In other human research, two other hypothalamic nuclei interstitial nuclei of the anterior hypothalamus [INAH] 2 and 3 and part of the bed nucleus of the stria terminalis BST have been reported to be sexually dimorphic in the human. Sexual differentiation to the human brain takes place much later than originally claimed. The cell number rapidly increases in boys and girls at the same rate until 2 to 4 years of age. After that age period, a decrease in cell number takes place in girls, but not in boys. This causes the sexual differentiation of the SDN. He obtained brains from 41 deceased hospital patients.
The subjects were classified into three groups. The first group comprised 19 gay men who had died of AIDS -related illnesses. The second group comprised 16 men whose sexual orientation was unknown, but whom the researchers presumed to be heterosexual.
Biology and sexual orientation
Six of these men had died of AIDS-related illnesses. The third group was of six women whom the researchers presumed to be heterosexual. One of the women had died of an AIDS-related illness. The HIV-positive people in the presumably heterosexual patient groups were all identified from medical records as either intravenous drug abusers or recipients of blood transfusions. Two of the men who identified as heterosexual specifically denied ever engaging in a homosexual sex act.
The records of the remaining heterosexual subjects contained no information about their sexual orientation; they were assumed to have been primarily or exclusively heterosexual "on the basis of the numerical preponderance of heterosexual men in the population".
However, the INAH3 group appeared to be twice as big in the heterosexual male group as in the gay male group; the difference was highly significant, and remained significant when only the six AIDS patients were included in the heterosexual group. However, other studies have shown that the sexually dimorphic nucleus of the preoptic area, which include the INAH3, are of similar size in homosexual males who died of AIDS to heterosexual males, and therefore larger than female.
This clearly contradicts the hypothesis that homosexual males have a female hypothalamus. Furthermore, the SCN of homosexual males is extremely large both the volume and the number of neurons are twice as many as in heterosexual males. These areas of the hypothalamus have not yet been explored in homosexual females nor bisexual males nor females.
William Byne and colleagues attempted to identify the size differences reported in INAH 1—4 by replicating the experiment using brain sample from other subjects: The researchers found a significant difference in INAH3 size between heterosexual men and heterosexual women. The INAH3 size of the homosexual men was apparently smaller than that of the heterosexual men, and larger than that of the heterosexual women, though neither difference quite reached statistical significance. The results for INAH3 weight were similar to those for INAH3 size; that is, the INAH3 weight for the heterosexual male brains was significantly larger than for the heterosexual female brains, while the results for the gay male group were between those of the other two groups but not quite significantly different from either.
The neuron count also found a male-female difference in INAH3, but found no trend related to sexual orientation. A study, Garcia-Falgueras and Swaab asserted that "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. In many species, a prominent feature of sexual differentiation is the presence of a sexually dimorphic nucleus SDN in the preoptic hypothalamus, which is larger in males than in females. Roselli et al. Neurons of the oSDN show aromatase expression which is also smaller in male-oriented rams versus female-oriented rams, suggesting that sexual orientation is neurologically hard-wired and may be influenced by hormones.
However, results failed to associate the role of neural aromatase in the sexual differentiation of brain and behavior in the sheep, due to the lack of defeminization of adult sexual partner preference or oSDN volume as a result of aromatase activity in the brain of the fetuses during the critical period. Having said this, it is more likely that oSDN morphology and homosexuality may be programmed through an androgen receptor that does not involve aromatisation. Most of the data suggests that homosexual rams, like female-oriented rams, are masculinized and defeminized with respect to mounting, receptivity, and gonadotrophin secretion, but are not defeminized for sexual partner preferences, also suggesting that such behaviors may be programmed differently.
Although the exact function of the oSDN is not fully known, its volume, length, and cell number seem to correlate with sexual orientation, and a dimorphism in its volume and of cells could bias the processing cues involved in partner selection. More research is needed in order to understand the requirements and timing of the development of the oSDN and how prenatal programming effects the expression of mate choice in adulthood. The early fixation hypothesis includes research into prenatal development and the environmental factors that control masculinization of the brain.
Some studies have seen pre-natal hormone exposures as the primary factor involved in determining sexual orientation. One explanation for these differences is the idea that differential exposure to hormone levels in the womb during fetal development may change the masculinization of the brain in homosexual men.
The concentrations of these chemicals is thought to be influenced by fetal and maternal immune systems, maternal consumption of certain drugs, maternal stress, and direct injection. This hypothesis is connected to the well-measured effect of fraternal birth order on sexual orientation. Daryl Bem , a social psychologist at Cornell University , has theorized that the influence of biological factors on sexual orientation may be mediated by experiences in childhood.
A child's temperament predisposes the child to prefer certain activities over others. Because of their temperament, which is influenced by biological variables such as genetic factors, some children will be attracted to activities that are commonly enjoyed by other children of the same gender. Others will prefer activities that are typical of another gender. This will make a gender-conforming child feel different from opposite-gender children, while gender-nonconforming children will feel different from children of their own gender.
According to Bem, this feeling of difference will evoke psychological arousal when the child is near members of the gender which it considers as being 'different'. Bem theorizes that this psychological arousal will later be transformed into sexual arousal: This proposal is known as the "exotic becomes erotic" theory.
