[2] Scientists do not know the exact cause of sexual orientation, but they theorize that it is the result of a complex interplay of genetic, hormonal, and environmental influences.

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 fetus and fetal brain.

Lesbians on average were found to have significantly more masculine digit ratios, a finding which has been replicated numerous times in studies cross-culturally,[19] but sometimes contradicted,[20] such as when ethnicity is taken into account.

Dissection studies found that gay men had significantly smaller sized INAH-3 than heterosexual males, which is shifted in the female typical direction, a finding first demonstrated by neuroscientist Simon LeVay, which has been replicated.

[27]: 107–110  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.

This research found that straight men had right hemispheres 2% larger than the left, described as a modest but "highly significant difference" by LeVay.

[28] On grounds of evolutionary plausibility, Gavrilets, Friberg and Rice argue that all mechanisms for exclusive homosexual orientations likely trace back to their epigenetic model.

[35][36][37] Women with congenital adrenal hyperplasia (CAH), which effects production of sex steroids, report more male typical play behaviors and show less heterosexual interest.

[38] 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".

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.

"[42] 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.

Bem also acknowledged that gay men were more likely to have older brothers (the fraternal birth order effect), which appeared to contradict an unfamiliarity with males.

Bem cited cross-cultural studies which also "appear to contradict the EBE theory assertion", such as the Sambia tribe in Papua New Guinea, who segregate boys from females during adolescence and ritually enforce homosexual acts among teenagers (they believe this is important for male growth potential), yet once these boys reached adulthood, only a small proportion of men continued to engage in homosexual behaviour - similar to levels observed in the United States.

[43]: 164  Social psychologist Justin Lehmiller stated that Bem's theory has received praise "for the way it seamlessly links biological and environmental influences" and that there "is also some support for the model in the sense that childhood gender nonconformity is indeed one of the strongest predicators of adult homosexuality", but that the validity of the model "has been questioned on numerous grounds and scientists have largely rejected it.

"[44] 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.

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.

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".

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.

[6][43]: 20  Instead, research has demonstrated that non-heterosexuals, especially men, are more likely to be targeted for childhood sexual abuse due to their gender nonconforming behavior, which is visible from a young age and is a strong predictor of adult homosexuality.

Among the Sambia of New Guinea, beginning between age 7 and 10, all boys are required to engage in ritual sexual contacts with older male youths for several years before they have any access to females, yet the vast majority of these boys become heterosexual men,[43]: 20 [46] while only a small number of males have homosexual orientations, at a similar level as found in Western cultures.

These include personality traits such as one's level of agreeableness or propensity to risk taking, which have been found to be heightened in studies of lesbians; this may make them more susceptible to being abused.

[6] A 2016 review authored by six experts in the fields of genetics, psychology, biology, neuroscience and endocrinology concluded that they favored biological theories for explaining sexual orientation, and that, compared to males, "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".

According to neuroscientist Simon LeVay, a controversy arose because the conclusion relied on an unusual statistical technique and it is argued she did not apply this correctly.

[6]: 83 The American Psychiatric Association states: "...no specific psychosocial or family dynamic cause for homosexuality has been identified, including histories of childhood sexual abuse".

[56] 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.

[6] Between 1939 and 1960, around two million expectant mothers in the United States and Europe were prescribed a synthetic estrogen known as diethylstilbestrol (DES) in the belief that it would prevent miscarriages.