title: Sex ratio of older siblings in heterosexual and homosexual, right-handed and non-right-handed men.
abstract: This study investigated why older brothers, which increase the odds of homosexuality in later-born males who are right-handed, have no effect or the opposite effect on later-born males who are non-right-handed. The specific question was whether the different results for the non-right-handed men have to do with the heterosexual non-right-handers or the homosexual non-right-handers. The human sex ratio at birth (106 males per 100 females) was used as a gold standard for determining which groups differ from the general population and in which direction. All usable data from previous studies were combined to obtain the largest possible sample (N = 8,201). The observed ratio of older brothers to older sisters was 105 for the heterosexual right-handers, 128 for the homosexual right-handers, 127 for the heterosexual non-right-handers, and 96 for the homosexual non-right-handers. The ratios for the homosexual right-handers and the heterosexual non-right-handers differed significantly from the expected value. These results suggest that both heterosexual and homosexual non-right-handers contribute to the older brothers x handedness x sexual orientation interaction.
authors.affiliation: Law and Mental Health Program, Centre for Addiction and Mental Health, 250 College Street, Toronto, Ontario, M5T 1R8, Canada. Ray_Blanchard@camh.net
authors.forename: Ray
authors.initials: R
authors.lastname: Blanchard
mesh_terms: D001722:Birth Order; D005260:Female; D007839:Functional Laterality; D020010:Heterosexuality / Q000706:statistics & numerical data*; D018451:Homosexuality, Male / Q000706:statistics & numerical data*; D006801:Humans; D008297:Male; D009864:Ontario / Q000453:epidemiology; D015203:Reproducibility of Results; D012107:Research Design; D012744:Sex Ratio; D012801:Sibling Relations; D035781:Siblings; D012938:Social Perception