Indonesia: Homosexual men displayed a higher number of older brothers than heterosexual men, even when sibship size was controlled for; men with older brothers seem also more feminine than those without older brothers

Male Homosexual Preference: Femininity and the Older Brother Effect in Indonesia. Sarah Nila et al. Evolutionary Psychology, November 19, 2019. https://doi.org/10.1177/1474704919880701

Abstract: 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.

Keywords: homosexuality, sexual dimorphism, fraternal birth order effect, sexual orientation, facial morphology

Discussion

Faces manipulated to resemble the average face of homosexual men are perceived as facially feminized, which indicates that homosexual men are facially feminized compared to heterosexual men, although higher facial femininity was not captured by the morphological analyses. Homosexual men displayed a greater number of older brothers, which was also associated with higher perceived femininity. They did not display a greater number of other sib categories.
When the shape of photographed men’s faces was partially transformed using an average homosexual face, the resulting faces appeared more feminized compared to similar transformation using an average heterosexual face. As age and masculinity are positively related in men (Boothroyd et al., 2005), a different mean age between the two groups of men used to build the average faces could trigger an unwanted difference in femininity/masculinity. However, the average faces were constructed with homosexual and heterosexual men sampled to minimize the difference in the age distribution (mean and variance). The resulting difference in mean age was less than 2 months, which is probably too small to generate a perceptible difference in masculinity. It is, however, possible that another variable, independent of sexual orientation, generated masculinity/femininity differences between the two samples, although this variable was not salary, education level, or ethnic origin, as these variables were not significantly different between the two groups. When the morphological difference between males and females was maximized during the discriminant analysis, homosexual men were not distributed differently compared to heterosexual men. Thus, any feminization displayed by homosexual men is not readily captured by the set of point coordinates or by their linear combinations. In this data set, when homosexual men were removed, heterosexual men and females were significantly differentiated by the discriminant analysis, although ∼20% of individuals were not correctly assigned. Other studies in occidental populations using similar morphological procedures to sex assign individuals typically report incorrect assignation within a range of 3–19% (e.g., Lee et al., 2014; Scott, Pound, Stephen, Clark, & Penton-Voak, 2010). This suggests that the present morphological analysis did not fully capture sexual facial differentiation. Alternatively, a real overlap could exist between male and female facial shapes.
Homosexual men displayed a greater number of older brothers than heterosexual men, suggesting that the older brother effect exists in Indonesian populations. The greater number of older brothers was present independently of possible higher fecundity observed in the families of homosexual men, as sib number was controlled for. A greater number of older sisters were also found, although it was no longer significant when sib number was controlled for, suggesting that this older sister effect is possibly driven by higher fecundity associated with the families of homosexual men. However, overall sibship size was not significantly differing between homosexual and heterosexual men. In Samoa, an older sister effect has been reported, although it is unclear if it remains after taking sib number into account. Thus far, the older brother effect has been found in all the populations in which it has been looked for (Western countries, Turkey, Iran, Hong Kong, Samoa, and Indonesia), suggesting that it is a general feature associated with MHP, although there is perhaps one counterexample (Brazil).
Using an index of facial femininity, homosexual men with more older brothers were not more feminized. As this femininity index does not capture differences between homosexual and heterosexual men, this result is preliminary. It has been shown that homosexuals with more older brothers are more feminine, the measure of femininity being a preference for the receptive role in anal intercourse (Blanchard, 2018a, 2018c), although a replication was equivocal (Swift-Gallant, Coome, Monks, & VanderLaan, 2018). The link between number of older brothers and femininity of homosexuals is not settled.
Independent of sexual orientation, men with older brothers seem more feminized than those without older brothers, consistent with the known effect of maternal parity on life-history traits (Skjærvø & Røskaft, 2013) and the hypothesis of the immune origin of the older brother effect. The possible maternal immune reaction primarily alters the development of sexually dimorphic brain structures relevant to sexual orientation, although other direct or indirect feminization effects are possible (Bogaert & Skorska, 2011). For example, birth weight is lower for newborn males with older brothers, but not for newborn females with older brothers or older sisters (Côté, Blanchard, & Lalumière, 2003). The higher influence of previous brothers to reduce male birth-weight, compared to female birth-weight, has been repeated in large samples (e.g., Nielsen et al., 2008) and is consistent with a lower birth weight of homosexual men relatively to heterosexuals (Xu, Norton, & Rahman, 2019).
Taken together, these results suggest the presence of a feminizing factor associated with male homosexuality that is partially determined by male birth order. This is consistent with the findings from Western societies and, thus, argues for a common pathway that could apply to various populations. Indeed, there are several lines of evidence showing that some specific aspects of the MHP are found cross-culturally. For example, early cross-gender or atypical behavior has been retrospectively assessed among men showing MHP in Brazil, Guatemala, Philippines, Thailand, Turkey, and United States (Cardoso, 2005, 2009; Whitam & Mathy, 1986; Whitam & Zent, 1984). These behaviors are displayed early during childhood and are found in distinct cultures; thus, providing another argument for a biological basis of this sexual preference. The developmental pathway of MHP could, therefore, rely on the same biological basis in distinct populations.
A feminizing factor is only a proximate explanation for the presence of MHP, and a more global framework is required to understand why such a feminizing factor exists. Interestingly, a sexually antagonistic gene that favors MHP in males and promotes fecundity (i.e., the ability to have children) in females has been proposed (Camperio-Ciani, Corna, & Capiluppi, 2004; Iemmola & Camperio-Ciani, 2009, but see Blanchard, 2012). Several studies support this hypothesis, and other studies have provided results that are consistent with predictions from this hypothesis (for a review, see Barthes, Crochet, & Raymond, 2015). The nature of the antagonistic factor has not yet been identified, but it has been proposed that it proximally enhances femininity in both sexes, resulting in the opposite effect on expected reproduction in each sex (Barthes, Godelle, & Raymond, 2013). Thus, under the sexually antagonistic gene hypothesis, the higher femininity of homosexual men, including their feminized sexual orientation, is seen as a pleiotropic cost of selection for higher fertility in females. In women, femininity of various traits is associated with fertility and is considered attractive (Hill & Hurtado, 1996; Law-Smith et al., 2006; Manning, Scutt, Whitehouse, & Leinster, 1997; Pawlowski, Boothroyd, Perrett, & Kluska, 2008; Rhodes, Simmons & Peters, 2005; Singh, 1993; Sugiyama, 2005).
Similarly, the immune origin of the older brother effect remains a proximate explanation and a broader context is required to understand why male birth rank interferes with sexual orientation in men. Birth order is obviously not heritable; thus, this trait cannot evolve by natural selection. However, the ability to generate a birth order effect is potentially heritable. Curiously, it is unclear whether the FBO effect should be seen as a feminizing effect that increases with male birth rank or as an anti-feminizing effect that decreases with birth order. The former phenomenon would be consistent with a mechanism that decreases competitive ability in later-born offspring, which would be useful to reduce, for example, the cost of sib competition in males. The latter phenomenon could operate when the firstborn males have special reproductive importance, for example, in societies promoting primogeniture (eldest son as the primary heir). However, the present study does not confirm such an association: Primogeniture has not been described in traditional Indonesian culture, particularly on the island of Java (Gultom, 2017), despite a significant FBO effect. The same situation is found in Turkey, where primogeniture was not traditionally enforced, but a significant FBO effect has been described (Blanchard, 2018a). Whether such a feminizing effect according to FBO exists in other mammals or not has apparently not been investigated (to our knowledge), although this would help to better understand this phenomenon in humans.