresses itself in a very clear way, before socialization and despite the social environment not encouraging it, has suggested that some even occurred on one event. Ray Blanchard has reviewed literature of one, for example, separated through different amniotic sacs, could be explained in a variety of manners, and how these environmental, non-genetic influences effect sexual orientation is not clear. It is also unclear if maternal immune responses to male fetuses, which have been found in scientific labs, could apply to identical twins.

The lower concordance between twins has led some biologists to propose an epigenetic model, which protects the fetus against intersex conditions, making it maintained at a similarly high rate. A variety of scientists explain that other fitness reducing traits are persistent at small but high portions, which would generally be selected against through evolution, but which persist none the less because they provide a protection against intersex conditions.

Additionally, if parental factors were to play a role in sexual orientation, it would be expected that concordance for sexual orientation would be much higher since twins receive the same rearing and upbringing environment. However, sexual orientation is similar in concordance rates to left handedness, and a variety of prenatal conditions.

Definition of environmental factors
When biologists and geneticists talk about environmental influences on sexual orientation, they are talking about any variable that is not defined by genes. Despite common belief, environment does not simply refer to family upbringing, or what many people refer to as "nurture". The environment is a vast and unexplored complex process, including prenatal environment (the womb, gestation and fetal development) which involves a variety of factors. These processes are also dependent on numerous environmental impacts on a pregnant mother. Environment also refers to a huge complexity of postnatal influence, including proximity, chemicals and other variables listed below. Most traits have around a 50 percent heritability component. In addition, a trait can be innate without being genetic, for example, handedness has the same concordance as sexual orientation, but has a weak genetic link. Results from decades long twin and adoption studies have found that twins separated at birth are remarkably similar, and that adopted children do not correlate (a correlation of 0.0) with their adoptive parents, and instead correlate with their biological parents, suggesting against the influence of parenting on all psychological traits.

While many have reverted to genetic determinism, this is a misunderstanding of genetics. Most traits are not informed by one gene, but rather, thousands of genes acting together in complex manner that scientists are working to understand. What is known, however, is how much environments effect it. Most research has shown that you can determine which environments are playing that role. Most traits are "polygenic", meaning they are informed by thousands of genes together. For example, height is 90% hereditable yet genes for height are lacking.

Many scientists say that the concept of "nature versus nurture" is misleading since results from large twin studies have shown that the shared environment, consisting of parenting and culture, has very little influence on most psychological traits in general. The environment includes a large and vastly unexplored landscape.

In genetic research, the environment generally consists of any factor acting on a person that is not set by DNA, but may still be biological in nature. Scientists caution that many people misconstrue the meanings of genetic and environmental. Environmental influence does not automatically imply that the social environment influences or contributes to the development of sexual orientation. Hypotheses for the impact of the post-natal social environment on sexual orientation are weak, especially for males. There is, however, a vast non-social environment that is non-genetic yet still biological, such as prenatal development (in the womb), where most of the available evidence points.

Included in the individual environment are non-social factors occurring in prenatal development (e.g. fluctuations in hormone levels in the womb, as well as hormone uptake), food, proximity to water, whether parents smoked or not, and a variety of other variables. The shared environment includes variables which are common between twins, this could include rearing styles, parenting, societal acceptance.

Examples of environmental factors:• Fetal environment (including hormone exposure and timing, maternal immune responses, chance events, etc)

• Chemicals

• Diet, food, ingredients

• Exposure to pathogens in development (viruses, bacteria, germs which influence neural development)

• Random developmental noise

• Peers

• Parenting

• Culture

• Where you grew up

• Proximity to waterScientists have urged caution against interpreting "environmental" factors on sexual orientation to mean social environment. Dean Hamer has said that "having the measles as a child" is an environmental impact, and J. Michael Bailey believes that almost all of it is established before birth, but due to a large unexplored territory of variables. Genetic determinism is not a correlated finding, because many genes – a smaller influence, but that another impact, probably hormonal, immunological impacts, and a variety of differences. A variety of experiments point towards a prenatal and nonsocial influence on men. According to sexologist Ray Blanchard, "scientists have had 100 years to provide any reliable evidence that upbringing effects sexual orientation, and the evidence they have provided is not convincing". Many of the variables above could also interplay with a pregnancy, for example, consumption of food during pregnancy could be linked with anything. Exposure to measles during early development, although not conclusive, is an example of any neural development. Twin studies of environment on sexual orientation indicate that the shared environment and fetal development shows no influence on male sexual orientation, and a minor influence on female behavior. This may indicate that women are more likely to experiment with same-sex behavior, and indicates that family and culture does not influence same sex behavior in men.

