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Differences in Homosexual Biological Substrate

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Difference in Hypothalamus

There could be difference in the brain structures of homosexual and heterosexual men and women. LeVay (1991) identifies that there is a difference in the hypothalamus of heterosexual and homosexual men. The male-typical sexual behavior is determined by the anterior of the hypothalamus in the brain. Accordingly, the researchers examined the volumes of cell structures of the tissues taken from three different groups of people. These groups were women, heterosexual men, and homosexual men. The cell groups were interstitial nuclei of the anterior hypothalamus (INAH) 1, 2, 3 and 4. There were no differences among the groups with respect to INAH 1, 2 and 4. In line with the findings of the previous research, the INAH 3 in homosexual men was twice as large as that in women. It was also more than twice as large in homosexual men as compared to heterosexual men. It was found that INAH 3 was dimorphic in men, and it changed according to the sexual orientation of the man. This proved that sexual orientation had biological substrate.

Evidence Supporting the Finding

The human hypothalamus is different with respect to gender and sexual orientation. Swaab, Gooren and Hofman (1992a) found that in line with many mammalian species the hypothalamus underwent sexual differentiation due to the interaction of sexual hormones and brain development. The researchers found sexually dimorphic nucleus (SDN) in male human adults in the hypothalamus preoptic area that was twice as large as the female one. It was also found that the magnitude of this sex difference in hypothalamus varied with age. Moreover, the extant literature supported the fact that two other nuclei (INAH 2 and 3) and part of the bed nucleus in stria terminalis were sexually dimorphic in humans.

Still, they could not find any difference in the numbers of cells of SDN in homosexual and heterosexual men. Accordingly, the researchers believed that it refuted Dörner's hypothesis that male homosexuals had a female hypothalamus.

After they have examined homosexual men, it was found out that suprachiasmatic nucleus (SCN) in these men had twice bigger number of cells than those of straight men. Similarly, enlarged SCN was also found in one woman with Prader-Willi Syndrome that explained that the development of SCN was connected to sex hormones. The recent study has shown that INAH 3 is twice as large in homosexual men as in heterosexual men. It was also found that SCN was large while SDN was small in primary male-to-female transsexuals. On the other hand, SCN was small while SDN large in secondary male-to-female transsexuals. However, this observation was not confirmed due to the lack of data. Yet it was clear that there was significant difference in the number of cells and in the volumes of various hypothalamic nuclei due to gender and sexual orientation differences. Still, the functional implications of these findings were not clear.

Brain research indicates that there are differences in brain structure and functions due to gender and sexual orientation of the persons. Swaab, Gooren, and Hofman (1995) revealed that there were structural differences in hypothalamus depending on the  biological sex and sexual orientation. It became clear that there were differences in various hypothalamus nuclei cell numbers and volumes in men with various sexual orientation. It was found that a cluster of cells in hypothalamus contained twice as many cells in young men in comparison to those of women. This cluster of cells was the sexually dimorphic nucleus (SDN). However, the magnitude of this difference in SDN depended on age. Yet, it was also proved that two other nuclei in hypothalamus, namely INAH 2 and 3 and the bed nucleus of stria terminalis (BST), were sexually dimorphic in humans. This shows that human males have different brain structures due to their sexual orientation.

However, the researchers still could not find any difference in the cell numbers of SDN with respect to homosexual and heterosexual men. This contradicts Dörner's hypothesis that the female hypothalamus, called the SCN, had half as many cells as that of the heterosexual men. the researchers corroborated the research by LeVay referring to the fact that the INAH 3, another nucleus, was twice as big in heterosexuals as that of homosexuals. They did not either contradict the findings revealed in the research by Allen and Gorski that the anterior commisure in homosexual men was twice as big as in heterosexual men or women. They also discussed preliminary research on male-to-female transsexuals. Still, the functional implications of these findings in determining the adult sexual orientation were not known.

There is a need to reappraise the biologic theories with respect to human sexual orientation. Byne and Parsons (1993) explain that there was a tendency to attribute sexual orientation to biologic changes. However, there was no evidence to support the claim that sexual orientation was determined by the biologic changes. This was similar to the situations with the non-supported psychosocial theories. While it is clear that there has to be a biologic substrate of sexual orientation behavior, it has also become clear that the interest for the biological nature of sexual orientation was a result of dissatisfaction with the psychosocial theories of sexual orientation. Still, no experimental data was available to corroborate biologic reasons of sexual orientation. Thus, a biological theory could not be expounded. The alternative model suggested that it was temperamental and personality traits that merged with familial and social milieu forming the sexual orientation of the person. Among other important factors are heritability and developmental impact of hormones. Accordingly, it is clear that homosexuality is inherited, that denies the fact that genes and hormones determine sexual orientation per se.

