Katana VentraIP

Biology and sexual orientation

The relationship between biology and sexual orientation is a subject of on-going research. While scientists do not know the exact cause of sexual orientation, they theorize that it is caused by a complex interplay of genetic, hormonal, and environmental influences.[1][2][3] However, evidence is weak for hypotheses that the post-natal social environment impacts sexual orientation, especially for males.[4]

Biological theories for explaining the causes of sexual orientation are favored by scientists.[1] These factors, which may be related to the development of a sexual orientation, include genes, the early uterine environment (such as prenatal hormones), and brain structure. While the evolutionary explanation for heterosexuality in organisms that reproduce sexually is straightforwardly understood to be a psychological adaptation resulting from greater reproductive success,[5] evolutionary explanations for homosexuality rely upon other mechanisms of evolution such as kin selection and inclusive fitness or antagonistic pleiotropy that favors heterozygotes producing homosexuality as a by-product.

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.

[127]

The of the hypothalamus was found by Swaab and Hopffman to be larger in gay men than in non-gay men;[128] the suprachiasmatic nucleus is also known to be larger in men than in women.[129][130]

suprachiasmatic nucleus

Gay men report, on average, slightly longer and thicker penises than non-gay men.

[131]

The average size of the 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.[48]

INAH 3

The was found to be larger in gay men than women and heterosexual men,[66] but a subsequent study found no such difference.[132]

anterior commissure

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 ).[133]

prenatal hormonal theory of sexual orientation

The (eyeblink following a loud sound) is similarly masculinized in lesbians and bisexual women.[134]

startle response

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).[135][136]

[63]

The , a region of the brain, is more active in gay men than non-gay men when exposed to sexually arousing material.[137]

amygdala

between the index and ring fingers have been reported to differ, on average, between non-gay and lesbian women.[138][139][140][141][142][143][144][145][146][147]

Finger length ratios

Gay men and lesbians are significantly than non-gay men and women;[148][149][150] Simon LeVay argues that because "[h]and preference is observable before birth...[151] [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.[48]

more likely to be left-handed or ambidextrous

Gay men have increased ridge density in the fingerprints on their left thumbs and .[152]

little fingers

Length of limbs and hands of gay men is smaller compared to height than the general population, but only among white men.

[152]

Some studies have found correlations between physiology of people and their sexuality; these studies provide evidence which suggests that:


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".[76]: 123