Fraternal Birth Order Effect: General Overview of Theories

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The argument of how an individual’s sexual orientation is determined is constantly up for debate. Some might argue that one is born gay while others may disagree and say that sexual orientation is a choice. A theory called “the older brother effect” may be used to argue that sexual orientation may be biological rather than social according to researchers in Toronto (LeVay, 2016, p. 135). They believe that on average gay men have more older brothers compared to straight men. Because this theory cannot apply to females it has also been called the fraternal birth order effect. This theory was proposed individually by Eliot Slater, Pat Moran, and Edward Hare during the 1960s to the 70s; But it was researched more in depth by Blanchard, Kenneth Zucker, and Anthony Bogaert since the early 1990s. They have published at least 25 papers on this particular subject (LeVay, 2016, p. 135).

Eliot Slater created The Slater’s Index which is used to represent the position of a person’s birth order. To use this you divide the person’s total amount of siblings with the number of the person’s older siblings. Although, this index cannot be applied to men without any siblings for obvious reasons. According to the Toronto research group’s research, their study showed that from their samples of gay men, the Slater’s Index was slightly higher compared to those of straight men. The reason for gay men having a higher Slater’s Index than the straight men in their study could have been due to the fact that they could have more older brothers, more older sisters, fewer younger brothers, or fewer younger sisters (LeVay, 2016, p. 135-6). According to Bogaert and Blanchard, each older brother increases the chance that a man will be gay with the same set percentage of the previous value which is 33% (LeVay, 2016, p. 139). However, some other studies show different results. For example, a study done by Schwartz and his colleagues found that the first two older brothers only have a small impact but grows with three or more older brothers.

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Blanchard has emphasized that relying only on studies conducted on Western populations have limitations because male androphilia can be expressed differently depending on the individual’s culture (Vanderlaan & Vasey, 2009). In non-Western cultures there are gender categories for androphilic males that are outside of simply “men” and “women.” One study done by Doug P. Vanderlaan and Paul L. Vasey investigated birth order and sexual orientation in androphilic and gynephilic males in Samoa. Androphilic males in Samoa are referred to as fa’afafine which directly translates into “in the manner of a woman” (Vanderlaan & Vasey, 2009). However, fa’afafines do not limit themselves to only presenting themselves in a feminine manner. They present themselves in a variety of ways and can range from being very feminine to very masculine. This particular study was done on 133 men who self-identified as fa’afafine and 208 self-identified straight men by using questionnaires asking questions about sexual orientation and age. The study showed that although patterned differently, the older brother effect was apparent in the Samoan population as well. The gynephilic males showed to have fewer older brothers than expected, fa’afafines tended to be born later among their brothers compared to the gynephilic males, and the mothers of fa’afafine tended to have more children than the mothers of gynephilic males (Vanderlaan & Vasey, 2009).

It is important to note that the older brother effect is not the only factor in determining whether or not a man will be gay. But Blanchard, Bogaert, James Cantor, and Andrew Paterson used statistical techniques and estimated that around 15% of gay mens’ sexual orientation was determined by the older brother effect. This estimate was then increased to 29% by Blanchard and Bogaert using different samples as well as methods. However, these estimates were criticized due to the fact that there were historical trends in family size (LeVay, 2016, p. 140). In the early 19th century the average number of children an American woman would have was 7 which has since dropped to 2 today. If in fact 29% of gay men owe their sexual orientation to the older brother effect, then the number of gay men should have been much higher two centuries ago. Bogeart explained his side and believed that homosexuality in the U.S. has declined but this statement has been proven to be unlikely.

So what causes the older brother effect? A possible explanation is that the older brother effect does not necessarily increase a man being interested in the same sex but rather that he will admit to same-sex attraction when asked in a survey (LeVay, 2016, p. 141). This particular behavior can also be due to the fact that they are trying to stand out from the first born son by showing more rebellion or being open to new experiences and ideas. A biological factor for this theory can also be determined by the mother’s immune system. According to a study done by Bogeart and his colleagues the mother of gay men have immune systems that “remembers” the earlier pregnancy which could influence sexual orientation (LeVay, 2016, p. 143). When a woman is pregnant with males she may be exposed to certain antigens. Then they may develop antibodies so that when the mother’s immune system comes across the antigens again, the antibodies are able to block their function resulting in the fetus being less stereotypically masculine than their heterosexual counterparts.

In conclusion, although the older brother effect theory is certainly interesting and can explain homosexuality in males it is also fair to say that it is not always the case. However, it is important to acknowledge the research and the findings done by the mentioned researchers. Sexual orientation is still a widely talked about subject that always has new findings everyday. With improving technology and research, in the future, we will be able to learn more in depth about sexual orientations.

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Fraternal Birth Order Effect: General Overview of Theories. (2022, July 14). Edubirdie. Retrieved July 17, 2024, from
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