Birth Order and Sibling Sex Ratio of Children and Adolescents Referred to a Gender Identity Service

Doug P Vanderlaan,Hayley Wood,Kenneth J Zucker,Ray Blanchard,Jerson Laks

doi:10.1371/journal.pone.0090257

Doug P Vanderlaan, Hayley Wood + Show 3 more

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https://doi.org/10.1371/journal.pone.0090257

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Abstract

In adult male samples, homosexuality is associated with a preponderance of older brothers (i.e., the fraternal birth order effect). In several studies comparing gender dysphoric youth, who are likely to be homosexual in adulthood, to clinical or non-clinical control groups, the findings have been consistent with the fraternal birth order effect in males; however, less is known about unique sibship characteristics of gender dysphoric females. The current study investigated birth order and sibling sex ratio in a large sample of children and adolescents referred to the same Gender Identity Service (N = 768). Probands were classified as heterosexual males, homosexual males, or homosexual females based on clinical diagnostic information. Groups differed significantly in age and sibship size, and homosexual females were significantly more likely to be only children. Subsequent analyses controlled for age and for sibship size. Compared to heterosexual males, homosexual males had a significant preponderance of older brothers and homosexual females had a significant preponderance of older sisters. Similarly, the older sibling sex ratio of homosexual males showed a significant excess of brothers whereas that of homosexual females showed a significant excess of sisters. Like previous studies of gender dysphoric youth and adults, these findings were consistent with the fraternal birth order effect. In addition, the greater frequency of only children and elevated numbers of older sisters among the homosexual female group adds to a small literature on sibship characteristics of potential relevance to the development of gender identity and sexual orientation in females.

Highlights

Numerous studies have shown that there is a unique relationship between older brothers and homosexuality in males
In line with the maternal immune hypothesis, these findings strongly suggest that the basis of the association between older brothers and male sexual orientation is prenatal in origin
Post hoc Scheffemultiple range tests showed that all groups differed significantly from one another for age at the p, .001 level, and homosexual females had significantly fewer siblings compared to heterosexual males (p = .02)

Summary

Introduction

Numerous studies have shown that there is a unique relationship between older brothers and homosexuality in males (for review, see [1,2,3]). The unique relationship between older brothers and male sexual orientation has been termed the fraternal birth order effect. The most prominent hypothesis regarding the fraternal birth order effect posits that maternal-fetal interactions are responsible for this association [4,5] This effect is hypothesized to reflect a mother’s immune response to the gestation of successive male fetuses. Some mothers experience an immune response to male-specific antigens linked to the Y chromosome, an immune response involving the production of anti-male antibodies This immune response is thought to become more likely with each successive male fetus gestated and increases the probability that the typical action of male-specific antigens in the developing fetal brain will be diminished. No laboratory evidence directly supporting the maternal immune hypothesis is available, there are three main lines of evidence that indirectly demonstrate that this hypothesis is tenable

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