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Homosexuality and Nurture

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. Blanchard R: Fraternal birth order and the maternal immune hypothesis of male homosexuality.

Horm Behav, 2001 Sep;40(2):105-14. In men, sexual orientation correlates with an individual’s number of older brothers, each additional older brother increasing the odds of homosexuality by approximately 33%. It has been hypothesized that this fraternal birth order effect reflects the progressive immunization of some mothers to Y-linked minor histocompatibility antigens (H-Y antigens) by each succeeding male fetus and the concomitantly increasing effects of such maternal immunization on the future sexual orientation of each succeeding male fetus.

According to this hypothesis, anti-H-Y antibodies produced by the mother pass through the placental barrier to the fetus and affect aspects of sexual differentiation in the fetal brain. This explanation is consistent with a variety of evidence, including the apparent irrelevance of older sisters to the sexual orientation of later born males, the probable involvement of H-Y antigen in the development of sex-typical traits, and the detrimental effects of immunization of female mice to H-Y antigen on the reproductive performance of subsequent male offspring.

The maternal immune hypothesis might also explain the recent finding that heterosexual males with older brothers weigh less at birth than heterosexual males with older sisters and homosexual males with older brothers weigh even less than heterosexual males with older brothers. Blanchard R, Bogaert AF: Homosexuality in men and number of older brothers. Am J Psychiatry, 1996 Jan;153(1):27-31. OBJECTIVE: This study investigated whether homosexual men have a higher mean birth order than heterosexual men primarily because they have more older brothers or because they have more older siblings of both sexes.

METHOD: For the main analyses, 302 heterosexual men were individually matched on year of birth with an equal number of heterosexual men. Each completed a self-administered, anonymous questionnaire concerning family background and other biodemographic information. RESULTS: Logistic regression analysis showed that homosexuality was positively correlated with the proband’s number of older brothers but not with older sisters, younger brothers, younger sisters, or parental age at the time of the proband’s birth.

Each additional older brother increased the odds of homosexuality by 33%. CONCLUSIONS: These results restrict the range of possible theories of the birth order phenomenon to those that can explain not only why older brothers increase the probability of homosexuality in later-born males but also why older sisters neither enhance this effect nor counteract it. Blanchard R, Bogaert AF: The relation of closed birth intervals to the sex of the preceding child and the sexual orientation of the succeeding child. J Biosoc Sci, 1997 Jan;29(1):111-8.

The study investigated whether homosexual men are, on average, born a shorter time after their next-older siblings than are heterosexual men. Because of mixed evidence that birth intervals are longer after a male child, the sex of the next-older sibling was included as a control variable. The probands were 220 heterosexual and 183 homosexual men with at least one older sibling examined in Southern Ontario in 1994-95. These completed a self-administered, anonymous questionnaire concerning their family background and other biodemographic information.

The results showed that birth interval was negatively correlated with sibship size, positively correlated with maternal age, and uncorrelated with paternal age. They also confirmed that birth intervals are longer after a male than after a female child. The mean birth intervals preceding heterosexual and homosexual males, however, were virtually identical, indicating that the association of short birth intervals with decreased sex hormone levels in cord blood is unrelated to the development of sexual orientation.

Blanchard R, Bogaert AF: Proportion of homosexual men who owe their sexual orientation to fraternal birth order: An estimate based on two national probability samples. Am J Hum Biol, 2004 Mar-Apr;16(2):151-7. Homosexuality in men correlates with an individual’s number of older brothers, greater numbers of older brothers being associated with a greater probability of homosexuality. There are reasons to believe that this relationship is causal rather than merely statistical, that is, that older brothers produce the increase in the probability of homosexuality for later-born males.

It is possible, under this assumption, to estimate the proportion of homosexual men who can attribute their sexual orientation to their birth order among their brothers (fraternal birth order). This statistic, the population attributable fraction (PAF), was computed on the combined archival data of 2,256 heterosexual and 71 homosexual men examined in survey studies of sexual behavior in the UK and the USA. The PAF was 28. 6%, with 95% confidence limits of 14. 8% and 48. 0%. These limits encompass the PAF of 15. 1% previously estimated with a Canadian sample.

