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

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.

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, 1 and the female fecundity effect (FFE) is the finding that the mothers of homosexual males produce more offspring than the mothers of heterosexual males. 2 There is a considerable amount of empirical evidence for the reproducibility of the FBOE [2][3][4][5] and the FFE [6][7][8]. Each effect relates to a specific biological theory of the aetiology of homosexuality in males-an immunological theory, in the case of the FBOE [9,10], and a genetic theory, in the case of the FFE [11].…”

Increased birth rank of homosexual males: disentangling the older brother effect and sexual antagonism hypothesis

“…Under this hypothesis, the OBE would be an adaptive plastic manipulation of the phenotype of male offspring by the mother. Nila et al (2019) have proposed that the OBE could decrease competitive ability of later-born sons, which would reduce the cost of sibling competition in males. Such a mechanism could be selected for in patrilocal societies, but probably not in matrilocal ones, where males usually migrate, thus reducing local competition.…”

“…This first is a statistical control of fecundity: raw number of older brothers are not transformed, but sibling size is used as a control variable, for example as a dependent variable in a regression (e.g. Ablaza et al, 2022; Nila et al, 2019). Second, transforming the raw data using various ad hoc metrics controlling for family size: the general form of these metrics is ( X + a )/( N + b ), where X is the number of older brothers (or any other sibling category under study), N is the total number of siblings, and { a, b } are two scalars.…”

The relationship between fraternal birth order and childhood sex‐atypical behavior among the Istmo Zapotec muxes

“…Semenyna, VanderLaan, & Vasey, 2017) also failed to find such an association among Samoan transgender androphilic males. An additional study found no association between facial femininity among Indonesian androphilic males and their number of older brothers (Nila et al., 2019). Given these results, it is presently unclear whether the fraternal birth order (FBO) differences found between transgender and cisgender androphilic males in Blanchard (2018) are directly related to differences in femininity among the two forms.…”

“…This is known as the fraternal birth order effect (FBOE). Among cisgender androphilic males, evidence for the FBOE has been found in Canada (Blanchard, 2004; Blanchard & Bogaert, 1996a; Blanchard, Cantor, Bogaert, Breedlove, & Ellis, 2006), China (Li & Wong, 2018), Greece (Apostolou, 2020), Indonesia (Nila, Barthes, Crochet, Suryobroto, & Raymond, 2018; Nila et al., 2019), 1 Italy (Camperio Ciani, Corna, & Capiluppi, 2004), the United Kingdom (King et al., 2005; Kishida & Rahman, 2015; Rahman, Clarke, & Morera, 2009; Xu, Norton, & Rahman, 2019), and the United States (Blanchard & Bogaert, 1996b, 1998; Wampold, 2018; Blanchard & Zucker, 1994; Zucker & Blanchard, 1994). Among transgender or highly femininized androphilic males, evidence for the FBOE has also been found in Brazil (VanderLaan et al., 2016), Canada (Blanchard & Sheridan, 1992; Blanchard, Zucker, Bradley, & Hume, 1995; VanderLaan, Blanchard, Wood, & Zucker, 2014), Samoa (Semenyna, VanderLaan, & Vasey, 2017; VanderLaan & Vasey, 2011; Vasey & VanderLaan, 2007), Iran (Khorashad et al., 2020), the Netherlands (Blanchard, Zucker, Cohen‐Kettenis, Gooren, & Bailey, 1996; Schagen, Delemarre‐van de Waal, Blanchard, & Cohen‐Kettenis, 2012), Spain (Gómez‐Gil et al, 2010), Turkey (Bozkurt, Bozkurt, & Sonmez, 2015), and the United Kingdom (Green, 2000).…”