Some studies have found correlations between physiology of people and their sexuality. These studies provide evidence which they claim suggests that:
Gay men report, on an average, slightly longer and thicker penises than non-gay men.[50]
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.[51]
The VIP SCN nucleus of the hypothalamus is larger in men than in women, and larger in gay men than in heterosexual men.[52]
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.[31]
The anterior commissure is larger in women than men and was reported to be larger in gay men than in non-gay men,[30] but a subsequent study found no such difference.[53]
Gay men's brains respond differently to fluoxetine, a selective serotonin reuptake inhibitor.[54]
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).[55]
The suprachiasmatic nucleus was found by Swaab and Hopffman to be larger in gay men than in non-gay men,[56] the suprachiasmatic nucleus is also known to be larger in men than in women.[57]
The startle response (eyeblink following a loud sound) is similarly masculinized in lesbians and bisexual women.[58]
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).[27][59][60]
The amygdala, a region of the brain, is more active in gay men than non-gay men when exposed to sexually arousing material.[61]
Finger length ratios between the index and ring fingers may be different between non-gay and lesbian women.[55][62][63][64][65][66]
Gay men and lesbians are significantly more likely to be left-handed or ambidextrous than non-gay men and women;[67][68][69] Simon LeVay argues that because "[h]and preference is observable before birth[70]... [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.[31]
A study of 50 gay men found 23% had counterclockwise hair whorl, as opposed to 8% in the general population. This may correlate with left-handedness.[71]
Gay men have increased ridge density in the fingerprints on their left thumbs and pinkies.[71]
Length of limbs and hands of gay men is smaller compared to height than the general population, but only among white men.[71]
A 2010 study stated: "The fetal brain develops during the intrauterine period in the male direction through a direct action of testosterone on the developing nerve cells, or in the female direction through the absence of this hormone surge. In this way, our gender identity (the conviction of belonging to the male or female gender) and sexual orientation are programmed or organized into our brain structures when we are still in the womb. There is no indication that social environment after birth has an effect on gender identity or sexual orientation."[33]
The fraternal birth order effect is the strongest known biodemographic predictor of sexual orientation.[3] According to several studies, each older brother increases a man's odds of having a homosexual orientation by 28â48%.[4][5][6][7][8] The fraternal birth order effect accounts for approximately one seventh of the prevalence of homosexuality in men.[9] There seems to be no effect on sexual orientation in women, and no effect related to the number of older sisters.[10][11]
The fraternal birth order effect has also been observed among male-to-female transsexuals: MtF transsexuals who are sexually interested in men have a greater number of older brothers than MtF transsexuals who are sexually interested in women. This has been reported in samples from Canada,[12] the United Kingdom,[13] the Netherlands,[14] and Polynesia.[15]
The effect has been found even in males not raised with their biological brothers, suggesting an in-utero environmental causation.[3] To explain this finding, a maternal immune response has been hypothesized.[16] Male fetuses produce H-Y antigens which may be involved in the sexual differentiation of vertebrates.[16] Other studies have suggested the influence of birth order was not due to a biological, but a social process.[17]