The results indicate that the proportion of homosexual men whose sexual orientation is attributable to fraternal birth order constitutes a minority, but not a negligible minority, of all homosexual men. The fraternal birth order effect may reflect the progressive immunization of some mothers to Y-linked antigens by each succeeding male fetus, and the concomitantly increasing effects of antimale antibodies on the sexual differentiation of the brain in each succeeding male fetus. Blanchard R, Cantor JM, Bogaert AF, Breedlove SM, Ellis L: Interaction of fraternal birth order and handedness in the development of male homosexuality.

Horm Behav, 2006 Mar;49(3):405-14. The present study investigated evidence for an interaction between two of the best established etiologic factors, or markers of etiologic factors, in the literature on male homosexuality: fraternal birth order and hand preference. By combining five samples, the authors produced study groups of 1774 right-handed heterosexuals, 287 non-right-handed heterosexuals, 928 right-handed homosexuals, and 157 non-right-handed homosexuals. The results showed a significant (P = 0.

004) handedness by older brothers interaction, such that (a) the typical positive correlation between homosexuality and greater numbers of older brothers holds only for right-handed males, (b) among men with no older brothers, homosexuals are more likely to be non-right-handed than heterosexuals; among men with one or more older brothers, homosexuals are less likely to be non-right-handed than heterosexuals, and (c) the odds of homosexuality are higher for men who have a non-right hand preference or who have older brothers, relative to men with neither of these features, but the odds for men with both features are similar to the odds for men with neither.

These findings have at least two possible explanations: (a) the etiologic factors associated with non-right-handedness and older brothers-hypothesized to be hyperandrogenization and anti-male antibodies, respectively-counteract each other, yielding the functional equivalent of typical masculinization, and (b) the number of non-right-handed homosexuals with older brothers is smaller than expected because the combination of the older brothers factor with the non-right-handedness factor is toxic enough to lower the probability that the affected fetus will survive. Blanchard R, Ellis L: Birth weight, sexual orientation and the sex of preceding siblings. J Biosoc Sci, 2001 Jul;33(3):451-67. This study’s first objective was to compare the mean birth weights of homosexual and heterosexual men and women.

Its second objective was to investigate whether prior male and female fetuses have different effects on the birth weight of subsequent fetuses. The subjects were 3229 adult men and women (the probands), who weighed at least 2500 g at birth, and whose mothers knew the sex of the child (or fetus) for each pregnancy prior to the proband. Information on birth weight, maternal gravidity and other demographic variables was reported on questionnaires completed by the probands’ mothers. The results confirmed earlier reports that boys with older brothers weigh less at birth than boys with older sisters, but they did not confirm reports that girls with older brothers weigh less than girls with older sisters.

The results did not show across-the-board differences in the mean birth weights of homosexual versus heterosexual women or homosexual versus heterosexual men. However, the homosexual males with older brothers weighed about 170 g less at birth than the heterosexual males with older brothers. It is suggested that this pattern of results may reflect a maternal immune response to Y-linked minor histocompatibility antigens (H-Y antigens). According to this hypothesis, when the maternal immune response is mild, it produces only a slightly reduced birth weight, but when it is stronger, it produces a markedly reduced birth weight as well as an increased probability of homosexuality.

Blanchard R, Zucker KJ, Siegelman M, Dickey R, Klassen P: The relation of birth order to sexual orientation in men and women. J Biosoc Sci, 1998 Oct;30(4):511-9. Homosexual men have a higher mean birth order than do heterosexual men, primarily because they have a greater number of older brothers. The purpose of this study was to determine whether the same difference occurs in homosexual vs heterosexual women. The probands were 964 homosexual and heterosexual, male and female volunteers, from whom birth order data were collected with self-administered questionnaires. The homosexual men had more older brothers than the heterosexual men, but they did not have more older sisters, younger brothers, or younger sisters.

The homosexual women did not differ from the heterosexual women with regard to any class of sibling. These results are consistent with the hypothesis that the high birth order of homosexual men reflects the progressive immunization of certain mothers to H-Y antigen by succeeding male fetuses, and the increasing effects of H-Y antibodies on sexual differentiation of the brain in succeeding male fetuses. Bogaert AF: Birth order and sibling sex ratio in homosexual and heterosexual non-white men. Arch Sex Behav, 1998 Oct;27(5):467-73. Researchers interested in the development of sexual orientation have investigated predominantly or exclusively White samples.

