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 only 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%.
As a counter-example, Bearman and Bruckner (2002), analyzed data from a large longitudinal study of adolescents. They found the data did not support genetic influence:
“ Among [identical] twins, 6.7% are concordant [that is, both express same-sex romantic attraction]. [Fraternal] twin pairs are 7.2% concordant. Full-siblings are 5.5% concordant. Clearly, the observed concordance rates do not correspond to degrees of genetic similarity. None of the comparisons between [identical] twins and others ... are even remotely significant. If same-sex romantic attraction has a genetic component, it is massively overwhelmed by other factors.[6] ”
Their conclusion is that the expression of same-sex attraction requires a social environment: "More plausible is the idea that genetic expression is activated only under strongly circumscribed social structural conditions. In contrast to other theories considered below, we assume that the close connection between gender identity and sexual identity is socially constructed."
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. ”
[edit] Criticisms of Twin Studies
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.
[edit] Chromosome linkage studies
Earlier chromosome studies of homosexuality in males have not been replicated, or have had doubt cast on these early suggestions. For example, 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.
However, 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.[16] Sanders et al. (1998) replicated the study, finding 66% Xq28 marker sharing in 54 pairs of gay brothers.[17] On the other hand, 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] Also, a study by Rice et al. in 1999 failed to replicate the Xq28 linkage results.[18]
Additionally, 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 much weaker link for Xq28 than reported by Hamer et al. However, they did find other markers with significant likelihood scores at 8p12, 7q36 and 10q26, the latter two having approximately equivalent maternal and paternal contributions.
[edit] 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] One problem in building consensus using this type of study stems from their heavy reliance on participants truthfully reporting their sexual orientation.