The Fraternal Birth-Order Effect as Statistical Artefact: Convergent Evidence from Probability Calculus, Simulated Data, and Multiverse Meta- Analysis

For a quarter of a century researchers investigating the origins of sexual orientation have largely ascribed to the fraternal birth order effect (FBOE) as a fact, holding that older brothers increase the odds of homosexual orientation among men through an immunoreactivity process. Here, we triangulate the empirical foundations of the FBOE from three distinct, informative perspectives: First, drawing on basic probability calculus, we deduce mathematically that the body of statistical evidence of the FBOE rests on the false assumptions that effects of family size should be controlled for and that this could be achieved through the use of ratio variables. Second, using a data-simulation approach, we demonstrate that by using ratio variables, researchers are bound to falsely declare corroborating evidence of an excess of older brothers at a rate of up to 100%, and that valid approaches attempting to quantify a potential excess of older brothers among homosexual men must control for the confounding effects of the number of older siblings. And third, we re-examine the empirical evidence of the FBOE by using a novel specification-curve and multiverse approach to meta-analysis. This yielded highly inconsistent and moreover similarly-sized effects across 64 male and 17 female samples (N = 2,778,998), compatible with an excess as well as with a lack of older brothers in both groups, thus, suggesting that almost no variation in the number of older brothers in men is attributable to sexual orientation.

A method yielding comparable estimates of the fraternal birth order and female fecundity effects in male homosexuality

The fraternal birth order effect (FBOE) is the finding that older brothers increase the probability of homosexuality in later-born males, and the female fecundity effect (FFE) is the finding that the mothers of homosexual males produce more offspring than the mothers of heterosexual males. In a recent paper, Khovanova proposed a novel method for computing independent estimates of these effects on the same samples and expressing the magnitude and direction of the effects in the same metric. In her procedure, only families with one or two sons are examined, and daughters are ignored. The present study investigated the performance of Khovanova's method using archived data from 10 studies, comprising 14 samples totalling 5390 homosexual and heterosexual subjects. The effect estimate for the FBOE showed that an increase from zero older brothers to one older brother is associated with a 38% increase in the odds of homosexuality. By contrast, the effect estimate for the FFE showed that the increase from zero younger brothers to one younger brother is not associated with any increase in the odds of homosexuality. The former result supports the maternal immune hypothesis of male homosexuality; the latter result does not support the balancing selection hypothesis.

Male Homosexual Preference: Femininity and the Older Brother Effect in Indonesia

Male homosexual preference (MHP) is an evolutionary enigma because it is partially heritable and imposes a fertility cost. In occidental societies, homosexual men are feminized at various levels and they have more older brothers than heterosexual men. To evaluate whether femininity and the fraternal birth order (FBO) effect are universal features of MHP or not, we collected original data from homosexual men, heterosexual men, and heterosexual women from Java (Indonesia). Facial photographs were used to test whether homosexual faces are feminized when compared with heterosexual ones. We found that faces manipulated to resemble the average face of homosexual men are perceived as facially feminized, suggesting that homosexual men are facially feminized compared to heterosexual men, although a higher facial femininity was not captured by morphological analyses. Then, family data were used to detect differences in siblings’ composition between homosexuals and heterosexuals. Homosexual men displayed a higher number of older brothers than heterosexual men, even when sibship size was controlled for, suggesting that the FBO effect exists in Indonesian populations. Independent of sexual orientation, men with older brothers seem more feminized than those without older brothers, consistent with the immune origin of the FBO effect. In conclusion, MHP in Indonesia is partially feminized and they have more older brothers. Such features are also associated with MHP in other cultural contexts, suggesting a cross-cultural effect of men homosexual preference. An evolutionary explanation is available for the feminizing effect, although the FBO effect remains unexplained even if proximal mechanisms start to be identified.