Boys who were sex-reassigned and raised as girls
Between the 1960‘s 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 and believed they could grow up to become heterosexual women. All available evidence indicates that the parents were dedicated to raising their sons as girls. In all seven published cases that have provided sexual orientation information, the subjects grew up to be attracted to females. Six researchers including J. Michael Bailey state this establishes a strong case that male sexual orientation is established due to factors before birth:

""This is the result we would expect if male sexual orientation were entirely due to nature, and it is opposite of the result expected if it were due to nurture, in which case we would expect that none of these individuals would be predominantly attracted to women. They show how difficult it is to derail the development of male sexual orientation by psychosocial means.""

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 had reverted to living as males at follow up, despite being surgically altered and reared as females.

The most famous and first documented case of this type of sex-reassignment was that of of David Reimer, who lost his penis in infancy after a botched circumcision. John Money – a prominent psychologist at Johns Hopkins Hospital – advised David's parents to castrate him and raise him as a girl. Money believed that gender and sexual orientation were learned, and informed the Reimer's that David would grow up to be a happy heterosexual woman who may one day marry a man. In the years following, Diamond found many other cases of boys who were sex-reassigned in infancy – all were attracted to women and had reverted to identifying as heterosexual men. These experiments have provided sexologists evidence that male sexual orientation and gender nonconformity are not learned patterns of behavior, but have a basis in uterine development. J. Michael Bailey and other researchers refer to these sex-reassignments as the near-perfect quasi-experiment in demonstrating the power of 'nature' over 'nurture' with regards to male sexual orientation.

Experiments
In addition to sex-reassignment experiments, similar intervention experiments were run to steer prepubescent boys away from gender nonconforming behavior and towards masculine behavior in order to "prevent" a homosexual outcome. The most prominent of these was run by Richard Green and George Rekers at UCLA, beginning in the 1970's. The most prominent of such cases was typified by the 'sissy boy experiment'. Murphy was given a chip for female behavior, and given a blue chip when he engaged in male typical behavior. He was beaten if he engaged in female typical behavior. In 2011, the case came to prominence when it was reported that Rekers had taken a vacation with a male prostitute, and that he had continued including Murphy's case as a "success" in one of his books as late as 2009, six years after Murphy's suicide. Green came to believe that sexual orientation has a biological underpinning.

During WWII, some German doctors believed that homosexuals were suffering from a low level of circulating testosterone, and reverted to injections, which only increased their sex drives. The available evidence suggseting a prenatal dependent, which is core for life long brain organisation.

Cultural
Research has found that peer influence has no effect on the tendency towards heterosexual or homosexual attraction in males, and sexual orientation is not transmitted or "contagious" through social networks. There is however evidence to say that female same-sex behaviour may be more fluid depending on the social and cultural context.

Significant research in Samoa has shown that gay men, which in Samoa are generally categorized as a Fa'afafine (a samoan third gender) are a core part of their family. The evidence available on "the prevalence of non-heterosexuality across cultures and time suggests that homosexual orientation does not increase in frequency with social tolerance, although its expression (in behavior and in open identification) may do so."

Due to social acceptance in larger cities in the United States, a higher proportion of people self-report a homosexual orientation than in rural areas. In addition to feeling more comfortable to identify with their attraction, others may move to cities from rural areas for this acceptance and additional peer networks between non-heterosexuals. Data scientist Seth Stephens-Davidowitz believes that the actual prevalence of gay men doesn't 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 people towards cities may be overstated, and says that in areas where there is a social stigma against homosexuality that "many more gay men are in the closet than are out".

Prenatal environment (gestation)
The prenatal environment provides a vast area for differences to occur, and is typically favored by most scientists including a variety of theories invoking prenatal hormones and fluctuations, immunological responses from the mother, intrusions of other environmental pathogens and so forth.

Maternal immune responses
The best demonstrated prenatal environmental occurrence is best described by the fraternal birth order effect, showing that men with more older brothers were more likely to be homosexual (or bisexual). This effect was only correlated with those with older biological brothers, not those with adoptive or step brothers supporting a nonsocial influence during gestation. Additionally, the effect is nullified in men with left handedness, which is also influenced by prenatal, non-genetic effects.

