Understanding Homosexuality: Moving on from Patterns to Mechanisms
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A correlation between male sexual preference (heterosexual or homosexual) and the number of older brothers (the fraternal birth order effect, FBOE) has been a subject of numerous studies since it was first identified by Blanchard and Sheridan (1992) and Blanchard and Bogaert (1996). The target article (Blanchard, 2017) builds on two and a half decades of work by Blanchard and coworkers to argue that “fraternal birth order is, by far, the most broadly established factor influencing sexual orientation in men.” Although we agree that the evidence is strong that the FBOE contributes to male homosexuality, we argue that the predominant cause of homosexuality is something else. Genetic evidence suggests only a minor contribution of genetics, but there is strong indirect evidence for an epigenetic causation (Rice, Friberg, & Gavrilets, 2012). This new hypothesis has at least one major favorable attribute: it can be readily tested experimentally using current technology on human stem cells...
We thank P. L. Vasey for useful comments and suggestions. This work was partially supported by the National Institute for Mathematical and Biological Synthesis through NSF Award EF-0830858, with additional support from the University of Tennessee, Knoxville (SG) and the Swedish Research Council (UF).
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Conflict of interest
The authors declare that they have no conflict of interest.
This article does not contain any studies with human participants or animals performed by the authors.
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