Sexual Orientation

Are Younger Brothers More Likely to Be Gay?

New research explores the effect of birth order on homosexuality in Indonesia.

Posted Dec 05, 2019

Source: monteiroleo/Pixabay

In 1996, the psychologists Ray Blanchard and Anthony Bogaert published a paper contending that homosexuality was more common in men who had older brothers. This, they claimed, had nothing to do with environmental factors. Instead, they suggested that the effect was biological in nature—having to do with the "maternal immune reaction to successive male pregnancies, with each male fetus increasing the likelihood of an immune response of the mother." Such immune responses, according to the researchers, could produce alterations in the development of brain structures related to sexual orientation and femininity.  

The effect, referred to as a Fraternal Birth Order effect, has been replicated in many Western societies. Now, new research appearing in the journal Evolutionary Psychology reproduced Blanchard and Bogaert's discovery in a sample of Indonesian men.

"In [Western] societies, homosexual men are feminized at various levels and they have more older brothers than heterosexual men," state the authors of the study. "To evaluate whether femininity and the fraternal birth order effect are universal features of male homosexual preference or not, we collected original data from homosexual men, heterosexual men, and heterosexual women from Java (Indonesia)."

The researchers examined data from 116 homosexual men and 62 heterosexual men from western and central Java. They tallied the number of older and younger biological siblings each participant had (including male and female siblings). The researchers also tallied participants' sexual orientation. Consistent with Blanchard and Bogaert's findings in Western societies, they found that homosexual men were more likely to have older brothers than heterosexual men. To be specific, homosexual men in their sample had an average of 0.96 older brothers while heterosexual men had an average of 0.68. 

Interestingly, the researchers also found that homosexual men were more likely to have older sisters, although this difference was smaller than the older brother difference.

The researchers then tested whether homosexuality was visible in the facial features of Indonesians. Using standardized photographs of heterosexual and homosexual Indonesian men, the researchers found that men with older brothers were perceived to have more feminine facial features than men without older brothers. They did not, however, find that homosexual men were any more feminized in their facial features than heterosexual men.

The authors write, "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."

Homosexuality has been a long-standing point of confusion for evolutionary psychologists. This is because it is difficult for a theory that places supreme emphasis on reproductive potential to explain behavior that does not appear to promote this goal. One idea, according to the researchers, is that the Fraternal Birth Order effect reduces sibling rivalry. In other words, the Fraternal Birth Order effect might be nature's attempt to minimize sibling conflict. Alternatively, it might be less about the feminization of younger brothers and more about the masculinization of older brothers. The researchers state, "It is unclear whether the Fraternal Birth Order 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."


Nila, S., Crochet, P.-A., Barthes, J., Rianti, P., Juliandi, B., Suryobroto, B., & Raymond, M. (2019). Male Homosexual Preference: Femininity and the Older Brother Effect in Indonesia. Evolutionary Psychology.