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Uber all member
Source - Evolutions Rainbow by JoanRoughgarden, 2004, University of California Press, pages 245-248
SEXUAL ORIENTATION
If outside behavior matches inside morphology, then gay and lesbian people may have unique bodies. If string players have special brain parts for left-handed fingering, and race jockeys special genes for a short physique, then perhaps people of same-sex sexuality have special brain parts and/or genes for sexuality too. The search for biological aspects of sexual orientation often confuses sexuality with transgender expression.
GAY BRAINS
Remember the three rice-grains of nerve cells in the preoptic/hypothalamus area at the base of the brain? These grains, called SDNPOA, BSTc, and VIP-SCN, are sexually dimorphic in humans. VIP-SCN size seems to align with sexual orientation in males. I bet you're guessing that gay males have a female-sized VIP-SCN. Nope. Gay males have an even bigger VIP-SCN than straight males, which is in turn bigger than the VIP-SCN of females. So much for the belief that gay men have female brains!(1) Specifically, straight males have about 2,500 cells, and females about 1,000 cells in this approximately 0.25 cubic millimeter cluster.(2) Gay males have a volume of VIP-SCN I.7 times as large, and with 2.1 times as many cells, as that of straight males.(3)
Another possible difference between gay and straight men comes from an unconfirmed study of a fourth and rarely mentioned rice-grain, the tiniest of all -a cluster of cells in the hypothalamus called INAH3. In heterosexual men, this grain averages 0.1 cubic millimeters; in heterosexual women, 0.05 cubic millimeters; and in gay men, also 0.05 cubic millimeters. This tiny feature in gay male brains has been singled out as matching that in women.(4) Thus gay males are closer to females in this rice-grain (INAH3), but farther from females in the other (VlP-SCN).
The brains of lesbian women appear to differ from those of straight women. Recall that men produce fewer clicking sounds in their internal ears than women do (see chapter 12). Lesbian and bisexual women produce fewer clicking sounds in their ears than straight women do, but more than men do.(5) Thus lesbian and bisexual women are intermediate between straight women and men in this regard. Indeed, ear clicking can change in an adult as a result of taking hormones. A transgendered woman who began taking estrogen prior to her sex reassignment surgery developed the ear clicking. So ear clicking does not necessarily say anything about how brain structure is organized.(6)
All in all variation in the many rice-grains of nerve cells shows that brains vary with sex, gender identity, and sexual orientation. Further analysis of brain states may reveal as many differences among people's brain as among people's faces.
GAY FAMILIES
Did Dad go fishing? Do you go fishing? Did Mom bake cookies? Do you bake cookies? Lots of traits run in families, like hobbies and styles of food preparation. Like hair color and eye color. Hobbies and cooking styles reflect shared environment; hair and eye color, shared genes. Being gay and lesbian runs in families too. Does shared sexual orientation in families reflect shared environment or shared genes, or both? The answer isn't clear. Here are some clues.
If a man is straight, there is a 4 percent his brother will be gay, the same percentage as in the general population. If a man is gay, the likelihood increases fivefold, to 22 percent. Whether a man is straight or gay has no statistical effect on whether his sister will be straight or lesbian.(7) These figures show that gay men cluster in families but do not say whether this stems from shared genes or a shared environment. Similarly, if a woman is lesbian, her sister is about twice as likely to be lesbian, but whether a woman is lesbian has a very small or undetectable statistical effect on whether her brother is gay or straight.(8) Gay men and lesbian women cluster independently.
Comparing identical and fraternal twins suggests some genetic component. In a 1991 study, 52 percent of identical male twins were both gay, while only 22 percent of fraternal twins were both gay.(9) In a 1993 study, 65 percent of identical male twins were both gay, and 29 percent of the fraternal twins were both gay.(10) Similarly, a 1993 study reported that 48 percent of identical female twins were both lesbian, and only 6 percent of fraternal twins were both lesbian.(11)
The studies just cited come from the United States. A 1992 British study, which looked at males and females together, found that 25 percent of identical twins were both homosexual, but only 2.5 percent of fraternal twins were homosexual.(12) A 1995 Australian study used a different method.(13) Instead of inviting twins to participate by placing advertisements in magazines and other sources likely to be seen by gay readers, the study used a preexisting list of twins. Based on a strict definition of whether twins could be scored as both being gay, the investigators reported that 20 percent of the identical male twins were both gay, 0[pah: zero] percent of the fraternal male twins were both gay, 24 percent of the identical female twins were both lesbian, and 11 percent of the fraternal female twins were both lesbian. .
The studies repeatedly show that identical twins are at least twice as likely both to be homosexual as fraternal twins. The chance that identical twins will both be gay ranges from about 25 percent to 50 percent, depending on the study, and is decidedly less than 100 percent. Thus, even though a genetic component may be present, other, presumably environmental factors account for 50 to 75 percent of the story.
Although comparisons between identical and fraternal twins suggest a genetic component in homosexuality, the possibility remains that identical twins are raised more similarly to one another than fraternal twins are, and that identical twins associate more closely and encounter more similar experiences while they are growing up than fraternal twins do. Further investigation of a genetic component should look at data from identical twins raised apart, because these dat will show the effects of shared genes in the absence of a common environment.
A 1986 study located six pairs of identical twins who were raised apart and had at least one member who was ay or lesbian. In all four female instances, one member was lesbian and the other straight. In one of the male instances, both members were gay-in fact, they didn't know of each other's existence until they happened to meet in a gay bar where people had been mistaking them for each other. In the other instance of identical male twins raised apart, one member was bisexual until age nineteen and then became exclusively gay, whereas the other was homosexual between ages fifteen and eighteen, then later married and regarded himself as straight. In this instance, both members exhibited at least partial same-sex sexual orientation. (14) Thus the data on sexual orientation in twins reared apart are perhaps suggestive of a possible genetic component for gay male sexuality, but much less so for lesbian sexuality .
Nonetheless, an important contrary fact remains. The 1991 study mentioned above also showed that an adopted brother of a gay man is twice as likely to be gay (11 percent) as an adopted brother of a straight man (5 percent). So unless the adoptive parents are somehow selecting babies likely to become gay, something about the environment into which the adopted child is placed is contributing to sexual orientation as much as any genes are.(15)
Substantial evidence points to both genetic and environmental components in the development of same-sex sexuality. No one who pushes one factor to the exclusion of the other can be correct.
