Sibling Sex Ratio and Birth Order in Early-Onset Gender Dysphoric Adolescents

Abstract

Several sibship-related variables have been studied extensively in sexual orientation research, especially in men. Sibling sex ratio refers to the ratio of brothers to sisters in the aggregate sibships of a group of probands. Birth order refers to the probands’ position (e.g., first-born, middle-born, last-born) within their sibships. Fraternal birth order refers to their position among male siblings only. Such research was extended in this study to a large group of early-onset gender dysphoric adolescents. The probands comprised 94 male-to-female and 95 female-to-male gender dysphoric adolescents. The overwhelming majority of these were homosexual or probably prehomosexual. The control group consisted of 875 boys and 914 girls from the TRAILS study. The sibling sex ratio of the gender dysphoric boys was very high (241 brothers per 100 sisters) compared with the expected ratio (106:100). The excess of brothers was more extreme among the probands’ older siblings (300:100) than among their younger siblings (195:100). Between-groups comparisons showed that the gender dysphoric boys had significantly more older brothers, and significantly fewer older sisters and younger sisters, than did the control boys. In contrast, the only notable finding for the female groups was that the gender dysphoric girls had significantly fewer total siblings than did the control girls. The results for the male probands were consistent with prior speculations that a high fraternal birth order (i.e., an excess of older brothers) is found in all homosexual male groups, but an elevated sibling sex ratio (usually caused by an additional, smaller excess of younger brothers) is characteristic of gender dysphoric homosexual males. The mechanisms underlying these phenomena remain unknown.

Do Children With Gender Dysphoria Have Intense/Obsessional Interests?

“…High sibling sex ratios have been hypothesized to reflect elevated intrauterine testosterone levels, which may be a risk factor for ASD (for more details, see Mouridsen, Rich, & Isager, 2010). In youth samples, a high sibling sex ratio is associated with ASD (Mouridsen et al, 2010) as well as GD in boys (Blanchard, Zucker, Bradley, & Hume, 1995;Schagen, Delemarre-van de Waal, Blanchard, & Cohen-Kettenis, 2012;Zucker et al, 1997) but not GD in girls (Schagen et al, 2012). Future research should consider whether ASD traits are similarly associated with size at birth and sibling sex ratios in gender-referred children.…”

Redesigning Fraternal Birth Order Studies from Top to Bottom

“…The new study by Schagen et al (2012) suggests that FBO correlates to certain personality traits which, in turn, correlate to an individual's position on the dominant/insertive-submissive/ receptive axis. Of course, it is possible that a person's position along each of these two axes is a fixed and essential trait, but it is also possible that an individual's position on one axis is fixed at birth, and that his position on the other axis is not, but the fixed characteristic influences the post-natal development of the variable characteristic.…”

“…According to Schagen et al (2012), there have been 12 previously published studies relating to the relationship between birth order and male transsexualism and, in nine of those studies, the researchers distinguished between two well-recognized male to female transsexual (''MtFs'') phenotypes: androphilic MtFs and gynephilic MtFs. These studies have consistently shown that late FBO is a marker for androphilic MtFs but not for gynephilic MtFs (e.g., Blanchard & Sheridan, 1992).…”

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

“…In Schagen et al, although gender dysphoric males had significantly greater numbers of older brothers than controls, they also had significantly fewer older and younger sisters than controls. In addition, both the older and younger sibling sex ratios were significantly elevated in Schagen et al [23]. Thus, none of the prior studies that utilized gender dysphoric youth samples showed a clear and unique relationship between male sexual orientation and the presence of older brothers.…”

“…Schagen et al compared the sibship compositions of 95 gender dysphoric and 914 non-clinical control peripubertal females in a Dutch sample [23]. The gender dysphoric females had significantly fewer older siblings, younger brothers, and total siblings.…”