Alternative Evidence

Both gender and sexual orientation could be defined by differences in the hypothalamus. Swaab, Gooren, and Hofman (1992b) explain that there is a theory of sexual orientation that results from the interaction between sex hormones and the developing brain. As it goes in the hypothesis of Dörner, there is a female differentiation of the hypothalamus in male homosexuals. They did not find evidence corroborating this hypothesis while checking sexually dimorphic nuclei (SDN) in men. Moreover, it was found that there was no difference in either SDN volume or the number of cells in homosexual and heterosexual men. Still, it became clear that there was a difference in the number of SCN cells depending on the sexual orientation of a person. Both volume and the number of SCN cells in homosexual men were twice bigger than that of the person of a traditional orientation. It is the age of 13-16 months when the SCN volumes and cell numbers reach their peak in values. At this stage, the volume and cell numbers of SCN reach the level that is present in homosexual men. On the contrary, for normal men this number decreases and makes about 35% of the peak value, which is the adult number of the SCN. Then, it is the SCN, rather than the SDN, that explains the difference in brain structure between homosexual and heterosexual men.

Then, it is clear that there should also be differences in brain functioning for homosexual and heterosexual persons. According to Kim, Yang, and Jeong (2011), there are different brain activation patterns in homosexual males and females with respect to their sexual orientation. They showed nude pics of the same sex to a homosexual male and a female and observed the brain activation patterns with a functional Magnetic Resonance Imaging (fMRI). It was observed that the sex hormone levels of both male and female homosexuals were normal and equal to their heterosexual counterparts. Likewise, the nude pics of the same sex resulted in huge activation in the brain parts of frontal cortex, occipital cortex, amygdala, hippocampus, parahippocampal gyrus, globus pallidus, and in other brain parts. All of these brain parts are sensitive to sexual arousal. Hence, they had more arousal associated with same sex than with the other sex. These findings showed different neural mechanisms in homosexuals.

These differences in brain functioning could be studied from different perspectives. Kinnunen, Moltz, Metz, and Cooper (2004) studied different brain activation in homosexuals and heterosexuals which was produced by selective serotonin reuptake inhibitor fluoxetine. It was considered that a number of studies looked at difference in brain nuclei due to sexual orientation. The researchers believed that there will also be neurotransmitter differences in addition to neuroanatomical differences. 40 mg of fluoxetine was administered to groups of homosexuals and heterosexuals and the cerebral metabolic changes were observed through fluorodoxyglucose positron emission tomography (FDG-PET). The observed metabolic differences were responsible for different underlying neurochemical differences in homosexual and heterosexual men.

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Better Position Regarding Homosexuals

The statements that the sexually dimorphic nucleus (SDN) is bigger in heterosexual males and smaller in homosexual males, so that it is just like one of females, make sense. Furthermore, the INAH 3 is also bigger in heterosexual males and is smaller in homosexual males and females. On the other hand, the alternative idea that suprachiasmatic nucleus (SCN) determines the sexual orientation of males is not very powerful. The SCN is also not sexually dimorphic while it is only the SDN and the specific INAH 3 that determines and distinguishes homosexual males from heterosexual males.

Moreover, it is only SDN and INAH 3 that are responsible for the determination of the sexual orientation of individuals. On the other hand, the other nuclei in humans are not the distinguishing factors of homosexuals and heterosexuals. Still, differences in cell numbers were found for SCN in males. At the same time, these differences were not enough to declare the SCN as sexually dimorphic. Consequently, the sexually dimorphic nature of the respective hypothalamic nuclei is important in determining changes with respect to sexual orientation.

Furthermore, it was also found that there was no significant evidence available to show biological substrate for sexual orientation as its determining factor. Then, it is clear that while there are significant differences in hypothalamic nuclei of homosexuals and heterosexuals, there is no need to postulate that such biologic substrate differences are responsible for the sexual orientation. This is also in line with the psychosocial theories of sexual orientation. Finally, Dörner's hypothesis that this change in size of the nucleus is not due to female hypothalamus in homosexual males has been rejected. On the contrary, the size of homosexual male hypothalamus nuclei is standalone and representative of their sexual orientation.

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