Representative samples, and especially population samples, are coveted sources of high-quality data in psychological science. In their recent article, Ablaza et al. (2021) used a population sample of marriages and registered partnerships in the Netherlands to examine the relationships between sibling characteristics and sexual orientation. We discuss the implications of this work for understanding the fraternal and sororal birth order effects on male and female same-sex attraction, and how both relate to maternal reproduction. The causal assumptions of these findings are explored, as well as the necessity of population-level data, and whether the data presented by Ablaza et al. are truly representative of the Dutch population.
Disclosure Statement
No potential conflict of interest was reported by the authors.
Notes
1 Terms related to sex (male/female), and gender (man, woman, transgender, non-binary, etc.) are often used interchangeably without adequate recognition that sex and gender do not align in all individuals (e.g., Zucker, Citation2017). Because sex and gender align in the overwhelming majority of individuals, we use man/male and woman/female interchangeably, and associated terms such as sister and brother to convey the biological sex of siblings.
2 Other genetic models for same-sex attraction have also been proposed, including overdominance (Zietsch et al., Citation2008), antagonistic pleiotropy (Zietsch et al., Citation2021), and polygenic effects (Ganna et al., Citation2019). These genetic effects could also be represented in diagrams like , but the framing of the FFE discussed presently, and by Ablaza et al. (Citation2021), is most strongly related to the SAGH.
Zietsch, B. P., Morley, K. I., Shekar, S. N., Verweij, K. J. H., Keller, M. C., Macgregor, S., Wright, M. J., Bailey, J. M., & Martin, N. G. (2008). Genetic factors predisposing to homosexuality may increase mating success in heterosexuals. Evolution and Human Behavior, 29(6), 424–433. https://doi.org/https://doi.org/10.1016/j.evolhumbehav.2008.07.002
Zietsch, B. P., Sidari, M. J., Abdellaoui, A., Maier, R., Långström, N., Guo, S., Beecham, G. W., Martin, E. R., Sanders, A. R., & Verweij, K. J. H. (2021). Genomic evidence consistent with antagonistic pleiotropy may help explain the evolutionary maintenance of same-sex sexual behaviour in humans. Nature Human Behavior, 5(9), 1251-1258. https://doi.org/https://doi.org/10.1038/s41562-021-01168-8
Ganna, A., Verweij, K. J. H., Nivard, M. G., Maier, R., Wedow, R., Busch, A. S., Abdellaoui, A., Guo, S., Sathirapongsasuti, J. F., 23andMe Research Team, Lichtenstein, P., Lundström, S., Långström, N., Auton, A., Mullan Harris, K., Beecham, G. W., Martin, E. R., Sanders, A. R., Perry, J. R. B., & Zietsch, B. P. (2019). Large-scale GWAS reveals insights into the genetic architecture of same-sex sexual behavior. Science, 365(6456), eaat7693. https://doi.org/https://doi.org/10.1126/science.aat7693
Ablaza, C., Kabátek, J., & Perales, F. (2021). Are sibship characteristics predictive of same sex marriage? An examination of fraternal birth order and female fecundity effects in population-level administrative data from the Netherlands. Journal of Sex Research.
Scott W. Semenyna is supported by a Postdoctoral Fellowship from the Social Sciences and Humanities Research Council (SSHRC) of Canada [Award #756-2020-0107]. Paul L. Vasey is supported by a Natural Sciences and Engineering Research Council (NSERC) Discovery Grant [RGPIN-2020-042444].
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