Behav Genet 2000 Jul;30(4):345-56

Measurement models for sexual orientation in a community twin sample.
Kirk KM, Bailey JM, Dunne MP, Martin NG.
Queensland Institute of Medical Research and Joint Genetics Program, The University of Queensland, Brisbane, Australia.

Multivariate structural equation modeling techniques have been applied to examine the causes of individual differences in responses to several items concerning sexual orientation. To minimize potential ascertainment and response biases, the study sample involved a large (N = 4901) community-based cohort of Australian twins aged 18-52 who answered an anonymous questionnaire on sexual behavior and attitudes. The statistical power of the analysis was increased by the availability of multiple measures of sexual orientation (behaviors, attitudes and feelings), providing stronger evidence for the existence of additive genetic influences on this phenotype than in a previous analysis (Bailey et al., 2000). Estimates of the heritability of homosexuality in this sample ranged between 50 and 60% in females but were significantly lower (heritability of approximately 30%) in males.
PMID: 11206089 [PubMed - indexed for MEDLINE]

Am J Psychiatry 2000 Nov;157(11):1843-6

Sexual orientation in a U.S. national sample of twin and nontwin sibling pairs.
Kendler KS, Thornton LM, Gilman SE, Kessler RC.
Department of Psychiatry, Medical College of Virginia, Virginia Commonwealth University, Richmond 23298-0126, USA.

OBJECTIVE: Although previous studies have suggested that sexual orientation is influenced by familial factors, which may be partly genetic, these studies have relied on unrepresentative and potentially biased samples. The authors attempted to estimate the role of genetic and environmental factors in the determination of sexual orientation in a more representative sample. METHOD: Sexual orientation was assessed by a single item on a self-report questionnaire in a U.S. national sample of twin and nontwin sibling pairs. Sexual orientation was classified as heterosexual or nonheterosexual (bisexual or homosexual). The authors compared the similarity of sexual orientation in the monozygotic twins to the similarity in the same-sex dizygotic twins, all dizygotic twins, the same-sex dizygotic twins and sibling pairs, and all dizygotic twins and sibling pairs. Biometrical twin analyses were performed. RESULTS: All analyses demonstrated familial resemblance for sexual orientation. Resemblance was greater in the monozygotic twins than in the dizygotic twins or in the dizygotic twins plus nontwin siblings. Biometrical twin modeling suggested that sexual orientation was substantially influenced by genetic factors, but family environment may also play a role. No evidence was found for a violation of the equal-environment assumption regarding monozygotic and dizygotic twin pairs. CONCLUSIONS: Familial factors, which are at least partly genetic, influence sexual orientation. The results of these analyses should be interpreted in the context of low statistical power and the use of a single item to assess the complex phenotype of sexual orientation.
PMID: 11058483 [PubMed - indexed for MEDLINE]

Orv Hetil 1998 Feb 1;139(5):247-9

[Genetic evaluation of male homosexuality].
[Article in Hungarian]
Gasztonyi Z.
OKI, Humangenetikai es Teratologai Osztaly, WHO Orokletes Artalmak Tarsadalmi Megelozese Egyuttmukodesi Kozpont, Budapest.

The family trees of 16 homosexual males are evaluated in the material of their Genetic Counselling Clinic. The familial cluster of three cases corresponded to the X-linked recessive inheritance. The results of family, twin and adoption studies are reviewed and the recent findings of molecular genetic and brain researches are summarised. Male homosexuality comprises of different subgroups, but one major entity is caused by X-linked recessive gene(s). This genetic background represent a predisposition which is triggered or suppressed by external factors.
PMID: 9489379 [PubMed - indexed for MEDLINE]

J Homosex 1995;28(1-2):91-113

A critique of the possibility of genetic inheritance of homosexual orientation.
Haynes JD.
State University of New York, College at Buffalo 14222, USA.

Many workers in human sexuality have tried to discover causes of sexual orientation. No one theory has proved to be satisfactory. Studies of monozygotic and dizygotic twins, some of whom have been reared separately and some together, suggest that there may be an inherited component of homosexuality. Other studies, particularly those concerned with the evolution of human sexuality, question such a possibility. A further question arises because a large part of the human population is neither exclusively homosexual nor exclusively heterosexual. This paper will examine the evidence for genetic inheritance presented by twin and family studies. It will explore ways in which a gene favoring a homosexual orientation but not reproduction could continue to exist in a population. The importance of defining terms that refer to sexual orientation will be discussed in the context of determining exactly what may be inherited. Finally, the effects of accepting genetic inheritance as the cause of sexual orientation will be discussed.
PMID: 7560926 [PubMed - indexed for MEDLINE]

Arch Sex Behav 1993 Jun;22(3):187-206

Homosexual orientation in twins: a report on 61 pairs and three triplet sets.
Whitam FL, Diamond M, Martin J.
Department of Sociology, Arizona State University, Tempe 85287-2101.