To address this imbalance, the relations between sexual orientation and two biodemographic variables, birth order and sibling sex ratio, were examined in a sample of non-White men. The men (N = 823) were interviewed by investigators at the Kinsey Institute for Sex and Reproduction from 1938 to 1963. A significantly later birth order was observed for non-White homosexual men relative to non-White heterosexual men. Non-White homosexual men also had an elevated sibling sex ratio. Results add to the generalizability of the birth order and sibling sex ratio effects previously observed to occur in (White) homosexual men. Bogaert AF: Interaction of older brothers and sex-typing in the prediction of sexual orientation in men. Arch Sex Behav, 2003 Apr;32(2):129-34.

Numerous studies indicate that homosexual men have, on average, more older brothers than do heterosexual men. One explanation of the “older brother” effect comes from D. J. Bem (1996), who argued that an increased number of older brothers in homosexual men can result if older brothers enhance a feminine boy’s sense of being different from (and hence his attraction to) other males. Thus, he argued that an interaction between older brothers and sex-typing will occur, such that when a boy is high in femininity, number of older brothers may strongly predict a homosexual orientation; and when a boy is low in femininity, number of older brothers may not or may only weakly predict a homosexual orientation.

In this study, the relations among sibling characteristics (e. g. , older brothers), childhood sex-typing, and sexual orientation were examined using a database of a large sample of homosexual and heterosexual men (N > 1,000) archived at the Kinsey Institute (A. P. Bell, M. S. Weinberg, & S. K. Hammersmith, 1981a). No significant Older Brother x Sex-Typing interaction effect was observed. These results join other recent evidence that postnatal (e. g. , learning/environmental) mechanisms probably do not underlie the older brother effect in men. Cantor JM, Blanchard R, Paterson AD, Bogaert AF: How many gay men owe their sexual orientation to fraternal birth order? Arch Sex Behav, 2002 Feb;31(1):63-71.

In men, sexual orientation correlates with the number of older brothers, each additional older brother increasing the odds of homosexuality by approximately 33%. However, this phenomenon, the fraternal birth order effect, accounts for the sexual orientation of only a proportion of gay men. To estimate the size of this proportion, we derived generalized forms of two epidemiological statistics, the attributable fraction and the population attributable fraction, which quantify the relationship between a condition and prior exposure to an agent that can cause it. In their common forms, these statistics are calculable only for 2 levels of exposure: exposed versus not-exposed. We developed a method applicable to agents with multiple levels of exposure–in this case, number of older brothers.

This noniterative method, which requires the odds ratio from a prior logistic regression analysis, was then applied to a large contemporary sample of gay men. The results showed that roughly 1 gay man in 7 owes his sexual orientation to the fraternal birth order effect. They also showed that the effect of fraternal birth order would exceed all other causes of homosexuality in groups of gay men with 3 or more older brothers and would precisely equal all other causes in a theoretical group with 2. 5 older brothers. Implications are suggested for the gay sib-pair linkage method of identifying genetic loci for homosexuality. Chivers ML, Bailey JM: Sexual orientation of female-to-male transsexuals: a comparison of homosexual and nonhomosexual types. Arch Sex Behav, 2000 Jun;29(3):259-78.

Homosexual and nonhomosexual (relative to genetic sex) female-to-male transsexuals (FTMs) were compared on a number of theoretically or empirically derived variables. Compared to nonhomosexual FTMs, homosexual FTMs reported greater childhood gender nonconformity, preferred more feminine partners, experienced greater sexual rather than emotional jealousy, were more sexually assertive, had more sexual partners, had a greater desire for phalloplasty, and had more interest in visual sexual stimuli. Homosexual and nonhomosexual FTMs did not differ in their overall desire for masculinizing body modifications, adult gender identity, or importance of partner social status, attractiveness, or youth.

These findings indicate that FTMs are not a homogeneous group and vary in ways that may be useful in understanding the relation between sexual orientation and gender identity. Purcell DW, Blanchard R, Zucker KJ: Birth order in a contemporary sample of gay men. Arch Sex Behav, 2000 Aug;29(4):349-56. The birth order of a contemporary North American sample of 97 gay men was quantified using Slater’s Index. For the 84 probands with at least one sibling, the results showed a late mean birth order compared with the expected value of . 50. Additional birth order indices derived from Slater’s Index suggested that the mean later birth order was accounted for more strongly by the proband’s number of older brothers than by his number of older sisters.

