Homosexuality and heterosexuality in identical twins

Homosexual orientation in twins: a report on 61 pairs and three triplet sets

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.

Sperm competition and the persistence of genes for male homosexuality

Homosexuality is increasingly recognized as having a genetic component. Why then does it persist, when common sense suggests that it should result in fewer offspring? Monozygotic-twin studies permit a rough estimate of the importance of genetics (70%) in the development of male homosexuality, and the proportion of homosexuals remains constant: Fisher's Theorem then tells us there is an advantage to the heterozygote, which we find need be no greater than 2%. Behavior and sperm competition suggest what this advantage might be.

Homosexuality, sexual dysfunction, and incest in male identical twins

This paper describes a set of adult male monozygotic twins concordant for homosexuality. When initially seen they were living together in a highly ambivalent incestual relationship. Both were dysfunctional sexually (impotent) outside the twinship. Individual and conjoint therapy has assisted each of them with separation and the establishment of more appropriate and sexually functional love relationships.

Is homosexuality familial? A review, some data, and a suggestion

This report summarizes evidence that sexual orientation is familial. Family studies report that homosexual subjects have more homosexual siblings than do heterosexual subjects and more than would be expected given population frequencies. Twin studies find in general a higher concordance in sexual orientation among monozygotic than among dizygotic twins. Evidence for associated psychopathology in homosexual subjects or their relatives is inconclusive. Possible sources of bias in these reports are discussed and a suggestion is offered that family studies should be undertaken in which the sexual orientation of relatives would be directly ascertained by interview.

A pair of monozygotic twins discordant for homosexuality: sex-dimorphic behavior and penile volume responses

In reports of identical twins discordant for homosexuality, the homosexual twins showed the effeminacy syndrome in childhood. This has been considered evidence that the homosexuality comes from the twin himself. The possibility that the heterosexual twin was denying homosexuality has never been excluded. A pair of identical male twins discordant for homosexuality are reported. They showed significant differences in their penile volume responses to moving pictures of male and female nudes indicative of sexual orientations consistent with their statements. The homosexual twin showed the effeminacy syndrome. Aspects of the syndrome can be induced in mammals by altering their hormonal environment during some critical period in their intrauterine development. Discordance for homosexuality in identical twins could be due to one's being exposed to a different hormonal level during such a critical period.

XYY karyotype in a pair of monozygotic twins: a 17-year life-history study

We have recently had the opportunity to investigate clinical and behavioural characteristics of a pair of 17-year-old twin boys who were discovered to have the XYY karyotype (18). Since these twins, known to this department since infancy, had been studied at the age of three, early direct findings as well as retrospective data are available regarding their symptoms and development.

Six homosexual siblings

A family consisting of several homosexual siblings is reported. All ten of the siblings in this family were to some degree homosexually experienced. Six were homosexual or predominantly homosexual, and five had a homosexual identity. The sex composition of the family was three females and seven males. Of the three females, one was homosexual, one was bisexual, and one had had some homosexual experience. There was one set of male dizygotic twins concordant for homosexuality in the family. All ten siblings were exposed to relatively similar family dynamics. The father's behavior toward the children and his wife was characterized by hostility, violence, and alcoholism. He was eventually murdered when the youngest child was 6 years old. The mother demonstrated qualities that were essentially opposite those of the father. She was described as affectionate and loving toward all the children, and she did not display violent behavior. The male homosexual sibling who was interviewed displayed a strong identification with his mother via a feminine self-identification, the adoption of the housewife role in a homosexual marriage, and the adoption of the passive role in anal intercourse. Only one other multiple homosexual sibling family has been reported in the scientific literature. This family consisted of 14 siblings, 11 male and three female. The four homosexual siblings in this family were all male and consisted of two sets of dizygotic twins. The behavior of the parents of this family was quite similar to the behavior of the parents in this study-a hostile and violent father and an affectionate and loving mother. These two multiple homosexual sibling families provide support for theories that emphasize the importance of early family relationships in the development of a homosexual adaptation. It is held that multiple homosexual sibling families can be employed as strategic data in testing various etiological theories of homosexuality.

A male monozygotic twinship discordant for homosexuality. A repertory grid study

The relative contribution of genetic and environmental factors to the development of homosexual behaviour is a controversial subject. The original suggestion that homosexuality is a purely inherited trait has been attributed to Krafft-Ebing (Kallmann, 1952). Perhaps the strongest support for this view was Kallmann's series of 40 male monozygotic twin pairs showing 100 per cent concordance for the overt practice and quantitative rating of homosexual behaviour (Kallmann, 1952). This report has been criticized, and Kallmann later conceded that the 100 per cent concordance was possibly a statistical artefact (Kallmann, 1960). Habel (1950), who obtained the index twins from a prison population, found concordant homosexuality in 3 out of 5 monozygotic pairs (60 per cent), but none of 5 dizygotic pairs. In a more recent study, Heston and Shields (1968) found concordant homosexuality in 2 out of 5 monozygotic pairs (40 per cent) and 1 out of 7 dizygotic pairs (14 per cent). Heston and Shields (1968) also report a family with a sibship of 14 which included 3 pairs of male monozygotic twins, in two of which both twins were homosexual and in the third both heterosexual; no environmental factors which differentiated the homosexual from the heterosexual sibs could be detected. These workers also refute the suggestion that the tendency for monozygotic twins to be more alike with regard to homosexuality than dizygotic twins is related not to genetic factors but to problems of sexual identification which predispose to homosexuality (Money, 1962) by pointing out that there is no evidence that monozygotic twins per se are especially prone to become homosexual.