if you have an identical twin that is gay and one that isn't then you simply aren't born gay. are you? identical twins are identical. every single identical twin that is born gay would have to share their sexuality with their sibling. otherwise the "born that way" theory simply doesn't play out.
http://en.wikipedia.org/wiki/Biology_and_sexual_orientation
[h=3]Twin studies[/h] A number of
twin studies have attempted to compare the relative importance of genetics and environment in the causation of sexual orientation. In a 1991 study, Bailey and Pillard found that 52% of
monozygotic (MZ) brothers and 22% of the
dizygotic (DZ) twins were concordant for homosexuality.[SUP]
[4][/SUP] 'MZ' indicates
identical twins with the same sets of genes and 'DZ' indicates
fraternal twins where genes are mixed to a similar extent as non-twin siblings. In 2000 Bailey, Dunne and Martin found similar results from a larger sample of 4,901 Australian twins.[SUP]
[5][/SUP] Self reported
zygosity, sexual attraction, fantasy and behaviours were assessed by questionnaire and zygosity was
serologically checked when in doubt. They found 20% concordance in the male identical or MZ twins and 24% concordance for the female identical or MZ twins. A meta-study by Hershberger (2001)[SUP]
[6][/SUP] 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.
Bearman and Bruckman (2002) criticized early studies of concentrating on small, select samples[SUP]
[3][/SUP] and non-representative selection of their subjects.[SUP]
[7][/SUP] They studied 289 pairs of identical twins (monozygotic or from one fertilized egg) and 495 pairs of fraternal twins (dizygotic or from two fertilized eggs) and found
concordance rates for same-sex attraction of only 7.7% for male identical twins and 5.3% for females, a pattern which they say "does not suggest genetic influence independent of social context."[SUP]
[3][/SUP]
A 2010 study of all adult twins in Sweden (more than 7,600 twins)[SUP]
[8][/SUP] 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 twins 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 (other than those shared with a twin), 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.
[h=4]Criticisms[/h] 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, sexual orientation cannot be purely caused by genetics.[SUP]
[9][/SUP]
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.[SUP]
[10][/SUP]
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".[SUP]
[11][/SUP] If one twin receives less testosterone and the other more, this could result in different levels of brain masculinisation.[SUP][
citation needed][/SUP]
