Does Birth Order Predict Sexual Orientation?

Does family birth order predict homosexual orientation?

If you read the popular press, you might think so. A recent report from Canadian psychologist, Anthony Bogaert, in the Proceedings of the National Academy of Sciences suggests a link for males, but not females, between adult sexual orientation and the number of older biological brothers. In other words, the more older biological brothers one has the more likely one is to be gay. Dr. Bogaert speculates a pre-natal cause for this observation, in part because the brothers had to be biologically related. Growing up with adopted or step-brothers did not predict sexual orientation. If the effect proves valid, then some gay little brothers might owe their sexuality in some way to the blood brothers who came before them.

However, the potential causal mechanism(s) for the fraternal birth order effect (FBOE) is unknown. Dr. Bogaert offers a theory he dubbed the maternal immune response. He speculates that when pregnant with a male, a woman’s body may identify proteins in male cells as foreign. With each new son, her body may develop a stronger immunity by producing antibodies which may in some way turn the fetus toward a homosexual outcome. Although there is no direct evidence for this hypothesis, news services worldwide validated the pre-natal theory with such headlines as “Sexual orientation of men determined before birth” (Reuters), “Men with older brothers more likely to be gay” (AP), and simply, “Born Gay: The Brother Factor” (Time Magazine).

So did the mainstream press get this one right? Does this study prove gay orientation is inborn?

Not really. First of all, in the studies where the FBOE has been found, the relationship explains very little. One effort to assess the extent of this effect estimates that only about 14% of gay men in North America, or about 1 million gay men, might owe their sexual orientation to the older brother effect. At best, this is highly speculative. Second, the reasons why even these men might be gay are not fully explained by this study. Stated technically, the study explains as little as 1% of the variation among all factors that might lead to homosexuality. For example, imagine a man without a coat standing outside in a howling snowstorm with an ice cube in his hand. (The news headline might read: “Hypothermia determined by ice cubes.”) However, the ice cube explains only a tiny fraction of why he might be shivering, a factor that sheds little insight on the larger picture. In context, the FBOE is an ice cube in the big picture of why someone might become gay.

Given the media treatment of the relatively small FBOE, what do you think the reaction would be to a larger but contradictory study that found no genetic or pre-natal effects? If you guessed the study would garner as much or more media interest, you’re not getting the point here.

Actually, we don’t have to guess. Another investigation, completely ignored by the media in 2002, casts doubt on the FBOE. Published in the American Journal of Sociology, Peter Bearman (Columbia) and Hannah Brückner (Yale) studied factors related to same-sex attraction in a large group of 20,745 adolescents. In contrast to Dr. Bogaert’s study of adults, the FBOE was not found. In fact, Bearman and Brückner identified only one significant sibling factor: males with an opposite-sex twin were more than twice as likely to report same-sex attraction compared to males with a male or female non-twin sibling. In direct contradiction to the FBOE, Bearman and Brückner found this caveat: the opposite-sex twin effect was eliminated by the presence of an older brother. Furthermore, they found no evidence for genetic or pre-natal effects.

Bearman and Brückner propose that in some cases, the presence of a twin sister with no older brother could push family and peer life away from culturally specific male gendered activities. Reviewing their results, they state: “Our results support the hypothesis that less gendered socialization in early childhood and preadolescence shapes subsequent same-sex romantic attraction.” They added, “If same-sex romantic attraction has a genetic component, it is massively overwhelmed by other factors.”

How many newspapers reported on this study? None. The study, according to Dr. Bearman, received no press. When I (Throckmorton) emailed to ask him about his study, he said I was only the second researcher to contact him. He could offer no reason for this oversight, other than the conclusions of his work were in direct contrast to conventional, dare we say, mainstream media wisdom.

The point here is not that biology is unimportant. It is possible that temperamental factors do have an impact in some, probably indirect, manner. Both nature and nurture research programs are important and since there are conflicting results, we must be open to additional study. This much is sure: sexuality is incredibly complex and socialization factors cannot be dismissed. Readers beware; one would never know that by reading the newspapers.

Here’s a question: why is it that one study that finds a weak sibling relationship and speculates a biological effect gets worldwide attention but another study that finds a weak sibling relationship but no evidence for a biological effect is completely ignored.

The answer to that question is probably worth a headline of its own.