Recent Findings on Fraternal Birth Order and Homosexuality in Males
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The fraternal birth order effect (FBOE) is the finding that greater numbers of older brothers are statistically associated with greater odds of homosexuality in later-born males. The reliability of this effect has recently been confirmed in separate meta-analyses of 30 convenience samples (Blanchard, 2018a) and six probability samples (Blanchard, 2018b), together totaling 9526 homosexual males and 458,138 heterosexual males. At the simplest level of analysis, homosexual males had more older brothers, adjusted for family size, than did heterosexual males in 35 of the 36 samples studied (p < .00000001).
The purpose of this Letter is to summarize findings on the FBOE from four new studies published after my 2018 meta-analyses. Two of the reviewed studies (Li & Wong, 2018; Xu, Norton, & Rahman, 2019) did not mention fraternal birth order in their titles, abstracts, or key words, making it possible they would be missed by typical search procedures. I therefore thought that this summary...