A couple days ago, I mentioned that gays’ adopted siblings are gay at six times the normal rate – and called this finding a “smoking gun” proving that family environment has a modest effect on sexual orientation.  This finding has bugged me for a while, so I finally turned to Michael Bailey, one of the original researchers.  Is it possible, I asked, that this “effect” merely reflects response bias?  Maybe gays were more likely to respond if they had gay siblings – biological or adopted.

Bailey’s answer: Not only is response bias possible; it’s his “best guess” for the finding.  He then pointed out something that I hadn’t previously noticed (or perhaps had forgotten): The original 1991 study explicitly noted the possibility of “ascertainment bias”:

There is an indirect indicator of proband cooperation: whether the proband consented to have his relative contacted… [P]robands with heterosexual adoptive brothers were significantly less likely to consent than probands with heterosexual twins; cooperation did not differ notably if relatives were homosexual.  If similar factors affect probands’ decisions regarding (1) allowing their relatives to be contacted and (2) their initial participation in the study, then our results would suggest that the proportion of heterosexual relatives was underestimated in the adoptive brothers, compared with twin subsamples.

Bottom line: I’m rewriting the section on family effects on sexual orientation.  The adoption evidence raises the possibility of a family effect, but contrary to what I originally wrote, there’s no “smoking gun.”