Zwillinge und Homosexualität

Eine Studie dazu, welchen Anteil Umwelt bzw. Gene an Homosexualität haben anhand einer Zwillingsstudie:

There is still uncertainty about the relative importance of genes and environments on human sexual orientation. One reason is that previous studies employed selfselected, opportunistic, or small population-based samples. We used data from a truly population-based 2005–2006 survey of all adult twins (20–47 years) in Sweden to conduct the largest twin study of same-sex sexual behavior attempted so far. We performed biometric modeling with data on any and total number of lifetime same-sex sexual partners, respectively. The analyses were conducted separately by sex. Twin resemblancewas moderate for the 3,826 studied monozygotic and dizygotic same-sex twin pairs. Biometric modeling revealed that, in men, genetic effects explained .34–.39 of the variance, the shared environment .00, and the individualspecific 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. Although wide confidence intervals suggest cautious interpretation, the results are consistent with moderate, primarily genetic, familial effects, and moderate to large effects of the nonshared environment (social and biological) on same-sex sexual behavior.

Quelle: Genetic and Environmental Effects on Same-sex Sexual Behavior: A Population Study of Twins in Sweden

Aus der weiteren Zusammenfassung:

It has been suggested that individual differences in heterosexual and homosexual behavior result from unique environmental factors such as prenatal exposure to sex hormones, progressive maternal immunization to sex-specific proteins, or neurodevelopmental instability (Rahman, 2005). Although the unique environmental variance component also includes measurement error, the present results support the notion that the individual-specific environment does indeed influence sexual preference. In conclusion, although confidence intervals were wide, we believe this study provides the most unbiased estimates presented so far of genetic and non-genetic contributions to same-sex sexual behavior. The results should inform further research on this complex trait

Und eine weiter Studie, die auf die Digit Ratio abstellt:

The 2nd to 4th digit ratio has been argued to reflect prenatal hormonal influences and is reportedly associated with various psychological and behavioral traits, such as sexual orientation, cognitive abilities, and personality. We examined genetic and environmental influences on the 2nd to 4th digit ratio (2D:4D) using a Japanese twin sample (N = 300). The genetic analysis showed substantial additive genetic influences for both right and left hand 2D:4D. The rest of the variance was explained mainly by environmental influences not shared within twin pairs. These findings were, in general, in accordance with preceding studies with primarily Caucasian twin samples. The bivariate genetic analysis revealed that the additive genetic influences were largely shared between the right and left hand, while the non-shared environmental influences were largely unique to each hand. Results from a comparison of opposite-sex and same-sex twins were not significant, although they were in the predicted direction according to the prenatal hormone transfer hypothesis. Female monozygotic twin pairs discordant in sexual orientation showed significant within-pair differences in left hand 2D:4D, where non-heterosexual twins had lower (more masculinized) 2D:4D. In addition, we found that non-heterosexual male MZ twins had larger (more feminized) 2D:4D than their heterosexual co-twins. These results suggest the existence of non-shared environmental influences that affect both 2D:4D and sexual orientation

Quelle: The Second to Fourth Digit Ratio (2D:4D) in a Japanese Twin Sample: Heritability, Prenatal Hormone Transfer, and Association with Sexual Orientation

Aus der Studie:

If the prenatal environment has some influence on 2D:4D, we would expect that even DZ co-twins are similar to each other because of the shared prenatal environment. The small shared environmental influences do not support the prediction. This suggests the possibility that twins do not share their prenatal hormonal environment. To test prenatal hormonal influences more directly, a second type of twin study compared twins from opposite-sex pairs (OS pairs) to those from same-sex pairs (SS pairs).(…) A recent meta-analysis showed that non-heterosexual females, including both homosexuals and bisexuals, had smaller (more masculine) 2D:4D than heterosexual females (Grimbos et al., 2010). Hall and Love (2003) conducted a within-pair comparison of 2D:4D of seven female MZ twin pairs with discordant sexual orientation. In each pair, one co-twin had a heterosexual orientation and the other had a homosexual orientation. In agreement with Grimbos et al.’s meta-analysis, Hall and Love reported that female homosexual MZ co-twins had more masculine 2D:4D compared with their heterosexual co-twins. A within-pair comparison of five female MZ twin pairs with concordant sexual orientation was not significant. Because MZ pairs share 100 percent of their genotypes, the differences between them must be explained by non-shared environmental influences. The environmental influences could include epigenetic effects. Previous results suggest that the same non-shared environmental factors that affect 2D:4D could also affect sexual orientation, indicating an overlap of environmental effect between the two phenotypes.

Dazu hatte ich auch bereits etwas in anderen Artikeln geschrieben:

Die Studien zeigen, dass die Gene und die pränatalen Hormone Gründe für Homosexualität haben können und Unterschiede zwischen Zwillingen gerade in diesem Bereich bestehen.