Biology and sexual orientation Totally Explained

Everything about Biology And Sexual Orientation totally explained

According to the American Psychological Association,

Most scientists today agree that sexual orientation is most likely the result of a complex interaction of environmental, cognitive and biological factors. In most people, sexual orientation is shaped at an early age. There is also considerable recent evidence to suggest that biology, including genetic or inborn hormonal factors, play a significant role in a person's sexuality. In summary, it's important to recognize that there are probably many reasons for a person's sexual orientation and the reasons may be different for different people.

According to the American Psychiatric Association,

No one knows what causes heterosexuality, homosexuality, or bisexuality.... Currently there's a renewed interest in searching for biological etiologies for homosexuality. However, to date there are no replicated scientific studies supporting any specific biological etiology for homosexuality.

A number of biological factors have been considered by scientists, such as prenatal hormones, chromosomes, polygenetic effects, brain structure effects and even viral influences.
In the fruitfly Drosophila melanogaster, mutant alleles of the fruitless gene were found to cause male flies to court and attempt to mate exclusively with other males. Likewise, mutations in the genderblind gene were effective in altering fly behavior toward a bisexual orientation.
Although much popular focus is on simplistic formulae like "the gay gene", the ultimate value of scientific research in this area is in creating a real understanding of the biological bases of sexual orientation generally, at present poorly understood for any sexual orientation. Study of how developmental mechanisms produce variations in sexual orientation ought to lead to a greater understanding of how many similar behavioral traits might work and develop.

Empirical studies

Twin studies

Researchers have traditionally used twin studies to try to isolate genetic influences from environmental or other influences. Many early twin studies in this area selected from non-representative samples, and gave non-representative results.
   Earlier twin studies gave indications that male homosexuality was genetically mediated. One common type of twin study compared the monozygotic (or identical) twins of people possessing a particular trait to the dizygotic (non-identical, or fraternal) twins of people possessing the trait. Since monozygotic twins have the same genotype (genetic makeup) while dizygotic twins share only, on average, 50% of their genotype, a difference in the prevalence of the trait in question between these types of twins provides evidence of a genetic component.
   A few such studies began to examine homosexuality in the early 20th century, using small, non-random samples.
   Bailey and Pillard (1991) in a study of gay twins found that 52% of monozygotic brothers and 22% of the dizygotic twins were concordant for homosexuality. Bailey, Dunne and Martin (2000) used the Australian twin registry to obtain a sample of 4,901 twins. Self reported zygosity, sexual attraction, fantasy and behaviours were assessed by questionnaire and zygosity was serologically checked when in doubt. MZ twin concordance for homosexuality was found to be 30%.
   Criticism of these earlier studies included: recruitment through gay media (which may result in higher response rates from twins who are both gay), and recruitment from twin registries (which may result in higher response rates from twins who act more similar to each other Dichorionic twins potentially have different hormonal environments and receive maternal blood from separate placenta. Monoamniotic twins share a hormonal environment, but can suffer from the 'twin to twin transfusion syndrome' in which one twin is "relatively stuffed with blood and the other exsanguinated". If one twin receives less testosterone and the other more, this could result in different levels of brain masculinisation.
   Overall, data appear to indicate that genetic factors may play some part in the development of sexual orientation, even if only a modest part. Further work is needed to more precisely quantify any genetic contribution to sexuality and to elucidate its mechanism.

Studies of brain structure

A number of sections of the brain have been reported to be sexually dimorphic; that is, they vary between men and women. There have also been reports of variations in brain structure corresponding to sexual orientation. In 1990, Swaab and Hofman reported a difference in the size of the suprachiasmatic nucleus between homosexual and heterosexual men. In 1992, Allen and Gorski reported a difference related to sexual orientation in the size of the anterior commissure.
   Early work of this type was also done by Simon LeVay - however, LeVay's assessments have not been replicated. LeVay studied four groups of neurons in the hypothalamus, called INAH1, INAH2, INAH3 and INAH4. This was a relevant area of the brain to study, because of evidence that this part of the brain played a role in the regulation of sexual behaviour in animals, and because INAH2 and INAH3 had previously been reported to differ in size between men and women.
   He obtained brains from 41 deceased hospital patients. The subjects were classified as follows: 19 gay men who had died of AIDS, 16 presumed heterosexual men (6 of whom had died of AIDS), and 6 presumed heterosexual women (1 of whom had died of AIDS).
   Byne and colleagues also weighed and counted numbers of neurons in INAH3, tests not carried out by LeVay. 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.

