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.

中文翻译：

---

男性同性恋偏好：印度尼西亚的女性气质和哥哥效应。

男性同性恋偏好（MHP）是一个进化之谜，因为它具有部分遗传性并会增加生育成本。在西方社会，同性恋男性在不同程度上女性化，他们比异性恋男性有更多的哥哥。为了评估女性气质和兄弟出生顺序（FBO）效应是否是 MHP 的普遍特征，我们收集了来自爪哇岛（印度尼西亚）的同性恋男性、异性恋男性和异性恋女性的原始数据。面部照片被用来测试同性恋面孔与异性恋面孔相比是否女性化。我们发现，被操纵成与同性恋男性的平均面孔相似的面孔被认为是面部女性化的，这表明与异性恋男性相比，同性恋男性在面部上女性化，尽管形态学分析没有捕捉到更高的面部女性化特征。然后，家庭数据被用来检测同性恋者和异性恋者兄弟姐妹构成的差异。即使在控制同胞大小的情况下，同性恋男性的哥哥数量也多于异性恋男性，这表明印度尼西亚人口中存在 FBO 效应。与性取向无关，有哥哥的男性似乎比没有哥哥的男性更加女性化，这与 FBO 效应的免疫起源一致。总之，印度尼西亚的 MHP 部分女性化，而且他们有更多的哥哥。这些特征也与其他文化背景下的 MHP 相关，表明男性同性恋偏好的跨文化影响。女性化效应可以用进化论来解释，尽管即使开始确定近端机制，FBO 效应仍然无法解释。