The present findings constitute a replication of a series of recent studies and add to the growing body of evidence that birth order is a reliable correlate of sexual orientation in males. Homosexuality and Nature Bailey JM, Bell AP: Familiality of female and male homosexuality. Behav Genet, 1993 Jul;23(4):313-22. We examined data from a large cohort of homosexual and heterosexual females and males concerning their siblings’ sexual orientations. As in previous studies, both male and female homosexuality were familial. Homosexual females had an excess of homosexual brothers compared to heterosexual subjects, thus providing evidence that similar familial factors influence both male and female homosexuality.

Furthermore, despite the large sample size, homosexual females and males did not differ significantly from each other in their proportions of either homosexual sisters or homosexual brothers. Thus, results were most consistent with the possibility that similar familial factors influence male and female sexual orientation. However, because results conflicted with those of some other studies, and because siblings’ sexual orientations were obtained in a manner likely to yield more errors than in these other, smaller studies, further work is needed using large samples and more careful methods before the degree of cofamiliality of male and female homosexuality can be resolved definitively. We also examined whether some parental influences comprised shared environmental effects on sexual orientation.

Scales attempting to measure such influences failed to distinguish subjects with homosexual siblings from subjects with only heterosexual siblings and, thus, did not appear to measure shared environmental determinants of sexual orientation. Bailey JM, Dunne MP, Martin NG: Genetic and environmental influences on sexual orientation and its correlates in an Australian twin sample. J Pers Soc Psychol, 2000 Mar;78(3):524-36. 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. Hamer DH, Hu S, Magnuson VL, Hu N, Pattatucci AM: A linkage between DNA markers on the X chromosome and male sexual orientation. Science, 1993 Jul 16;261(5119):321-7.

The role of genetics in male sexual orientation was investigated by pedigree and linkage analyses on 114 families of homosexual men. Increased rates of same-sex orientation were found in the maternal uncles and male cousins of these subjects, but not in their fathers or paternal relatives, suggesting the possibility of sex-linked transmission in a portion of the population. DNA linkage analysis of a selected group of 40 families in which there were two gay brothers and no indication of nonmaternal transmission revealed a correlation between homosexual orientation and the inheritance of polymorphic markers on the X chromosome in approximately 64 percent of the sib-pairs tested.

The linkage to markers on Xq28, the subtelomeric region of the long arm of the sex chromosome, had a multipoint lod score of 4. 0 (P = 10(-5), indicating a statistical confidence level of more than 99 percent that at least one subtype of male sexual orientation is genetically influenced. Hershberger SL, Segal NL: The cognitive, behavioral, and personality profiles of a male monozygotic triplet set discordant for sexual orientation. Arch Sex Behav, 2004 Oct;33(5):497-514. The neurohormonal theory of sexual orientation proposes that homosexual men and homosexual women are exposed prenatally to a hormonal environment that is similar to that of the other sex.

Prenatal exposure to an opposite-sex hormonal environment may lead the nervous system to develop in a manner consistent with the opposite sex. If this cross-sex exposure occurs, one prediction would be that the cognitive ability profile of homosexual men would be similar to that of heterosexual women. This study examined a set of male monozygotic triplets, aged 21 years, discordant for sexual orientation: 2 of the triplets were heterosexual, 1 was homosexual. The triplets were administered measures of 23 domains of cognitive ability, as well as measures of sexual orientation and masculinity/femininity. On the measures of cognitive ability, the triplets performed similarly, yet consistent differences were found between the 2 heterosexual triplets and the 1 homosexual cotriplet.

Differences having the same pattern were found for the number of Schafer homosexuality signs on the Rorschach, and on a homosexuality scale derived from items on the Minnesota Multiphasic Personality Inventory–2 (MMPI-2). Responses from the homosexual triplet were in a more feminine direction than responses from his 2 heterosexual cotriplets on measures of masculinity-femininity, which included measures derived from Rorschach responses, the MMPI-2 Masculinity-Femininity scale, the Bem Sex Role Inventory, and the Boyhood Gender Conformity Scale. Responses to the 16 Personality Factor Questionnaire also distinguished the 1 homosexual triplet from the 2 heterosexual cotriplets. These findings support the view that the prenatal hormonal environment may have enduring effects on selected behavioral traits.

