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. kathE@qimr.edu.au
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]