On The Influence of Genetic Factors on the Formation of Homosexuality by Data of Twin Studies

Abstract

Results of twin studies are presented; these demonstrate that in a number of cases genetic effects can play a role of mild predisposing factors for the development of homosexuality, but the main part in its formation is accounted for by psychological and social factors. The opinion that genetic factors play the only and dominant role in the genesis of homosexuality does not hold water due to the fact that if it were so then their concordance for homosexuality in monozygotic twins would be 100 %, but it is not observed in reality. The studies conducted with the correct selection of examinees revealed 20 % of the concordance for homosexuality in male monozygotic twins and 24 % in female ones (Bailey, J.M., et al. Genetic and environmental influences on sexual orientation and its correlates in an Australian twin sample. J. Pers. Soc. Psychol. 78(3), 524‑536). The use of Holzinger’s formula for analyzing the obtained numerical findings demonstrated that in the above case the proportion between heritable and environmental factors for male persons was 0.2 (20 %) versus 0.8 (80 %), for female persons it being 0.15 (15 %) versus 0.85 (85 %). Earlier twin studies (Bailey, J.M., Pillard, R.C. (1991). A genetic study of male sexual orientation. Arch. Gen. Psychiatry. 48(12), 1089–1096) revealed that their concordance for homosexuality in siblings (biological brothers, who are not twins) was lower than in adopted brothers (9.2 % versus 11 %), it contradicting to the idea of genetic determination of same-sex attraction. Moreover, attention is also attracted by the fact that dizygotic male twins demonstrated a significantly higher concordance for homosexuality than siblings (22 % versus 9.2 %). But it is known that dizygotic twins, like siblings, have on an average only 50 % of common genes. If there were genetic determination, such differences would not exist; the revealed difference demonstrates environmental effects, since it is evident that family upbringing of dizygotic twins is much more similar. Also it is necessary to pay attention to the fact that the rate of homosexuality in adopted homosexual brothers (11 %) considerably exceeded recent estimations of the part of homosexuals in the general population and was actually equal to the value for siblings, once again convincingly demonstrating a significant role of the environment in the formation of sexual orientation. We should not also ignore the fact that upbringing of monozygotic twins is even more similar than that of dizygotic ones; this phenomenon can cause their larger concordance for homosexuality.