Hu S, Pattatucci AM, Patterson C, Li L, Fulker DW, Cherny SS, Kruglyak L, Hamer DH: Linkage between sexual orientation and chromosome Xq28 in males but not in females. Nat Genet, 1995 Nov;11(3):248-56. We have extended our analysis of the role of the long arm of the X chromosome (Xq28) in sexual orientation by DNA linkage analyses of two newly ascertained series of families that contained either two gay brothers or two lesbian sisters as well as heterosexual siblings. Linkage between the Xq28 markers and sexual orientation was detected for the gay male families but not for the lesbian families or for families that failed to meet defined inclusion criteria for the study of sex-linked sexual orientation.

Our results corroborate the previously reported linkage between Xq28 and male homosexuality in selected kinships and suggest that this region contains a locus that influences individual variations in sexual orientation in men but not in women. McCormick CM, Witelson SF: Functional cerebral asymmetry and sexual orientation in men and women. Behav Neurosci, 1994 Jun;108(3):525-31. Functional cerebral asymmetry was assessed in 32 gay men and 32 heterosexual men and in 30 lesbians and 30 heterosexual women with a linguistic dichotic listening test. All groups showed the typical greater right-ear accuracy and, by inference, left-hemisphere representation for language functions. As shown repeatedly in previous studies (e. g. , M. P. Bryden, 1982), among heterosexuals, consistent-right-handers showed greater perceptual asymmetry than did non consistent-right-handers.

In contrast, gay men and lesbians did not show an association between hand preference and magnitude of perceptual asymmetry. The results indicate different patterns of functional cerebral asymmetry in gay men and lesbians compared with heterosexual people and specifically, less association between motoric and linguistic components of cerebral asymmetry. This suggestion of atypical patterns of functional asymmetries is consistent with previous results of an increased prevalence of left-hand preference among gay men and lesbians compared with the heterosexual population (C. M. McCormick, S. F. Witelson, & E. Kingstone, 1990; C. M. McCormick & S. F. Witelson, 1991).

The finding of an association between aspects of functional asymmetry, a neurological characteristics likely present from birth, and sexual orientation suggests that a neurobiological factor is involved in the origins of sexual orientation. Rahman Q: Fluctuating asymmetry, second to fourth finger length ratios and human sexual orientation. Psychoneuroendocrinology, 2005 May;30(4):382-91. Sexual orientation in humans may be influenced by levels of prenatal sex steroids which canalise neurodevelopment along sex-typical (heterosexual) or sex-atypical (homosexual) lines. Some evidence for sexual-orientation-related differences in putative somatic markers of prenatal sex hormones supports this view. A competing theory asserts that human homosexuality is due to developmental instability (DI) because it represents a shift from the species-typical pattern of heterosexual orientation.

Evidence for elevated rates of non-right handedness among homosexuals provides limited support for this account. The current study tested both theories by examining nine bilateral somatic traits in 120 healthy heterosexual and homosexual men and women in order to compute second to fourth finger length ratios (2D:4D), a measure ascribed to levels of prenatal sex steroids, and fluctuating asymmetry (FA), a measure of DI. Homosexual men and women had significantly lower right hand 2D:4D ratios (even after controlling for handedness, height and weight differences) in comparison to heterosexuals, but sexual orientation did not relate to composite FA scores.

The findings constrain the number of possible neurodevelopmental pathways responsible for sexual orientation in humans. Rahman Q, Wilson GD: Sexual orientation and the 2nd to 4th finger length ratio: evidence for organising effects of sex hormones or developmental instability? Psychoneuroendocrinology, 2003 Apr;28(3):288-303. It has been proposed that human sexual orientation is influenced by prenatal sex hormones. Some evidence examining putative somatic markers of prenatal sex hormones supports this assumption. An alternative suggestion has been that homosexuality may be due to general developmental disruptions independent of hormonal effects.

