THURSDAY, June 29 (HealthDay News) -- Having more biological older brothers, regardless of the amount of time spent raised together, is associated with male homosexuality, suggesting a prenatal origin to sexual orientation, according to a report published online June 28 in the Proceedings of the National Academy of Sciences Early Edition.
Previous research had found that having older male siblings increased the likelihood that a man would be homosexual, an idea known as the fraternal birth-order effect, Anthony F. Bogaert, Ph.D., of Brock University in St. Catharines, Ontario in Canada notes. To investigate the mechanism, he studied four samples of 944 heterosexual and homosexual men, of which three were archival samples. The fourth sample was composed of men raised in non-biological or blended families with half- or step-siblings or as adoptees.
Bogaert found that only the number of biological older brothers predicted the men's sexual orientation, regardless of the amount of time they were raised together.
"These results support a prenatal origin to sexual orientation development in men and indicate that the fraternal birth-order effect is probably the result of a maternal 'memory' for male gestations or births," Bogaert concludes.
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