Bem sought support from published literature but did not present new data testing his theory. A meta-analysis of 48 studies showed childhood gender nonconformity to be the strongest predictor of a homosexual orientation for both men and women. Sexual practices that significantly reduce the frequency of heterosexual intercourse also significantly decrease the chances of successful reproduction, and for this reason, they would appear to be maladaptive in an evolutionary context following a simple Darwinian model competition amongst individuals of natural selection—on the assumption that homosexuality would reduce this frequency.
Several theories have been advanced to explain this contradiction, and new experimental evidence has demonstrated their feasibility. Some scholars  have suggested that homosexuality is indirectly adaptive, by conferring a reproductive advantage in a non-obvious way on heterosexual siblings or their children. By way of analogy, the allele a particular version of a gene which causes sickle-cell anemia when two copies are present, also confers resistance to malaria with a lesser form of anemia when one copy is present this is called heterozygous advantage.
Scholars have also pointed out that Darwin himself described kin selection in The Origin of Species , so under a Darwinian model of evolution, not only individuals, but family groups bloodlines can compete for selection. Brendan Zietsch of the Queensland Institute of Medical Research proposes the alternative theory that men exhibiting female traits become more attractive to females and are thus more likely to mate, provided the genes involved do not drive them to complete rejection of heterosexuality.
In a study, its authors stated that "There is considerable evidence that human sexual orientation is genetically influenced, so it is not known how homosexuality, which tends to lower reproductive success, is maintained in the population at a relatively high frequency. Their results suggested that "genes predisposing to homosexuality may confer a mating advantage in heterosexuals, which could help explain the evolution and maintenance of homosexuality in the population". However, in the same study, the authors noted that "nongenetic alternative explanations cannot be ruled out" as a reason for the heterosexual in the homosexual-heterosexual twin pair having more partners, specifically citing "social pressure on the other twin to act in a more heterosexual way" and thus seek out a greater number of sexual partners as an example of one alternative explanation.
Also, the authors of the study acknowledge that a large number of sexual partners may not lead to greater reproductive success, specifically noting there is an "absence of evidence relating the number of sexual partners and actual reproductive success, either in the present or in our evolutionary past". The heterosexual advantage hypothesis was given strong support by the Italian study demonstrating increased fecundity in the female matrilineal relatives of gay men.
The "gay uncle hypothesis" posits that people who themselves do not have children may nonetheless increase the prevalence of their family's genes in future generations by providing resources e. This hypothesis is an extension of the theory of kin selection , which was originally developed to explain apparent altruistic acts which seemed to be maladaptive. The initial concept was suggested by J.
Haldane in and later elaborated by many others including John Maynard Smith , W. Hamilton and Mary Jane West-Eberhard. Vasey and VanderLaan tested the theory on the Pacific island of Samoa, where they studied women, straight men, and the fa'afafine , men who prefer other men as sexual partners and are accepted within the culture as a distinct third gender category. Vasey and VanderLaan found that the fa'afafine said they were significantly more willing to help kin, yet much less interested in helping children who aren't family, providing the first evidence to support the kin selection hypothesis.
The hypothesis is consistent with other studies on homosexuality, which show that it is more prevalent amongst both siblings and twins. It is speculated that environmental and hormonal stress factors linked to resource feedbacks may act as triggers. Since the hypothesis solves the problem of why homosexuality has not been selected out over thousands of years, despite it being antithetical to reproduction, many scientists consider it the best explanatory model for non-heterosexual behaviour such as homosexuality and bisexuality.
The natural bell curve variation that occurs in biology and sociology everywhere, explains the variable spectrum of expression. Vasal and VanderLaan provides evidence that if an adaptively designed avuncular male androphilic phenotype exists and its development is contingent on a particular social environment, then a collectivistic cultural context is insufficient, in and of itself, for the expression of such a phenotype. Some studies have found correlations between physiology of people and their sexuality; these studies provide evidence which suggests that:.
What do the new ‘gay genes’ tell us about sexual orientation? | New Scientist
Whether genetic or other physiological determinants form the basis of sexual orientation is a highly politicized issue. The Advocate , a U. Equal protection analysis in U. Evidence that sexual orientation is biologically determined and therefore perhaps immutable in the legal sense would strengthen the legal case for heightened scrutiny of laws discriminating on that basis.
The perceived causes of sexual orientation have a significant bearing on the status of sexual minorities in the eyes of social conservatives. However, it is not true.
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There is no 'cure' for homosexuality because it is not a disease. There are, however, different ways of living with homosexual desires. Some advocates for the rights of sexual minorities resist linking that cause with the concept that sexuality is biologically determined or fixed at birth.
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They argue that sexual orientation can shift over the course of a person's life. They conjure up the specter of the surgical or chemical "rewiring" of gay people, or of abortions of fetal homosexuals who have been hunted down in the womb. LGBT culture. From Wikipedia, the free encyclopedia.
Relationship under active research. Main article: Epigenetic theories of homosexuality. Fraternal birth order and sexual orientation. Prenatal hormones and sexual orientation.
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Main articles: Marriages, Families, and Relationships: Making Choices in a Diverse Society. Cengage Learning. Retrieved February 11, The most recent literature from the APA says that sexual orientation is not a choice that can be changed at will, and that sexual orientation is most likely the result of a complex interaction of environmental, cognitive and biological factors Principles and Practice of Psychiatric Nursing.
Elsevier Health Sciences. No conclusive evidence supports any one specific cause of homosexuality; however, most researchers agree that biological and social factors influence the development of sexual orientation. Psychological Science in the Public Interest.
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The Royal College of Psychiatrists.