This study investigated the ratio of the 2nd to 4th finger digits (the 2D:4D ratio), a measure often ascribed to the organisational actions of prenatal androgens, and the fluctuating asymmetry (FA-a measure of general developmental disruption) of these features, in a sample of 240 healthy, right handed and exclusively heterosexual and homosexual males and females (N=60 per group). Homosexual males and females showed significantly lower 2D:4D ratios in comparison to heterosexuals, but sexual orientation did not relate to any measures of FA. The evidence may suggest that homosexual males and females have been exposed to non-disruptive, but elevated levels of androgens in utero. However, these data also draw attention to difficulties in the interpretation of results when somatic features are employed as biological markers of prenatal hormonal influences. Rahman Q, Wilson GD, Abrahams S: Biosocial factors, sexual orientation and neurocognitive functioning. Psychoneuroendocrinology, 2004 Aug;29(7):867-81.

It has been proposed that sexual orientation related differences in cognitive performance are either due to the actions of prenatal factors early in development or the influence of gender role learning. This study examined the performance of 240 healthy, right-handed heterosexual and homosexual males and females (N = 60 per group) on a battery of cognitive tasks comprising mental rotation, judgement of line orientation (JLO), verbal fluency, perceptual speed and object location memory. Measures were also taken of the psychological gender, birth order, sibling sex composition and the 2nd to 4th finger length ratios of the right and left hands.

A series of stepwise regression analyses revealed that sex and sexual orientation were the strongest predictors of cognitive performance, with IQ also contributing considerable variance. Psychological gender (M/F scores) added a small, but significant, amount of variance to mental rotation and perceptual speed scores in addition to these main factors, but prenatal hormone related indices, such as 2nd to 4th finger ratios, birth order and sibling sex composition added no independent predictive power. These findings are discussed in relation to biosocial influences on cognitive differences between heterosexuals and homosexuals. Rice G, Anderson C, Risch N, Ebers G: Male homosexuality: absence of linkage to microsatellite markers at Xq28. Science, 1999 Apr 23;284(5414):665-7.

Several lines of evidence have implicated genetic factors in homosexuality. The most compelling observation has been the report of genetic linkage of male homosexuality to microsatellite markers on the X chromosome. This observation warranted further study and confirmation. Sharing of alleles at position Xq28 was studied in 52 gay male sibling pairs from Canadian families. Four markers at Xq28 were analyzed (DXS1113, BGN, Factor 8, and DXS1108). Allele and haplotype sharing for these markers was not increased over expectation. These results do not support an X-linked gene underlying male homosexuality. Turner WJ: Homosexuality, type 1: an Xq28 phenomenon. Arch Sex Behav, 1995 Apr;24(2):109-34.

Despite the absence of phenotypic manifestations in alternating generations characteristic of X-linked disorders, a thesis is presented that a major type of Kinsey grades 5 and 6 male homosexuality is determined by a gene in the Xq28 region. A total of 133 families in 78 kinshps of male and female homosexual probands, in addition to 116 families (including those of 40 famous homosexuals) from the literature, revealed an unbalanced secondary sex ratio in the maternal generation of male, but not of female, homosexuals. On the maternal side, in this study, the ratio of all uncles to all aunts of 90 males homosexuals was 132/209, chi 2 = 8. 52, p = 0. 004. On the maternal side for the total of all sources, the ratio of uncles to aunts of male homosexuals was 241/367, chi 2 = 13. 20; p < 0. 0001.

The male/female ratio of the total number of maternal sibships bearing homosexuals (310/628: 0. 491) was a measure of fetal wastage of the mothers’ male sibs; 49%. This ratio was very close to that of the total number of children born to fathers affected with any one of nine Xq28-linked male semilethal conditions (255/508: ratio 0. 556); for the difference between the two populations chi 2 = 0. 859, p = 0. 354. The male/female ratio of the total number of children born to female carriers of any one of these same conditions (1,232/1,062: ratio 1. 16), chi 2 = 13. 8 p < or = 0. 0001, is close to that of the total number of children in homosexual sibships: 511/413, chi 2 = 10. 4, p = 0. 005.

Between the number of children born to Xq28 mothers and to those born of mothers of homosexuals chi 2 = 0. 581, p = 0. 446. One may readily surmise that the maternal influence so often related to homosexuality may lie in the mother being a genetic carrier, with traits thereto associated. In this study, 65% of the mothers of homosexuals had no or only one live-born brother. Additional support for a genetic hypothesis is found in the occurrence of multiple instances–almost exclusively among maternal relatives–of infertility, spontaneous abortions, miscarriages, stillbirths, remaining single past age 30, and suicide. Of 109 male and 43 female homosexual index cases in the present series there were 6 instances of brother/sister homosexual sibships.

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