Sexual Orientation Are Younger Brothers More Likely to Be Gay?

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

Posted December 5, 2019 | Reregarded by Matt Huston


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In 1996, the psychologists Ray Blanctough and also Anthony Bogaert published a paper completing that homosexuality was more widespread in males who had actually older brothers. This, they asserted, had nopoint to execute via eco-friendly factors. Instead, they said that the result was biological in nature—having to do through the "maternal immune reaction to succeeding male pregnancies, with each male fetus enhancing the likelihood of an immune response of the mother." Such immune responses, according to the researchers, could create alterations in the breakthrough of brain structures pertained to sex-related orientation and also femininity.


The result, referred to as a Fraternal Birth Order result, has actually been replicated in many Western cultures. Now, new research study showing up in the journal Evolutionary Psychology recreated Blanchard and Bogaert"s discovery in a sample of Indonesian males.

You are watching: The fraternal birth-order effect refers to a factor that seems to be associated with:

"In cultures, homosexual men are feminized at various levels and also they have more older brothers than heterosexual males," state the authors of the research. "To evaluate whether femininity and also the fraternal birth order effect are universal features of male homosex-related choice or not, we built up original data from homosexual men, heterosexual guys, and heterosexual womales from Java (Indonesia)."


The researchers examined data from 116 homosex-related males and 62 heterosexual guys from western and main Java. They tallied the number of older and also younger organic siblings each participant had actually (including male and also female siblings). The researchers additionally tallied participants" sex-related orientation. Consistent via Blanchard and Bogaert"s findings in Western cultures, they found that homosexual men were even more most likely to have actually older brothers than heterosexual men. To be particular, homosex-related males in their sample had an average of 0.96 older brothers while heterosex-related men had actually an average of 0.68.


Interestingly, the researchers additionally found that homosexual men were even more likely to have actually older sisters, although this difference was smaller sized than the older brother distinction.

The researchers then tested whether homosexuality was visible in the facial functions of Indonesians. Using standardized photographs of heterosex-related and also homosex-related Indonesian guys, the researchers found that guys with older brothers were regarded to have more feminine facial attributes than guys without older brothers. They did not, however, find that homosex-related guys were any even more feminized in their facial functions than heterosexual males.

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The authors compose, "Taken together, these outcomes indicate the presence of a feminizing factor associated with male homosexuality that is partly determined by male birth order. This is continuous through the findings from Western societies and, for this reason, argues for a prevalent pathmethod that can apply to assorted populations."


Homosexuality has been a long-standing suggest of confusion for evolutionary psychologists. This is bereason it is difficult for a concept that places supreme focus on reproductive potential to explain actions that does not appear to promote this goal. One concept, according to the researchers, is that the Fraternal Birth Order effect reduces sibling rivalry. In other words, the Fraternal Birth Order impact could be nature"s attempt to minimize sibling problem. Additionally, it might be less about the feminization of younger brothers and also even more about the masculinization of older brothers. The researchers state, "It is unclear whether the Fraternal Birth Order effect must be seen as a feminizing effect that boosts with male birth rank or as an anti-feminizing impact that decreases through birth order."


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


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About the Author


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Mark Travers, Ph.D., is an Amerideserve to psychologist through degrees from Cornell College and also the University of Coloraperform Boulder.