In 2017, biochemical evidence for Ray Blanchard and Anthony Boegarts hypothesis was found in the laboratory, and further studies are needed to confirm it's effect. Their finding suggests that women with no sons also had levels of antibodies, meaning that a woman may become exposed to male linked proteins during any intercourse with men, and thus could invoke a response on a first born gay son. Many scientists have considered in the most well-established variable on sexual orientation. Since a substantial portion of all pregnancies are miscarried, without the mothers knowledge, this could explain a high portion of male homosexuality.

Hormones
Dennis McFadden has demonstrated that lesbians have masculinized inner ears, which resemble those of men. Gay men had hyper masculinized responses, indicating a correct exposure to hormone exposure, but McFadden theorizes that uptake or response to the androgen/testosterone may be at play. Additionally, he found that gay men processed sound waves across the brain in a female-typical manner, also set in birth rather than early environmental inputs. There is no evidence that gay men received too little hormones, and in adult gay men, testosterone levels are the same as heterosexual men. A variety of inputs, but this has little influence, since the importance of organizational during gestation seems to be important. Simon LeVay has suggested that subtypes of gay men may exist, ones who owe their sexual orientation to the fraternal birth order effect, hormone fluctuations, some genetic contribution or other levels of impact during fetal gestation. Additionally, a variety of lesbian women – floating on masculinized finger ratios, which are biological markers for a prenatal event: i.e. if a woman has a very masculinized finger ratio, it is reasonable to assume that she either received more average testosterone in utero, or, her androgen receptors took up more testosterone on average.

Relation to brain differences in humans and rams
In 1991, neuroscientist Simon LeVay found that gay men had INAH-3 which resembled that of women, a finding later replicated by William Byne, albeit with a smaller sample that he called a trend. Neuroscientist Dick Swaab has also demonstrated that these differences occur in male-to-female transsexuals, indicating that this variable may also be effective in gender identity. Neuroscientist Ivanka Savic has shown that gay men had female-typical amygdala's, a brain region constant across life, rather than something acquired through time. In the late 1990's, sheep farms had trouble – since a minority of male rams refused to mate with females. These express long term homosexual orientation, and show no interest in female rams.

Men versus women
Most scientists think the typology between men and women may be formed through different causes. J. Michael Bailey typically says that it is not processed in the same way, "for men, his arousal is his sexual orientation". A variety of researchers believe that lesbians were exposed to more testosterone in utero, pushing some regions of the fetal brain in a masculinized direction.

Animal experiments
Alongside humans, a variety of animals models provide some inducement. Charles Roselli has increased testosterone, which grew female male genitalia and sex differentiation, but when he induced testosterone in the second trimester, the ewes had regular female genitalia but had masculinized behavior. Roselli says this indicates the likely influence is in the second stage of fetal development, rather than a generalized differences.

Summary
There is no substantial evidence which suggests parenting, upbringing or early childhood environment and experiences influence male sexual orientation, and a very minor influence on women. The belief that homosexuality resulted from negative parent-child relationships became a prominent theory in the 1900's, when theorists came to blame a dysfunctional relationship between children and their parents for children's homosexuality, such as emotionally distant fathers and overbearing mothers. These hypotheses were based upon highly speculative theory rather than from systematic scientific studies. Many of the men and women in therapists offices were those who were unhappy about their sexual orientation, and thus were disproportionately reporting parental factors. In the 1980's, larger studies (notably Bell et al. 1981) also seemingly disproved the theory that homosexuality resulted from the quality of parent-child relationships. Instead, they found that negative parent-child relationships mainly occurred due to a negative reaction in response to childhood gender nonconformity – for example, fathers may react negatively to feminine behavior in their sons, a common trait which emerges very early in gay men. Bell found that homosexual men were happily adjusted and most came from good backgrounds. Additionally, the strained parent hypothesis has not held-up cross culturally, since homosexual men in other cultures such as Samoa and Scandinavian countries reported warmer fathers, indicating these cultures react more affectionately to feminine sons, yet homosexuality occurs at a similar rate as is found in Western countries studied. Research has found that gender nonconforming children are targeted for various types of abuse both within the family and from those outside, meaning negative childhood treatment can be wrongly attributed as the "cause" of a non-heterosexual sexual orientation, rather than something that occurs in response to childhood gender nonconformity that typically comes with it. Researchers have studied the relationship between absent fathers and male homosexuality through large longitudinal studies and databases, and found no correlation. Social theories were increasingly challenged by biologists, who found it implausible that humans would have evolved to require correct socialisation to be attracted to the opposite sex. Instead, a prenatal biological mechanisms in a very small proportion of humans could result in homosexual orientations.

