Fertility in the mothers of firstborn homosexual and heterosexual men.

This study tested the balancing selection hypothesis, that is, genes predisposing men to homosexuality escape elimination from the population because the decreased fertility of men with the heritable form of homosexuality is offset by an increased fertility among biological relatives who carry the same genetic variants. The index subjects (probands) were 40,197 firstborn heterosexual men and 4,784 firstborn homosexual men retrieved from six archival data sets, all of which had previously been used in published research. The measure of familial (specifically, parental) fertility was the proband's number of younger siblings. The results directly contradicted the prediction of the balancing selection hypothesis. In four of the six samples, the homosexual probands had significantly fewer younger siblings; in the other two samples, the means were not significantly different. It is possible that mothers who produce a homosexual son at their first delivery include a biologically distinct subpopulation of mothers of homosexual sons.