Twin studies Researchers have traditionally used twin studies to try to isolate genetic influences from environmental or other influences. One common type of
twin study compares identical
twins (known as
monozygotic or "MZ twins") who both have a particular trait to non-identical or
fraternal twins (known as
dizygotic or "DZ twins") with that same trait. Since identical twins have the same genetic makeup (
genotype) while non-identical twins share an average of 50% of their genes, a difference between these types of twins provides evidence of a genetic component. For example, if a high percentage of identical twins both have red hair (while a low percentage of non-identical twins both have red hair), that suggests that red hair has a genetic basis. On the other hand, if identical twins share a characteristic just as often as fraternal twins (such as love of music), that suggests that there is not a genetic basis for that trait.
A number of twin studies have attempted this kind of isolation. As Bearman and Bruckner (2002)
[6] describe it, early studies concentrated on small, select samples, which showed very high genetic influences; however, they were also criticized for non-representative selection of their subjects.
[7] Later studies, performed on increasingly representative samples, showed much lesser concordance among MZ twins, although still significantly larger than among DZ twins.
For example, a recent meta-study by Hershberger (2001)
[8] compares the results of eight different twin studies: among those, all but two showed MZ twins having much higher concordance of sexual orientation than DZ twins, suggesting a non-negligible genetic component. Two additional examples: Bailey and Pillard (1991) in a study of gay twins found that 52% of monozygotic (MZ) brothers and 22% of the dizygotic (DZ) twins were concordant for homosexuality.
[9] Also, Bailey, Dunne and Martin (2000) used the Australian twin registry to obtain a sample of 4,901 twins.
[10] 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%. Averaging over all studies suggests that roughly 50 percent of the variance in sexual orientation can be attributed to inherited factors.
A recent study of all adult twins in Sweden (more than 7,600 twins)
[11] found that same-sex behavior was explained by both heritable factors and individual-specific environmental sources (such as prenatal environment, experience with illness and trauma, as well as peer groups, and sexual experiences), while influences of shared-environment variables such as familial environment and societal attitudes had a weaker, but significant effect. Women showed a statistically non-significant trend to weaker influence of hereditary effects, while men showed no effect of shared environmental effects. The use of all adult twins in Sweden was designed to address the criticism of volunteer studies, in which a potential bias towards participation by gay twin may influence the results (see below).
“ Overall, the environment shared by twins (including familial and societal attitudes) explained 0-17% of the choice of sexual partner, genetic factors 18-39% and the unique environment 61-66%. The individual's unique environment includes, for example, circumstances during pregnancy and childbirth, physical and psychological trauma (e.g., accidents, violence, and disease), peer groups, and sexual experiences. [...] In men, genetic effects explained .34–.39 of the variance, the shared environment .00, and the individual-specific environment .61–.66 of the variance. Corresponding estimates among women were .18–.19 for genetic factors, .16–.17 for shared environmental, and 64–.66 for unique environmental factors.
Criticisms
Twin studies have received a number of criticisms including
self-selection bias where homosexuals with gay siblings are more likely to volunteer for studies. Nonetheless, it is possible to conclude that, given the difference in sexuality in so many sets of identical twins (who are genetically identical, and shared the same fetal environment), sexual orientation cannot be purely caused by genetics.
[12]
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. Gringas and Chen (2001) describe a number of mechanisms which can lead to differences between monozygotic twins, the most relevant here being chorionicity and amniocity.
[13] 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".
[14] If one twin receives less testosterone and the other more, this could result in different levels of brain masculinisation.
Chromosome linkage studies
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.
[15] Hamer
et al. 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. In another finding, 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. This was popularly (but inaccurately) dubbed as the 'gay gene' in the media, causing significant controversy.
A later analysis by Hu
et al. replicated and refined these findings. This study revealed that 67% of gay brothers in a new saturated sample shared a marker on the X chromosome at Xq28.
[16] Sanders
et al. (1998) replicated the study, finding 66% Xq28 marker sharing in 54 pairs of gay brothers.
[17] Although two other studies (Bailey
et al., 1999; McKnight and Malcolm, 2000) failed to find a preponderance of gay relatives in the maternal line of homosexual men
[17], a rigorous replication of the maternal loading was reported on samples in Italy in England. One study by Rice
et al. in 1999 failed to replicate the Xq28 linkage results.
[18] Meta-analysis of all available linkage data indicates a significant link to Xq28, but also indicates that additional genes must be present to account for the full heritability of sexual orientation.
Mustanski
et al. (2005) performed a full-genome scan (instead of just an X chromosome scan) on individuals and families previously reported on in Hamer
et al. (1993) and Hu
et al. (1995), as well as additional new subjects.
[19] With the larger sample set and complete genome scan, the study found somewhat reduced linkage for Xq28 than reported by Hamer
et al. However, they did find other markers with significant likelihood scores at 8p12, 7q36 and 10q26. Interestingly, one of the links showed highly significant maternal loading, thus further confirming the previous family studies.
Epigenetics studies
A recent 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. Bocklandt et al. (2006) reported that, in mothers of homosexual men, the number of women with extreme skewing of X chromosome inactivation is significantly higher than in mothers without gay sons. Thirteen percent of mothers with one gay son, and 23% of mothers with two gay sons showed extreme skewing, compared to 4% percent of mothers without gay sons.
[20]
Birth order
Main article:
Fraternal birth order and sexual orientation
Blanchard and Klassen (1997) reported that each older brother increases the odds of a man being gay by 33%.
[21][22] This is now "one of the most reliable epidemiological variables ever identified in the study of sexual orientation."
[23] 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.
[21]
Female fertility
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. The researchers concluded that there was genetic material being passed down on the X chromosome which both promotes fertility in the mother and homosexuality in her male offspring. The connections discovered, would explain about 20% of the cases studied, indicating that this is a highly significant but not the sole genetic factor determining sexual orientation.
[24].
Pheromone studies
Recent research conducted in Sweden
[25] 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.
[26] The conclusion, that sexual attraction, whether same-sex or opposite-sex oriented, operates similarly on a biological level, does not mean that there is 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.[
citation needed]
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.
[27] In 1992, Allen and Gorski reported a difference related to sexual orientation in the size of the
anterior commissure.
[28]
Early work of this type was also done by
Simon LeVay. 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.
[29]
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).
[29] The AIDS patients in the heterosexual groups were all identified from medical records as intravenous drug abusers or recipients of blood transfusions, though only 2 of the men in this category had specifically denied homosexual activity. The records of the remaining heterosexual subjects contained no information about their sexual orientation; they were assumed to have been mostly or all heterosexual "on the basis of the numerical preponderance of heterosexual men in the population."
[29] LeVay found no evidence for a difference between the groups in the size of INAH1, INAH2 or INAH4. 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 6 AIDS patients were included in the heterosexual group. The size of the INAH3 in the homosexual male brains was similar to that in the heterosexual female brains.
William Byne and colleagues attempted to replicate the differences reported in INAH 1-4 size using a different sample of brains from 14 HIV-positive homosexual males, 34 presumed heterosexual males (10 HIV-positive), and 34 presumed heterosexual females (9 HIV-positive). They found a significant difference in INAH3 size between heterosexual men and 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.
[30]
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.
[30]
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