Early theories
Psychiatrist Bernard Zuger was one of the first scientists to reject the nurturist view, by examining the the parental environment, found that such dynamics were not the cause of homosexuality, but an effect. Long-term interviews with some fifty-five children (some of whom Zuger would follow for thirty years) showed that in virtually every case the boys demonstrated very early feminine play preferences, interests, and behavior. The father’s efforts to bond over masculine interests were rebuffed by the child, and the father—rejected—would emotionally withdraw; thus creating the observed pattern of a distant father and overbearing mother. Zuger however, suspected a biological basis for homosexuality that contradicted the universally accepted nurture view. J. Michael Bailey has described the freudian view as "morally bankrupt" and false.

The 'nurture' view was dominant in the 1900's, which lead to a variety of experiments in which infant boys were sex-reassigned at birth or in infancy (genitalia removed), reared as girls and given hormones, which did not succeed in making them attracted to men. Despite these castrations, this did not make them feminine in behavior either, indicating that the gender nonconformity of gay men is not the result of rearing, but of a early brain arrangement in the womb. Bailey has said that "compelling evidence that male sexual orientation is fixed early in development, probably before birth and certainly before childhood adversity could plausibly affect it". This view is supported by many other prominent sexologists. Sex researcher Debra W. Soh says that people wrongly prescribe upbringing, when the data indicates that parents actually respond in reaction to the nature of their children rather than children becoming traumatized as a result of parenting. In the 2000's, large twin studies were able to provide insight into which environments were influencing same-sex behavior. They have demonstrated that the shared environment, which includes rearing styles, parenting and societal attitudes play no role in male same sex behavior, and a very minor role in female same sex behavior. This is inline with the finding that male sexual orientation is thought to be much more directional, whereas female sexuality is thought to show more fluidity, although many lesbians reflect no interest in men.

Scientists also say that societal acceptance of homosexuality or 'learned' behavior based on up bringing plays no role. In cultures where boys are encouraged to fellatio on older men, but this does not play a role in adult sexual orientation. In British boarding schools, boys may engage in homosexual behavior but once they leave school and go out into the general public, the same proportion of men are gay as is found in the general population and have no interest in men. Similarly, while heterosexual men in prison may engage in homosexual behavior, when they leave prison, their attractions are towards women. In other words, homosexuality is not learned, but some heterosexual men may engage in situational homosexuality due to limited access to females. Many researchers say that male sexuality is more 'fixed' than females due to human evolution

Evolutionary psychologists generally believe the differences in female and male sexual orientation are a result of our evolution, where female sexuality may have evolved to be more fluid since it helps in pair bonding. Some believe that female bisexuality is also an indicator of extreme heterosexuality, since heterosexual men are attracted to female same sex attraction.

The misuse of scientific research to make arguments is a key concern for scientists. For example, a 2006 danish study reports that they are "significantly" more likely to have absent fathers, however this only correlates with a 6% absent father rate among gay men compared with a 5% absent father rate for straight men. The meaning of significant in scientific terms has little applicability to drawing conclusions in general public.

Youth bullying
Suicide rates are highest among LGBT youth, and this is largely explained for social environmental reasons.

Discrimination
Bailey writes "Gay men who were gender nonconforming boys and who came out early are more likely to say that they contemplated or attempted suicide than masculine gay men who came out later. We don't know why, but it seems likely to have something to do with the stigmatization of gender nonconformity."

Biological factors
J. Michael Bailey, a professor of psychology best known for his research on sexual orientation, has suggested that some biological factors among gay people make them more prone to suicide. These are some genetic contributors, and brain differences. First, because females are more likely to have sexual orientation issues, thus neuroticism is "feminized" in gay men. In this sense, gay men may have elevated rates of suicide due to combining female typical traits with male typical propensity to commit suicide. These factors however, interplay with the social factors above. Bailey says that regardless of this, it is not unique since many characteristics are associated with an increased risk for certain problems, writing: ""Men commit much more violent crime than women, but women are more depressed than men. Young people are more likely to be delinquent, old people to be demented. Samoan adolescents commit suicide at much higher rates than Americans do. But we don't conclude from these facts that being male, female, young, old, or Samoan is a mental illness. Rather, the problems are being violent, depressed, delinquent, demented, or suicidal. Homosexuality, per se, is not a problem."