Whether homosexuality has biological involvement has remained a matter of debate among researchers over the years. Though there is no scientific consensus regarding the influence of biology on determining sexual orientation of a person, researchers consider various biological factors that include prenatal hormones, viral influences, polygenetic effects, brain structure and chromosomes.
From the middle of the 19th century to the beginning of the 20th century, various researchers like Karl Heinrich Ulrichs, Richard von Krafft-Ebing, Magnus Hirschfield, Havelock Ellis, Carl Jung and Sigmund Freud have put forward several significant theories in this regard. Many gender variant gay people too have contributed to their studies. According to the scientists of the ‘American Academy of Pediatrics’, instead of a single ‘gay gene’, an interaction among biological, genetic and environmental factors determine the sexual orientation of a gay person. People were asked to rate themselves on a scale that ranged from ‘totally straight’ to ‘completely gay’ in the oft-cited ‘Sexual Behavior in the Human male’ and ‘Sexual Behavior in the Human Female’ proffered by Alfred C. Kinsey in the years 1948 and 1953, respectively.
As per some recent suggestions, an excess of genes coding for female-like traits is represented by male homosexuality. Thus, gay men are tipped over a ‘liability threshold’ into homosexuality. The viability of their offspring has increased as a result of women choosing ‘gay-enabled’ men.
Twin studies affirm the genetically mediated nature of male homosexuality. In 1952, Kallman reported the first comprehensive twin study on sexual orientation of gay people. Bailey and Pillard established the fact that 52% of monozygotic brothers and 22% of the dizygotic twins were concordant for homosexuality in their study of gay twins in 1991. In order to get a sample of 4,901 twins, Bailey, Martin and Dunne made use of the Australian twin registry in 2000. In their study, 30% monozygotic twin concordance for homosexuality was found.
Sexual dimorphism is very much evident from the structures of different sections of human brain. Different brain structures leads to variations in sexual orientation. As per researches, the size of suprachiasmatic nucleus and anterior commissure plays significant role in determining sexuality of gay men. As the hypothalamus part of our brain regulates sexual behavior, Simon LeVay made the four groups of neurons of this area, known as INAH1, INAH2, INAH3 and INAH4, the subject of his study. He reported that the size of INAH3 in the brains of gay males and straight females was similar. Further researches by William Byne and his colleagues found that INAH3 size differed considerably between heterosexual males and females as well as between homosexual men and heterosexual women.
In the field of chromosome linkage studies, Dean Hamer came into the conclusion that the number of gay male cousins and uncles of a gay man is more from the maternal side than the paternal side of the family. A linkage examination of a sample of 76 gay brothers and their families was the basis of the findings of Hamer, published in 1993. In 1998, Sanders and others reproduced his study. It has also been presumed that mediated by H-Y antigens in the immune system of the mother, a complex genetic interaction somehow influences male homosexuality. In 1997, Blanchard and Klassen reported that the more the number of older brothers, the chance of being gay becomes more.
As per the reports of Bocklandt, Hamer, Villain and Horvath in 2006, extreme skewing of X chromosome inactivation is quite common among some mothers of gay babies. By the fraternal birth order effect, this extreme skewing may have its influence on male sexual orientation. Another study in 2004 found that the number of offspring is more among the female relatives of gay men than that of straight males. It has also been inferred that compared to straight people, gay men and women are more likely to be non-right handed.
In Sweden, researchers have recently revealed that a region of hypothalamus of gay men and straight women get activated in the same way by the odor of men’s sweat. The role of parental hormones in determining gay men’s sexual orientation cannot be ignored. Prenatal development and the environmental aspects that control the brain’s masculinization are incorporated under the early fixation hypothesis. According to this study, in homosexual men, masculinization of the brain can be blocked due to disparity in exposure to hormonal levels in the womb at the time of fetal development. This hypothesis is related to the research of fraternal birth order.
The pathogenic theory of homosexuality states that a bacterium or virus may be the reason behind this condition. As per the imprinting or critical period theory, gender characteristics of a person if shaped within the first few years of life after birth. This procedure is similar to the imprinting method monitored in the animal kingdom.
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