Thoughts on the recent “Older Brother” study by Anthony Bogaert

I originally wrote this for Writers On The Loose. I decided to cross-post it here to my own blog.

Yesterday, Zjabs wrote a column in which he linked to an L.A. Times article about a study suggesting a link between birth order amongst males born to the same mother and the probability that each male would be gay. Given my own recent column on the origins of sexual orientation, I thought it appropriate to take a closer look at the study. To that effect, I did a Google search and found a reprint of the researcher’s own write-up on the study. I would encourage anyone who is interested in this topic at all to take the time to read this more scientific article, as it provides a lot more details and gives more clarity as to exactly what conclusions can and can not be reached from this study.

To that effect, I think that Ms. Kaplan, the author of the L.A. Times article, has done the study a great disservice. In the very first part of the article, she suggests, “A mother’s antibodies may change with each boy, raising chances the next will be homosexual.” In including this statement in her article at the outset, Ms. Kaplan gives the impression that this is the conclusion reached in Bogaert’s study. This is entirely untrue. While it is true that Bogaert mentions this possibility, he also makes it clear that this is merely speculation on one possible explanation behind the real conclusion of his study. Indeed, Bogaert indicates that there is no direct evidence at this time to support maternal antibodies as a contributing factor in sexual orientation. So in this sense, Ms. Kaplan has run out ahead of the scientists she is talking about.

To be clear, Bogaert’s study is simply a statistical analysis of data on four sample sets of men. The relevant data concerned ages of the participants’ mothers at the time of their birth, the number of older and younger siblings of each sex they had, and the amount of time they were reared with each sibling as children. The study also incidated whether each sibling was a “biological” sibling (birthed by the same mother) or a “non-biological” sibling. This data was run through a number of statistical analyses to see if there was any strong correlation between a number of factors and the sexual orientation of the men. The only strong correlation found was that, statistically speaking, men who had a large number of older biological brothers were more likely to be gay.

In order to further determine whether this correlation was due to prenatal factors involved in the birth order or some other factors (such as the number of brothers raised with), Bogaert included a number of other factors in his statistical analysis. Bogaert spends a good deal of time explaining the rationale he used in determing what factors to analyze in order to exclude post-natal explanations, and I would direct everyone to his write-up for that information rather than trying to reproduce it here. I will say, however, that I found his approach rather thorough.

It is important to understand that what Bogaert’s analysis shows is that (1) there is an apparent link between birth order amongst biological brothers and the probability that each of them will be gay and (2) that link appears to be related to pre-natal factors (such as the speculation about the mother’s antibodies during pregnancy) rather than post-natal ones (such as childrearing factors or the psychological effects of interractions between brothers). It in no way casts any light on the subject of what that pre-natal factor (or factors, as there’s no reason to assume there’s only one factor involved). Indeed, Bogaert indicates that this is an area of research for other people — most likely those in the fields of biology and chemistry — and even cites some research being done in that area.

Bogaert’s statistical analysis itself will need to be examined more closely and duplicated. Most likely, further such analyses will need to be done to expand this study and address any gaps or methodological errors in it. Indeed, this study itself is a follow-up study of a previous one Bogaert had done along the same lines. So any attempt to read this particular study as “final proof” would be a tragic misunderstanding of the research process. Nonetheless, this study is vital in that it strongly indicates a valuable area for further research.

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