Prenatal hormones and sexual orientation

[1]: 25  Prenatal factors that affect or interfere with the interaction of these hormones on the developing brain can influence later sex-typed behavior in children.

[1]: 24  This hypothesis is originated from experimental studies in non-human mammals, yet the argument that similar effects can be seen in human neurobehavioral development is a much debated topic among scholars.

[2] It is impossible to completely rule out the social environment or the child's cognitive understanding of gender when discussing sex typed play in androgen-exposed girls.

[2] An endocrinology study by Garcia-Falgueras and Swaab postulated that "In humans, the main mechanism responsible of [sic] sexual identity and orientation involves a direct effect of testosterone on the developing brain.

[7] As of 2006 results from studies in humans had found conflicting evidence regarding the effect of prenatal exposure to hormones and psychosexual outcomes; Gooren noted in 2006 that studies in subprimate mammals are invalid measures of human sexual differentiation, as sex hormones follow a more "on-off" role in sex-typed behavior than is found in primates.

[8] Some studies do suggest that prenatal stress significantly increases the likelihood of homosexuality or bisexuality, although varying evidence exists for which trimester is most important.

[1]: 24  Studies of endocrinology have found implications for amphetamines and thyroid-gland hormones to increase homosexuality in female offspring as well, although it has not been examined in conjunction with prenatal stress levels.

[1]: 24 Some have postulated that postnatal (e.g., social and environmental factors) development can play a role in the sexual orientation of an individual, yet solid evidence of this has yet to be discovered.

Anti-male antibodies might bind to these molecules and thus interfere with their role in normal sexual differentiation, leading some later born males to being attracted to men as opposed to women.

"[9] Garcia-Falgueras and Swaab state that "The ... fraternal birth order effect ... is putatively explained by an immunological response by the mother to a product of the Y chromosome of her sons.

Some did not find any statistically significant difference in either the sibling composition or rate of older brothers of gay and straight men,[13][14] including large, nationally representative studies in the US and Denmark.

Other correlates to handedness (e.g., cerebral laterality, prenatal hormonal profiles, spatial ability) have been linked to sexual orientation, either empirically and/or theoretically.

[9] As with other purported marks indicating higher incidence of homosexuality, however, the link with handedness remains ambiguous and several studies have been unable to replicate it.

Rh is a factor in blood, and in cases where the mother is absent of this (Rh-) while carrying an Rh+ fetus, an immune response may develop with deleterious effects.

The report of 12 cases with same-sex attraction/gender dysphoria born to mothers with thyroid diseases was first presented in EPA Congress, Vienna (2015) and published as an article in the same year.

According to Turkish child & adolescent psychiatrist Osman Sabuncuoglu, who generated the theory, maternal thyroid dysfunction may lead to abnormal deviations from gender-specific development in the offspring.

Likewise, increased rates of autism spectrum disorder in children born to mothers with thyroid dysfunction and overrepresentation of ASD individuals in gender dysphoria populations suggest such an association.

[29][30] A commentary by Jeffrey Mullen, published shortly after the 2015 article, underlined the importance of Prenatal Thyroid Model and supported developments in this field.

[32] As a tertiary source, an authoritative book on the subject of interplay between endocrinology, brain and behavior has also cited the thyroid-homosexuality proposal article in the latest edition.

[36] Most importantly, a genome-wide genetic association study on male homosexuals identified a significant region on Chromosome 14 which is related to autoimmune thyroid dysfunction in human beings.

[49] Typically, this belief is traced to the early "inversion theory" of sex researchers who state that homosexuality is a result of biological abnormalities that "invert" sexual attraction and personality.

[49] Backing this up are reports that lesbians display more masculinized 2D;4D digit ratios than heterosexual women, based on data gathered from at least six different laboratories.

[49] Girls with congenital adrenal hyperplasia (an autosomal recessive condition which results in high androgen levels during fetal development) have more masculinized sex role identities and are more likely to have a homosexual sexual orientation as adults than controls.

However, the degree to which the girls' genitals are masculinized does not correlate with their sexual orientation, suggesting that prenatal hormones are a stronger causal factor, not parental influence.

[56] Prenatal androgen exposure has been associated with an increased chance of patient-initiated gender reassignment to male after being initially raised as female in early childhood or infancy.

"[59][better source needed] While studies have found that xenoestrogens and xenoandrogens can alter the brain's sexual differentiation in a number of species used as animal models,[60][better source needed] from the data in hand to date, it is "misleading ...to expect EDCs to produce profiles of effects, such as sexually dimorphic behaviors, as literal copies of those produced by native hormones.