Wednesday, January 30, 2008
SDN. homosexuality twins
Twin studies. Most twin studies have focused on the concordance rate for homosexuality. This is the likelihood that, if one twin is gay, his or her co-twin will be gay too. If genes influence sexual orientation, the concordance rate should be higher for twin pairs who are monozygotic (“identical”) than for twin pairs who are dizygotic (“fraternal”). That’s because monozygotic twins share all the same genes, whereas dizygotic twins share only about half their genes. If genes absolutely determined sexual orientation the concordance rate for monozygotic twins should be 100%.
One early study did report a near-100% concordance rate for male monozygotic twins (Kallmann 1952). More recent studies have come up with much lower figures, but have generally reported higher concordance rates for monozygotic than for dizygotic twins, consistent with a genetic influence on sexual orientation. In one study the concordance rate was 52% for male monozygotic twins compared with 22% for male dizygotic twins (Bailey and Pillard 1995). A comparable study of female twins came up with concordances of 48% and 16% respectively (Bailey, Pillard et al. 1993).
Although these studies suggest that there is a substantial influence of genes on sexual orientation in both men and women, there are problems of interpretation. For one thing, it is difficult to get from the concordance rates to a measure of heritability (meaning, simply put, the fraction of the total causation of homosexuality that is genetic). If it is the case that monozygotic twins experience a more similar environment than do dizygotic twins (being treated more similarly by their parents, for example), and these environmental factors influence sexual orientation, then the concordance rate would be higher for monozygotic twins for that reason alone. There is in fact no reason to think that this scenario is the case, but it is a theoretical possibility.
Another problem has to do with ascertainment bias. Typically, researchers do these twin studies by advertising for individuals who are gay and have a twin, then they check on the other twin’s sexual orientation. But if the likelihood that a person responds to the ad is affected by whether his/her twin is also gay or not, this could throw off the statistics. To get away from this problem, Bailey and colleagues did one study using a pre-existing twin registry (Bailey, Dunne et al. 2000). This study came up with lower concordance rates than previous studies, especially in women. Interestingly, the researchers found that childhood gender nonconformity—a common precursor of adult homosexuality—was significantly heritable in both sexes.
There is one small study of monozygotic twins reared apart (Eckert, Bouchard et al. 1986). Of four female pairs in which one twin was lesbian, none of the co-twins were lesbian. Of two male pairs in which one twin was gay, one of the co-twins was also gay, while the other was bisexual.
Comment: There remains considerable uncertainty about the heritability of homosexuality: it is probably significantly heritable in men but may be only slightly heritable or not heritable at all in women.
One early study did report a near-100% concordance rate for male monozygotic twins (Kallmann 1952). More recent studies have come up with much lower figures, but have generally reported higher concordance rates for monozygotic than for dizygotic twins, consistent with a genetic influence on sexual orientation. In one study the concordance rate was 52% for male monozygotic twins compared with 22% for male dizygotic twins (Bailey and Pillard 1995). A comparable study of female twins came up with concordances of 48% and 16% respectively (Bailey, Pillard et al. 1993).
Although these studies suggest that there is a substantial influence of genes on sexual orientation in both men and women, there are problems of interpretation. For one thing, it is difficult to get from the concordance rates to a measure of heritability (meaning, simply put, the fraction of the total causation of homosexuality that is genetic). If it is the case that monozygotic twins experience a more similar environment than do dizygotic twins (being treated more similarly by their parents, for example), and these environmental factors influence sexual orientation, then the concordance rate would be higher for monozygotic twins for that reason alone. There is in fact no reason to think that this scenario is the case, but it is a theoretical possibility.
Another problem has to do with ascertainment bias. Typically, researchers do these twin studies by advertising for individuals who are gay and have a twin, then they check on the other twin’s sexual orientation. But if the likelihood that a person responds to the ad is affected by whether his/her twin is also gay or not, this could throw off the statistics. To get away from this problem, Bailey and colleagues did one study using a pre-existing twin registry (Bailey, Dunne et al. 2000). This study came up with lower concordance rates than previous studies, especially in women. Interestingly, the researchers found that childhood gender nonconformity—a common precursor of adult homosexuality—was significantly heritable in both sexes.
There is one small study of monozygotic twins reared apart (Eckert, Bouchard et al. 1986). Of four female pairs in which one twin was lesbian, none of the co-twins were lesbian. Of two male pairs in which one twin was gay, one of the co-twins was also gay, while the other was bisexual.
Comment: There remains considerable uncertainty about the heritability of homosexuality: it is probably significantly heritable in men but may be only slightly heritable or not heritable at all in women.
# posted by roodbont @ 3:24 PM 
