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'Have another drink. Enjoy yourself.' 'Which? The two are mutually exclusive.' |
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Wednesday, July 22, 2009
WIKI: 2D:4D digit ratio The best, non-invasive, marker of prenatal hormone exposure is the ratio of the second and fourth finger lengths (2D:4D ratio), a known sexually dimorphic measure (males showing lower ratios than females). Patients with androgen over-exposure (such as in congenital adrenal hyperplasia) show lower 2D:4D ratios, providing evidence linking prenatal androgen exposure as key to this feature. Additionally, the 2D:4D ratio has been shown to be affected by variation in the androgen receptor gene. The ratio of testosterone to estrogen in amniotic fluid has also been found to be negatively correlated with the 2D:4D ratio. Independent studies indicate that homosexual women have masculinized (lower) ratios and homosexual men show either hyper-masculinized or feminized ratios. These findings reinforce the prenatal androgen model - abnormal prenatal hormone exposure is related to the development of human homosexuality. _______________ Fraternal birth order effect Studies show that homosexual men have higher numbers of older male siblings. This finding led to the development of the fraternal birth order effect theory of homosexuality, stating that a mother becomes progressively immunized to successive male children, leading to increased chances of homosexuality in later male children. The mechanism involves the mother producing increasing anti-male antibodies to male-specific antigens expressed in male fetuses. These antibodies are thought to block the full masculinization of the fetal brain by “binding to and inactivating male-specific molecules located on the surface of fetal brain cells” thus preventing the morphogenesis of masculinized sexual preferences. Estimations indicate that there is a 33% increase in chances of homosexuality in a male child with each older male sibling. A BBC study reported that the fraternal birth order effect does not apply to left-handed homosexual males. Support for this theory is given by data indicating that the effect holds true only for biological brothers and the chances of male homosexuality is not increased by the number of older stepbrothers or adoptive siblings. This theory does not apply to the development of female homosexuality. _______________ Developmental instability and handedness The chances of being left-handed are increased in homosexual populations. Homosexual men have an approximate 34% odds ratio of being non-right-handed while lesbian women have 91% odds of being so, both compared to heterosexual populations. It has been proposed that non-right-handedness (including ambidexterity) is related to homosexuality through developmental instability. Developmental instability refers to the level of vulnerability to environmental and genetic stresses and perturbations during development. _______________ Response to visual sexual stimuli A recent functional magnetic resonance imaging study has demonstrated that upon viewing of both heterosexual and homosexual erotic visual stimuli, only those images corresponding to the subject’s sexual orientation produced hypothalamic activation patterns associated with sexual arousal. The response of heterosexuals viewing heterosexual adult videos showed the same pattern of sexual arousing neural processing as homosexuals viewing same-sex adult videos, while the viewing of the opposite orientation’s images did not elicit the same response. _______________ http://en.wikipedia.org/wiki/Neuroscience_and_sexual_orientation
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