In addition to this, of the men LeVay used in his studies, the sexual histories of the men he assumed to be heterosexual were unknown.

Chromosome linkage studies

In 1993, Dean Hamer published findings from a linkage analysis of a sample of 76 gay brothers and their families. Hamer et al. (1993) found that the gay men had more gay male uncles and cousins on the maternal side of the family than on the paternal side. Gay brothers who showed this maternal pedigree were then tested for X chromosome linkage, using twenty-two markers on the X chromosome to test for similar alleles. Thirty-three of the forty sibling pairs tested were found to have similar alleles in the distal region of Xq28, which was significantly higher than the expected rates of 50% for fraternal brothers. A later analysis by Hu et al. revealed that 67% of gay brothers in a new saturated sample shared a marker on the X chromosome at Xq28. Sanders et al. (1998) replicated the study, finding 66% Xq28 marker sharing in 54 pairs of gay brothers. These studies only examined homosexuality in males.
   However, two later studies (Bailey et al., 1999; McKnight and Malcolm, 2000) failed to find a preponderance of gay relatives in the maternal line of homosexual men. More recently, Mustanski (2005) failed to find the Xq28 marker in a complete genome scan of gay men’s DNA. Mustanski did however find autosomal markers at 7q36, 8p12 and 10q26.
   The evidence for the Xq28 marker is therefore preliminary and has yet to be fully proved or disproved. Even at face value, the discovery of the Xq28 region would only show one genetic correlate of male homosexuality. Hamer's study was important though, as it was the first experiment to claim such a correlation. These findings don't suggest that the Xq28 region is necessary for homosexuality or singularly causes homosexuality, but rather that it might be one of many factors which influence sexual orientation in some males.
   A recent study supports X-linkage from a different perspective. Women have two X chromosomes, one of which is "switched off". The inactivation of the X chromosome occurs randomly throughout the embryo, resulting in cells that are mosaic with respect to which chromosome is active. In some cases though, it appears that this switching off can occur in a non-random fashion. Bocklandt et al (2006) reported that the number of women with extreme skewing of X chromosome inactivation is significantly higher in mothers of homosexual men than in age-matched controls without gay sons. 4% of controls showed extreme skewing compared to 13% of the mothers with gays sons and 23% of mothers with two or more gay sons.
   Also, male homosexuality appears likely to be influenced by a complex genetic interaction which may be mediated by H-Y antigens in the mother’s immune system (see below). Whichever genes are implicated they almost certainly cause male brains to differentiate in a female typical direction.
   As for female homosexuality, there remains little evidence from replicated genetic linkage studies.

Maternal linkage, birth order, and female fertility

Blanchard and Klassen (1997) reported that each older brother increases the odds of being gay by 33%. This is now "one of the most reliable epidemiological variables ever identified in the study of sexual orientation." To explain this finding, it has been proposed that male fetuses provoke a maternal immune reaction that becomes stronger with each successive male fetus. Male fetuses produce HY antigens which are "almost certainly involved in the sexual differentiation of vertebrates." It is this antigen which maternal H-Y antibodies are proposed to both react to and 'remember'. Successive male fetuses are then attacked by H-Y antibodies which somehow decrease the ability of H-Y antigens to perform their usual function in brain masculinisation.
Homosexuals of either sex are more likely than the general population to be non-right handed (see Handedness and sexual orientation)

Pheromones correlation

Recent research conducted in Sweden has suggested that gay and straight men respond differently to two odors that are believed to be involved in sexual arousal. The research showed that when both heterosexual women (lesbians were included in the study, but the results regarding them were "somewhat confused") and gay men are exposed to a testosterone derivative found in men's sweat, a region in the hypothalamus is activated. Heterosexual men, on the other hand, have a similar response to an estrogen-like compound found in women's urine. The conclusion, that sexual attraction, whether same-sex or opposite-sex oriented, operates similarly on a biological level, doesn't mean that there's necessarily a biological cause for homosexuality. Researchers have suggested that this possibility could be further explored by studying young subjects to see if similar responses in the hypothalamus are found and then correlating this data with adult sexual orientation.

Early fixation hypothesis

The early fixation hypothesis includes research into prenatal development and the environmental factors that control masculinization of the brain. Studies have concluded that there's empirical evidence to support this hypothesis, including the observed differences in brain structure and cognitive processing between homosexual and heterosexual men. One explanation for these differences is the idea that differential exposure to hormone levels in the womb during fetal development may block or exaggerate 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 also connected to the fraternal birth order research.

