Further Data Concerning Blanchard’s (2011) “Fertility in the Mothers of Firstborn Homosexual and Heterosexual Men”

Abstract

Blanchard (2011) reanalyzed previously published data to examine thebalancingselectionhypothesis,whichconcerns thepersistence of male homosexuality. This hypothesis is based on several findings suggesting that male homosexuality is to some degree heritable (LeVay, 2011). However, since homosexual men are less fecund (i.e., produce feweroffspring) thanheterosexualmen, some compensatory mechanism must exist. The balancing selection hypothesis states that the decreased fecundity of men who inherit a genetic contribution to homosexuality is counterbalanced by an increased fecundity of their relatives carrying the same genetic variants (Hamer & Copeland, 1994). Blanchard (2011) noted that the balancing selection hypothesis cannot simply be tested by comparing the number of siblings of heterosexual and homosexual participants because of a robust finding termed the fraternal birth order effect. The odds of a male being homosexual increase with each older biological brother he has. Because having more older brothers correlates with larger sibships, thefinding thathomosexualmenhavemore siblings than heterosexual men could be an artifact of the fraternalbirthordereffect.However, therearefirstbornsonswhoare homosexual and their sexual orientation should not be due to this effect. Thus, an approach to the problem of how to test the balancing selection hypothesis is to compare the number of siblings offirstbornheterosexualandhomosexualmen.Thebalancingselection hypothesis implies that firstborn homosexual men should have more siblings compared with firstborn heterosexual men. If they do, it also implies that the mothers of these homosexual men are generally more fecund than the mothers of the heterosexual men. To date, three studies have used this rationale to compare sibships of firstborn heterosexual and homosexual men. Two of these studies found differences in the predicted direction, with firstborn homosexual men having more siblings than firstborn heterosexual men (Camperio-Ciani, Corna, & Capiluppi, 2004; IemmolaChowever, thedifferencewas statistically significant in only one (Iemmola & Camperio Ciani, 2009).Blanchard (2011)conductedcorrespondinganalysesacross six archival data sets. These results contradicted predictions; in the majority of these data sets, firstborn homosexual men had fewer siblings than firstborn heterosexual men. Thus, there is conflicting evidence whether findings based on the above methodology are in line with the balancing selection hypothesis. The present research reanalyzed data of another study on sexual orientation and the balancing selection hypothesis (Schwartz, Kim, Kolundzija, Rieger, & Sanders, 2010). In the datasetbySchwartzetal. (2010),380heterosexualmenand283 G. Rieger (&) Department of Human Development, Cornell University, B42 Martha Van Rensselaer Hall, Ithaca, NY 14853, USA e-mail: gerulf@cornell.edu