Reasons for Caution About the Fraternal Birth Order Effect
The etiology of sexual orientation is an intriguing and poorly understood topic. Genes appear to play an important role, which raises the Darwinian paradox as to why genes for homosexuality would be maintained in the population (Zietsch et al., 2008). In terms of non-genetic influences, harsh childhood environments and childhood sexual abuse are associated with adult homosexuality, though the causality needs clarification and the mechanisms are not known (Zietsch et al., 2012). Also in this mix is the fraternal birth order effect (FBOE)—the focus of the target article (Blanchard, 2017). Numerous studies have found that homosexual men have a disproportionate number of older brothers relative to heterosexual men, though, again, the mechanisms by which this variable might relate to development of sexual orientation are unknown (notwithstanding the “maternal immune hypothesis” [Blanchard & Bogaert, 1996], for which I know of no direct evidence).
The target article concludes, based on a...
Thanks to Fiona Barlow and Kana Imuta for helpful comments on an earlier draft. Funding was provided by Australian Research Council (Grant No. FT160100298).
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