Imprinting/critical period

This type of theory holds that the formation of gender identity occurs in the first few years of life after birth. It argues that individuals can be predisposed to homosexual orientation by biological factors but are triggered in some cases by upbringing. Part of adopting a gender identity involves establishing the gender(s) of sexual attraction. This process is analogous to the "imprinting" process observed in animals. A baby duckling may be genetically programmed to "imprint" on a mother, but what entity it actually imprints upon depends on what objects it sees immediately after hatching. Most importantly, once this process has occurred, it can't be reversed, any more than the duckling can hatch twice.
   A sort of reverse sexual imprinting has been observed in heterosexual humans; see the section on the "Westermarck effect" in Behavioral imprinting.
   Several different triggers for imprinting upon a particular sexual orientation have been proposed.
   One hypothesis is that something about what young children see in the gender-roles behavior of adults, or some differences (possibly unconscious) in the way adults treat young children, somehow influences or determines a child's eventual sexual orientation.

Exotic becomes erotic

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 physiological arousal when the child is near members of the gender which it considers as being 'different'. Bem theorizes that this physiological arousal will later be transformed into sexual arousal: children will become sexually attracted to the gender which they see as different ("exotic"). This theory is known as Exotic Becomes Erotic (EBE) theory.
The theory is based in part on the frequent finding that a majority of gay men and lesbians report being gender-nonconforming during their childhood years. A meta-analysis of 48 studies showed childhood gender nonconformity to be the strongest predictor of a homosexual orientation for both men and women. Fourteen studies published since Bailey & Zucker's 1995 also show the same results. In one study by the Kinsey Institute of approximately 1000 gay men and lesbians (and a control group of 500 heterosexual men and women), 63% of both gay men and lesbians reported that they were gender nonconforming in childhood (for example, didn't like activities typical of their sex), compared with only 10-15% of heterosexual men and women. There are also six "prospective" studies--that is longitudinal studies that begin with gender-nonconforming boys at about age 7 and follow them up into adolescence and adulthood. These also show that a majority (63%) of the gender nonconforming boys become gay or bisexual as adults. There are very few prospective studies of gender nonconforming girls. In a group of eighteen behaviorally masculine girls (mean age of assessment: 9 years), all reported a homosexual sexual orientation at adolescence, and eight had requested sex reassignment.

Sexual orientation and evolution

Sexual practices that significantly reduce the frequency of heterosexual intercourse also significantly decrease the chances of successful reproduction, and for this reason, they'd appear to be maladaptive in an evolutionary context following a simple Darwinian model of Natural Selection—on the assumption that homosexuality would reduce this frequency.
Those who believe that homosexuality is purely genetic argue that maladaptive traits will only be removed from a population if the trait is under simple, direct selection, if it derives from a heritable component of a genotype and if the intensity of selection is greater than other evolutionary forces like genetic drift, or inclusive fitness.
Some scholars have suggested that homosexuality is adaptive in a non-obvious way. By way of analogy, the allele (a particular version of a gene) which causes sickle-cell anemia when two copies are present may also confer resistance to malaria with a lesser form of anemia when one copy is present (this is called heterozygous advantage).
The so-called "gay uncle" theory posits that people who themselves don't have children may nonetheless increase the prevalence of their family's genes in future generations by providing resources (food, supervision, defense, shelter, etc.) to the offspring of their closest relatives. This hypothesis is an extension of the theory of kin selection. Kin selection was originally developed to explain apparent altruistic acts which seemed to be maladaptive. The initial concept was suggested by J.B.S. Haldane in 1932 and later elaborated by many others including John Maynard Smith and West Eberhard. This concept was also used to explain the patterns of certain social insects where most of the members are non-reproductive.
The primary criticism of this theory has to do with the fact that children share on average 25% of their genes with their uncles and aunts, but on average 50% with their parents. This means that to be adaptive, a "gay uncle" would need to somehow assist an extra two nieces or nephews, on average, to reach adulthood for every one of their own offspring they give up. Critics of the theory find this trade-off to be unlikely to produce a net reproductive gain.

Politics

The issue of genetic or other physiological determinants as the basis of sexual orientation is a highly politicised issue. The Advocate, a U.S. gay and lesbian newsmagazine, reported in 1996 that 61% of its readers believed that "it would mostly help gay and lesbian rights if homosexuality were found to be biologically determined". A cross-national study in the United States, the Philippines, and Sweden found that those who believed that "homosexuals are born that way" held significantly more positive attitudes toward homosexuality than those who believed that "homosexuals choose to be that way" and/or "learn to be that way".

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