The Australian Twin Registry study by Bailey, Dunne and Martin

Source: Dept of Psychology, Northwestern University
March 2000

Genetic and Environmental Influences on Sexual Orientation and Its
Correlates in an Australian Twin Sample

Bailey, J. Michael1, Dunne, Michael P., Martin, Nicholas G.

Abstract: We recruited twins systematically from the Australian Twin Registry and assessed their sexual orientation and 2 related traits: childhood gender nonconformity and continuous gender identity. Men and women differed in their distributions of sexual orientation, with women more likely to have slight-to-moderate degrees of homosexual attraction, and men more likely to have high degrees of homosexual attraction. Twin concordances for nonheterosexual orientation were lower than in prior studies. Univariate analyses showed that familial factors were important for all traits, but were less successful in distinguishing genetic from shared environmental influences. Only childhood gender nonconformity was significantly heritable for both men and women. Multivariate analyses suggested that the causal architecture differed between men and women, and, for women, provided significant evidence for the importance of genetic factors to the traits’ covariation.

The causes of sexual orientation have provoked intense scientific interest, inspiring both empirical work and theory. This interest stems, in part, from the mostly mistaken belief that different etiological accounts of sexual orientation have different social and ethical implications.But there are also legitimate and important scientific reasons for interest in the issue. Sexual orientation is a fundamental aspect of human sexuality, guaranteeing that for the large majority, men mate with women. Furthermore, sexual orientation is empirically closely linked to some aspects of gender roles, including childhood play behavior and gender identity and aspects of adult sex-typed behavior as well, particularly occupational and recreational interests. Thus, illuminating the origins of sexual orientation could also shed light on the development of other important sex differences.

Empirical research about the origins of sexual orientation has been organized, generally, around the nature–nurture dichotomy, motivated by two different theoretical approaches. The first, sometimes called the “neurohormonal” or “neuroendocrine” theory (Ellis & Ames, 1987; Meyer-Bahlburg, 1987), examines the possibility that homosexual people have been subject to atypical levels of hormones in development, thus causing sexatypical neural diffentiation. LeVay’s finding that for one hypothalamic nucleus, gay men are more similar to heterosexual women than to heterosexual men is perhaps the most important finding motivated by this perspective (LeVay, 1991). The second approach, behavioral genetics, has focused on whether sexual orientation is familial, and if so, whether familial aggregation is attributable to genetic or shared environmental factors. It is important to emphasize that the two approaches are not competing theories but represent different levels of analysis. The present study is primarily an example of the second approach, although some of the variables considered (e.g., childhood gender nonconformity) are also highly relevant to the neurohormonal approach.

Editor’s note: Click austtwins_baileydunnemartin for the pdf file containing the full paper.

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1 Response to “The Australian Twin Registry study by Bailey, Dunne and Martin”


  1. 1 Phil 29 March 2013 at 12:21 am

    This article should really show the statistics gathered from the results of this study. It shows that only about 9% of twins share sexual orientation when one twin is homosexual.


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