Twin pairs in which at least one twin is homosexual were solicited through announcements in the gay press and personal referrals from 1980 to the present. An 18-page questionnaire on the "sexuality of twins" was filled out by one or both twins. Thirty-eight pairs of monozygotic twins (34 male pairs and 4 female pairs) were found to have a concordance rate of 65.8% for homosexual orientation. Twenty-three pairs of dizygotic twins were found to have a concordance rate of 30.4% for homosexual orientation. In addition, three sets of triplets were obtained. Two sets contained a pair of monozygotic twins concordant for sexual orientation with the third triplet dizygotic and discordant for homosexual orientation. A third triplet set was monozygotic with all three concordant for homosexual orientation. These findings are interpreted as supporting the argument for a biological basis in sexual orientation.
PMID: 8494487 [PubMed - indexed for MEDLINE]

Arch Gen Psychiatry 1993 Mar;50(3):217-23

Heritable factors influence sexual orientation in women.
Bailey JM, Pillard RC, Neale MC, Agyei Y.
Department of Psychology, Northwestern University, Evanston, Ill 60208.

Homosexual female probands with monozygotic cotwins, dizygotic cotwins, or adoptive sisters were recruited using homophile publications. Sexual orientation of relatives was assessed either by asking relatives directly, or, when this was impossible, by asking the probands. Of the relatives whose sexual orientation could be confidently rated, 34 (48%) of 71 monozygotic cotwins, six (16%) of 37 dizygotic cotwins, and two (6%) of 35 adoptive sisters were homosexual. Probands also reported 10 (14%) nontwin biologic sisters to be homosexual, although those sisters were not contacted to confirm their orientations. Heritabilities were significant using a wide range of assumptions about both the base rate of homosexuality in the population and ascertainment bias. The likelihood that a monozygotic cotwin would also be homosexual was unrelated to measured characteristics of the proband such as self-reported history of childhood gender nonconformity. Concordant monozygotic twins reported similar levels of childhood gender nonconformity.
PMID: 8439243 [PubMed - indexed for MEDLINE]

Br J Psychiatry 1992 Mar;160:407-9

Homosexuals who are twins. A study of 46 probands.
King M, McDonald E.
Royal Free Hospital School of Medicine, London.

Forty-six homosexual men and women who were twins took part in a study of their sexuality and that of their co-twin. Discordance for sexual orientation in the monozygotic pairs confirmed that genetic factors are insufficient explanation of the development of sexual orientation. There was a high level of shared knowledge of sexual orientation between members of twin pairs, and a relatively high likelihood of sexual relations occurring with same sex co-twins at some time, particularly in monozygotic pairs. The implications of these results for the study of the origins of sexual orientation and for twin research are discussed.
PMID: 1562871 [PubMed - indexed for MEDLINE]

Arch Gen Psychiatry 1991 Dec;48(12):1089-96

Comment in:

A genetic study of male sexual orientation.
Bailey JM, Pillard RC.
Department of Psychology, Northwestern University, Evanston, Ill 60208.

Homosexual male probands with monozygotic cotwins, dizygotic cotwins, or adoptive brothers were recruited using homophile publications. Sexual orientation of relatives was assessed either by asking relatives directly, or when this was impossible, asking the probands. Of the relatives whose sexual orientation could be rated, 52% (29/56) of monozygotic cotwins, 22% (12/54) of dizygotic cotwins, and 11% (6/57) of adoptive brothers were homosexual. Heritabilities were substantial under a wide range of assumptions about the population base rate of homosexuality and ascertainment bias. However, the rate of homosexuality among nontwin biological siblings, as reported by probands, 9.2% (13/142), was significantly lower than would be predicted by a simple genetic hypothesis and other published reports. A proband's self-reported history of childhood gender non-conformity did not predict homosexuality in relatives in any of the three subsamples. Thus, childhood gender nonconformity does not appear to be an indicator of genetic loading for homosexuality. Cotwins from concordant monozygotic pairs were very similar for childhood gender nonconformity.
PMID: 1845227 [PubMed - indexed for MEDLINE]

Behav Genet 1991 Jan;21(1):75-96

Sexual orientation, sexual identity, and sex-dimorphic behaviors in male twins.
Buhrich N, Bailey JM, Martin NG.
Department of Psychiatry, St. Vincent's Hospital, Sydney, Australia.

Sexual orientation, sexual identity, and sex-dimorphic behaviors were assessed concurrently and retrospectively, for childhood, in 95 pairs of male monozygotic (MZ) twins and 63 pairs of dizygotic (DZ) twins. There was a significantly higher rate of adult homosexuality among the MZ than among DZ twins. We employed a model-fitting approach using LISREL to test for genetic and environmental influences on variation for each trait singly and on the covariation among all six traits (three for childhood and three for adulthood). Univariate analyses confirmed the presence of familial factors for five of the six variables but were generally unable to distinguish shared environmental from genetic influences. Hierarchical tests of multivariate models supported the existence of an additive genetic factor contributing to the covariance among the variables. More restrictive multivariate models yielded a significant genetic influence on sexual orientation. Because of the different rates of orientation by zygosity and because of the restrictive nature of some of the multivariate models, our results are best considered tentative but do suggest that further biometrically oriented studies of sexual orientation and its correlates would be worthwhile.
PMID: 2018464 [PubMed - indexed for MEDLINE]