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Biological theories for explaining the causes of sexual orientation are favored by experts and involve a complex interplay of genetic factors, the early uterine environment and brain structure. A number of twin studies have attempted to compare the relative importance of genetics and environment in the determination of sexual orientation. Biometric modeling revealed that, in men, genetic effects explained . 39 of the variance , the shared environment . 00, and the individual-specific environment . Corresponding estimates among women were . 17 for shared environmental, and . Twin studies have received a number of criticisms including self-selection bias where homosexuals with gay siblings are more likely to volunteer for studies.

They hypothesized that “while genes predisposing to homosexuality reduce homosexuals’ reproductive success, they may confer some advantage in heterosexuals who carry them”. Sexual Hormones and the Brain: An Essential Alliance for Sexual Identity and Sexual Orientation, in Endocrine Development, vol. Biometric modeling revealed that, in men, genetic effects explained . Although consistent differences have been identified, including the size of the brain and of specific brain regions, male and female brains are very similar. 51 in Lesbian, Gay, and Bisexual Identities and Youth: Psychological Perspectives, edited by Anthony R.

Nonetheless, it is possible to conclude that, given the difference in sexuality in so many sets of identical twins, sexual orientation cannot be attributed solely to genetic factors. Another issue is the recent finding that even monozygotic twins can be different and there is a mechanism which might account for monozygotic twins being discordant for homosexuality. Chromosome linkage studies of sexual orientation have indicated the presence of multiple contributing genetic factors throughout the genome. In 1993 Dean Hamer and colleagues published findings from a linkage analysis of a sample of 76 gay brothers and their families. A later analysis by Hu et al. Results from the first large, comprehensive multi-center genetic linkage study of male sexual orientation were reported by an independent group of researchers at the American Society of Human Genetics in 2012.

In addition to sex chromosomal contribution, a potential autosomal genetic contribution to the development of homosexual orientation has also been suggested. In a study population composed of more than 7000 participants, Ellis et al. The biology of sexual orientation has been studied in detail in several animal model systems. In the common fruit fly Drosophila melanogaster, the complete pathway of sexual differentiation of the brain and the behaviors it controls is well established in both males and females, providing a concise model of biologically controlled courtship. In interviews to the press, researchers have pointed that the evidence of genetic influences should not be equated with genetic determinism. According to Dean Hamer and Michael Bailey, genetic aspects are only one of the multiple causes of homosexuality. A study suggests linkage between a mother’s genetic make-up and homosexuality of her sons.


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. This is now “one of the most reliable epidemiological variables ever identified in the study of sexual orientation”. However, the maternal immune hypothesis has been criticized because the prevalence of the type of immune attack proposed is rare compared with the prevalence of homosexuality. In 2004, Italian researchers conducted a study of about 4,600 people who were the relatives of 98 homosexual and 100 heterosexual men.

Female relatives of the homosexual men tended to have more offspring than those of the heterosexual men. Female relatives of the homosexual men on their mother’s side tended to have more offspring than those on the father’s side. 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 and gay men are exposed to a testosterone derivative found in men’s sweat, a region in the hypothalamus is activated.



There have also been reports of variations in brain structure corresponding to sexual orientation. In 1990, Dick Swaab and Michel A. Research on the physiologic differences between male and female brains are based on the idea that people have male or a female brain, and this mirrors the behavioral differences between the two sexes. Some researchers state that solid scientific support for this is lacking. Although consistent differences have been identified, including the size of the brain and of specific brain regions, male and female brains are very similar. He studied four groups of neurons in the hypothalamus called INAH1, INAH2, INAH3 and INAH4.

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. Six of these men had died of AIDS-related illnesses. 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.