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.