We conducted follow-up, pairwise comparisons on the NLGN4Y results. The largest of these effects, using statistical effect size estimates appropriate to the nonparametric statistics we employ here, would be designated as medium (e.g., rs approximately = 0.30) ( Fig. The expected ordering of groups was confirmed for anti-NLGN4Y isoform 1 ( P = 0.000096) ( Fig. In this analysis, we also controlled for total number of pregnancies (live births and miscarriages) because this reproductive variable differed among the groups and was related to anti-NLGN4Y levels ( Materials and Methods and Tables S4 and S5). Mothers of gay sons with older brothers were predicted to have the highest concentration of antibodies, but we also predicted mothers of gay sons with no older brothers to have higher concentrations than mothers of heterosexual sons and women with no sons, on the assumption that the former could be immunized on a first male pregnancy or, for example, include a subset of mothers immunized by miscarried and possibly undetected male fetuses. In line with the maternal immune hypothesis, the predicted ordering for antibody concentrations to anti-NLGN4Y was as follows: women with no sons < mothers of heterosexual sons < mothers of gay sons with no older brothers < mothers of gay sons with older brothers. Given that the expected sex difference was confirmed for NLGN4Y, we then examined anti-NLGN4Y levels among women ( n = 142) using an omnibus, two-tailed, Jonckheere–Terpstra test for ordered alternatives. Finally, an incremental maternal immune response occurs to a Y-linked protein (SMCY/H-Y) in relation to prior male fetuses ( 21), a phenomenon that, at times, may underlie widespread alteration of fetal development and induce miscarriage ( 22). There is also evidence that maternal immune products, including antibodies, can enter the fetal compartment and pass the blood/brain barrier of the developing fetal brain ( 16– 20). Evidence in favor of a maternal immune hypothesis underlying FBO includes the finding that fetal material enters maternal circulation at parturition ( 12), as exemplified by male microchimerism occurring in mothers of sons ( 13, 14) and by hemolytic disease of the newborn ( 15). FBO is independent of potential confounds, such as maternal age ( 1, 11), and likely operates during prenatal life and not childhood ( 11). Although sexual orientation is likely determined by multiple (potentially independent) factors and FBO is associated with only a proportion of men’s sexual orientation ( 4– 6), the effect has been confirmed many times, including by independent investigators and in non-Western samples ( 2, 3, 7– 10). The results suggest an association between a maternal immune response to NLGN4Y and subsequent sexual orientation in male offspring.Ī fraternal birth order (FBO) effect exists in the sexual orientation of men, but not of women, with older brothers (and no other sibling characteristic) increasing the odds of male homosexuality ( 1– 3). In addition, after statistically controlling for number of pregnancies, mothers of gay sons, particularly those with older brothers, had significantly higher anti-NLGN4Y levels than did the control samples of women, including mothers of heterosexual sons.
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Results indicated women had significantly higher anti-NLGN4Y levels than men.
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Plasma from mothers of sons, about half of whom had a gay son, along with additional controls (women with no sons, men) was analyzed for male protein-specific antibodies. Immune assays targeting two Y-linked proteins important in brain development-protocadherin 11 Y-linked (PCDH11Y) and neuroligin 4 Y-linked (NLGN4Y isoforms 1 and 2)-were developed. This explanation posits that some mothers develop antibodies against a Y-linked protein important in male brain development, and that this effect becomes increasingly likely with each male gestation, altering brain structures underlying sexual orientation in their later-born sons. We conducted a direct test of an immunological explanation of the finding that gay men have a greater number of older brothers than do heterosexual men.