Fraternal Birth Order Effects on Personality: Will Reasonable Claims Require Extraordinary Evidence?
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Blanchard’s (2017)fourth meta-analysis of the fraternal birth order effect centers on the use of a novel metric, the older brother ratio, a measure which controls for the confounded variable of family size. If being born to a mother who has previously given birth to more boys increases the likelihood that man will be sexually attracted to other men, then there ought to be a bias toward gay men being born into large families and, as such, family size will be confounded with sexual orientation if the fraternal birth order effect is real. Given the relative rarity of men with a same-sex attraction in the general population, it is difficult to collect perfectly balanced groups of gay and straight men. Drawing gay and straight men from populations with different family sizes creates a problem when measuring the effect size. The probability that a study will find spurious relationships with all classes of siblings, not just older brothers, increases if gay men are drawn from larger...
This project was supported by funds from the Natural Sciences and Engineering Research Council (RGPIN 249685) awarded to Peter L. Hurd.
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