Ray Blanchard1 Centre for Addiction and Mental Health—Clarke Site, and Department of Psychiatry, Faculty of Medicine, University of Toronto, 250 College Street, Toronto, Ontario M5T 1R8, Canada
Received August 9, 2000, accepted March 1, 2001
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 sextypical 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.
Researchers began asking at least 60 years ago whether older siblings increase or decrease the odds of homosexuality in later born children. Since 1989, the
author, his colleagues, and other researchers have investigated this question in subjects examined in recent years and in subjects examined decades ago; in groups collected in England, The Netherlands, Canada, and the United States; in psychiatric patients and in nonpatient volunteers; in subjects examined in adulthood and in subjects examined in childhood; in transsexual subjects and in subjects contented with their assigned gender; and in men sexually attracted to adults as well as men attracted to children. In order to provide an overview of the results of the author’s research program, the birth order data from each study have been converted into a common metric, namely Slater’s Index (Slater, 1962). Slater’s Index equals the number of siblings older than the subject divided by the subject’s total number of siblings, that is, older siblings/(older siblings 1 younger siblings). This index cannot be calculated for only-children; for all other individuals, regardless of sibship size, it expresses birth order as a quantity between 0 and 1, where 0 corresponds to ﬁrstborn and 1 corresponds to lastborn. The results are shown in Fig. 1 (see also Table 1). In this ﬁgure, taller bars denote later births. The results show that, in every one of the 14 samples, the homosexual subjects were born later in their sibships than the heterosexual subjects. One can compute, as a kind of simple meta-analysis, the probability that the same group would show the higher mean 14 of 14 times purely by chance. That probability is about 1 in 10,000. These data therefore establish beyond much doubt that homosexual males do, on average, have higher birth orders than comparable heterosexuals. Because the sexual orientation of a newborn boy cannot operate backward in time to affect his number of older siblings, this ﬁnding implies