Saturday, September 9, 2006

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 likelier one is to be gay. Dr. Bogaert speculates a prenatal cause for this, in part because the brothers had to be biologically related. Growing up with adopted or stepbrothers did not predict sexual orientation. If the effect proves valid, some little brothers might somehow owe their homosexuality to 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 that may somehow turn the fetus toward a homosexual outcome.

Though there is no direct evidence, news services worldwide validated the prenatal 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 homosexual 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 only about 14 percent of homosexual 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 even these men might be homosexual are not fully explained. Stated technically, the study explains as little as 1 percent 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 homosexual.

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 prenatal effects? If you guessed the study would garner as much or more media interest, you’re not getting the point.

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 of Columbia and Hannah Bruckner of 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, Drs. Bearman and Bruckner 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 nontwin sibling. In direct contradiction to the FBOE, Drs. Bearman and Bruckner 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 prenatal effects.

Drs. Bearman and Bruckner propose that in some cases, a twin sister with no older brother could push family and peer life away from culturally specific male-gendered activities. In review, 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. According to Dr. Bearman, the study received no press. When I (Dr. Warren Throckmorton) e-mailed 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 that the conclusions of his work were in direct contrast to conventional, dare we say, mainstream media wisdom.

The point is not that biology is unimportant. Temperamental factors may 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 newswire.

Why does one study that finds a weak sibling relationship and speculates a biological effect get worldwide attention while another study that finds a weak sibling relationship but no evidence for a biological effect is completely ignored? The answer is probably worth a headline of its own.


Drs. Warren Throckmorton and Gary Welton are on the psychology faculty at Grove City College, Pennsylvania. Comments may be directed to Dr. Throckmorton at See:

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