Handedness and fraternal birth order impact on sexual orientation

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

Horm Behav. 2001 Sep;40(2):105-14.

Blanchard R.

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.

Centre for Addiction and Mental Health-Clarke Site, Department of Psychiatry, Faculty of Medicine, University of Toronto, 250 College Street, Toronto, Ontario M5T 1R8, Canada. Ray_Blanchard@camh.net

Review and theory of handedness, birth order, and homosexuality in men.

Laterality. 2008 Jan;13(1):51-70.

Blanchard R.

Research has repeatedly shown that older brothers increase the odds of homosexuality in later-born males. This phenomenon has been called the fraternal birth order effect. The most highly developed explanation of this phenomenon is the maternal immune hypothesis, which proposes that the fraternal birth order effect reflects the progressive immunisation of some mothers to male-specific antigens by each succeeding male foetus and the concomitantly increasing effects of anti-male antibodies on the sexual differentiation of the brain in each succeeding male foetus. Recent studies indicate that older brothers increase the odds of homosexuality in right-handed males but not in non-right-handed males. The present article explores how the maternal immune hypothesis might be extended or modified to account for the apparent interaction of older brothers and handedness. Two possibilities are considered: (1) non-right-handed foetuses are insensitive to the presence of maternal anti-male antibodies, and (2) mothers of non-right-handed foetuses do not produce anti-male antibodies.

Centre for Addiction and Mental Health, Toronto, ON, Canada. Ray_Blanchard@camh.net

The sex ratio of older siblings in non-right-handed homosexual men.

Arch Sex Behav. 2008 Dec;37(6):970-6. Epub 2006 Nov 28.

Blanchard R, Lippa RA.

This study tested the prediction, based on prior research, that non-right-handed homosexual men will report fewer than expected older brothers. Participants were 2486 heterosexual and homosexual, right-handed and non-right-handed, male and female adults, representing five samples collected for various projects by the second author. Data on sibship composition, sexual orientation, and hand-preference were gathered in the original research using on-line (Internet) or self-administered paper-and-pencil questionnaires. The non-right-handed homosexual men reported 83 older brothers per 100 older sisters, which was significantly lower than the human sex ratio of 106 live-born males per 100 live-born females. In contrast, the right-handed homosexual men reported 125 older brothers per 100 older sisters, which was significantly higher than the expected ratio. One possible explanation of these results is that older brothers increase the odds of homosexuality in right-handed males but decrease the odds of homosexuality in non-right-handed males. A second possibility is that older brothers decrease the probability that non-right-handed homosexual males will be represented in survey research. The latter scenario could arise if the combination of some biological factor associated with older brothers and some biological factor associated with non-right-handedness is so toxic that it kills the fetus or predisposes the individual to a condition (e.g., mental retardation, major mental illness) that makes him less likely to be available for research recruitment at Gay Pride parades (etc.) than other members of the gay community.

Law and Mental Health Program, Centre for Addiction and Mental Health, College Street Site, 250 College Street, Toronto, Ontario, M5T 1R8, Canada. Ray_Blanchard@camh.net

Sex ratio of older siblings in heterosexual and homosexual, right-handed and non-right-handed men.

Arch Sex Behav. 2008 Dec;37(6):977-81.

Blanchard R.

This study investigated why older brothers, which increase the odds of homosexuality in later-born males who are right-handed, have no effect or the opposite effect on later-born males who are non-right-handed. The specific question was whether the different results for the non-right-handed men have to do with the heterosexual non-right-handers or the homosexual non-right-handers. The human sex ratio at birth (106 males per 100 females) was used as a gold standard for determining which groups differ from the general population and in which direction. All usable data from previous studies were combined to obtain the largest possible sample (N = 8,201). The observed ratio of older brothers to older sisters was 105 for the heterosexual right-handers, 128 for the homosexual right-handers, 127 for the heterosexual non-right-handers, and 96 for the homosexual non-right-handers. The ratios for the homosexual right-handers and the heterosexual non-right-handers differed significantly from the expected value. These results suggest that both heterosexual and homosexual non-right-handers contribute to the older brothers x handedness x sexual orientation interaction.

Law and Mental Health Program, Centre for Addiction and Mental Health, 250 College Street, Toronto, Ontario, M5T 1R8, Canada. Ray_Blanchard@camh.net

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.

Blanchard R, Bogaert AF.

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.

Department of Psychiatry, University of Toronto, Toronto, Ontario, Canada. Ray_Blanchard@camh.net