PubMed · 13534211
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D E BARTON, F N DAVID. 1958. A test for birth order effect.. https://doi.org/10.1111/j.1469-1809.1958.tb01420.x
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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.
Two of the most consistent correlates of sexual orientation in men are handedness and fraternal birth order (i.e., number of older brothers). In the present study, the relationship among handedness, older brothers, and sexual orientation was studied in 4 samples of heterosexual and gay or bisexual men (N = 944). Unlike previous studies, which have only observed an increased rate of non-right-handedness in gay or bisexual men relative to heterosexual men, an elevated rate of extreme right-handedness was found in gay or bisexual men relative to heterosexual men. The results also demonstrated that older brothers moderate the relationship between handedness and sexual orientation. Specifically, older brothers increase the odds of being gay or bisexual in moderate right-handers only; in both non-right-handers and extreme right-handers, older brothers do not affect (or decrease) the odds of being gay or bisexual. The results have implications for an early neurodevelopmental origin to sexual orientation in men.
This study tested the prediction, based on prior research, that non-right-handed homosexual men will report fewer than expected older brothers. Participants were 2486 heterosexual and homosexual, right-handed and non-right-handed, male and female adults, representing five samples collected for various projects by the second author. Data on sibship composition, sexual orientation, and hand-preference were gathered in the original research using on-line (Internet) or self-administered paper-and-pencil questionnaires. The non-right-handed homosexual men reported 83 older brothers per 100 older sisters, which was significantly lower than the human sex ratio of 106 live-born males per 100 live-born females. In contrast, the right-handed homosexual men reported 125 older brothers per 100 older sisters, which was significantly higher than the expected ratio. One possible explanation of these results is that older brothers increase the odds of homosexuality in right-handed males but decrease the odds of homosexuality in non-right-handed males. A second possibility is that older brothers decrease the probability that non-right-handed homosexual males will be represented in survey research. The latter scenario could arise if the combination of some biological factor associated with older brothers and some biological factor associated with non-right-handedness is so toxic that it kills the fetus or predisposes the individual to a condition (e.g., mental retardation, major mental illness) that makes him less likely to be available for research recruitment at Gay Pride parades (etc.) than other members of the gay community.