[quote=Tlaloc]Actually there is evidence that homosexuality can have genetic roots.
No, there isn’t.
[quote=Tlaloc]Specifically there are twin studies which show higher incidence of both twins being homosexual if the twins are identical as opposed to non-identical.
The findings of twins studies by J. Michael Bailey and Richard Pillard are far from conclusive, and are at odds with genetic theory. There were also methodological problems with their study.
“The sampling method employed in this study falls short of the ideal genetic epidemiological study, which would involve systematic sampling from a well-specified population. In particular, although all recruiting advertisements stated that [subjects] were desired regardless of the sexual orientation of their relatives, there is no guarantee that volunteers heeded this request.” Source: Bailey and Pillard, “A Genetic Study of Male Sexual Orientation,” Archives of General Psychiatry, 48 (December 1991).
Follow up research confirmed that Bailey and Pillard did not employ “a systematically ascertained sample of twins. Subjects were recruited through advertisements placed in homosexual-oriented periodicals and, therefore, may not be typical of the homosexual population at large.” Source: Byne and Parsons, “Human Sexual Orientation: The Biologic Theories Reappraised,” Archives of General Psychiatry, 50 (March 1993).
From the previous source: “The concordance rate for homosexuality in nontwin biologic brothers was only 9.2 percent - significantly lower that that required by a simple genetic hypothesis.” “Furthermore, the fact that the concordance rates were similar for nontwin biologic brothers (9.2 percent) and genetically unrelated adoptive brothers (11.0 percent) is at odds with a simple genetic hypothesis, which would predict a higher concordance rate for biologic siblings.”
Bailey and Pillard, in “A Genetic Study of Male Sexual Orientation,” admit that twin studies have failed to demonstrate a genetic cause: “Buhrich et al reported a twin study of sexual orientation and related behaviors. They found a strong familial resemblance, but had insufficient power to determine whether that correlation was due to genetic or environmental factors or both.”
There is also Dean Hamer’s work. Hamer does not claim to have found a genetic cause for homosexuality.
“Although the observed rates of homosexual orientation in the maternally derived uncles and male cousins of gay men were higher than in female and paternally related male relatives, they were lower than would be expected for a simple Mendelian trait.” Source: Hamer, et al. “A Linkage Between DNA Markers on the X Chromosome and Male Sexual Orientation,” Science 261 (1993).
“At present, we can say nothing about the fraction of all instances of male homosexuality that are related or unrelated to the Xq28 candidate locus.” Same source as above.
“Given the overall complexity of human sexuality, it is not surprising that a single genetic locus does not account for all the observed variability.” Same source as above.
It is also noteworthy that there hasn’t been any success in replicating Hamer’s original study. See George Rice, Carol Anderson, Neil Risch, and George Ebers, “Male Homosexuality: Absence of Linkage to Microsatellite Markers at Xq28,” Science, 284 (April 1999). From this study: “These results do not support an X-linked gene underlying male homosexuality.”
There is no convincing scientific evidence of a genetic cause for homosexuality. The primary causes are social and psychological.
– Mark L. Chance.