Wheaton provost Stanton L. Jones corrects an article in the New York Times and cites evidence that homosexuality is not genetic after all: Identical twins, who have exactly the same genes, are not very likely (contrary to some reports) to both be gay.
Frank Bruni, in his essay “Genetic or Not, Gay Will Not Go Away“(New York Times, January 28, 2012), makes a broad point regarding which I am in complete agreement: Our societal, legal, and cultural debates will not be solved by science. But when you do cite the science, you ought to get it right. . . .
In support of the argument that at least sometimes sexual orientation is a condition of birth, Bruni describes how “One landmark study looked at gay men’s brothers and found that 52% of identical twin brothers were also gay.” This brief explanation both fails as a description of that 20+ year old study and fails to reflect the better research published since.
Bruni gets the number right; the 1990 landmark study by Bailey and Pillard reported a 52% “probandwise concordance” for homosexual orientation among genetically identical sibling groups, but this does not mean what Bruni says it means. A proband wise concordance is a technical calculation, one that in this case results from the following actual results: There were 41 genetically identical sibling groups (40 identical twin pairs and one triplet trio) and of these 41 groups, only in 14 of the groups did the genetically identical brothers match for sexual orientation; in the remaining 27 sets the identical twin brothers did not match.
But this 1990 study was actually based on a sample that was apparently distorted by volunteer bias and hence not representative of homosexual persons in general. Bailey’s own study of a decade later, and the recently published “gold standard” study by Långström et. al. of the Swedish Twin Registry, both found even lower matching among identical twins with much larger and more representative samples. Both studies reported about 10% matching (for Långström, 7 identical twin pairs matched with both identical brothers gay out of 71 total pairs of male identical twin pairs).
So in plain English, the best contemporary scientific findings are that when one identical twin brother is gay, the probabilities of the second twin being gay are approximately 10%. This suggests that the contribution of genetics to the determination of homosexual orientation is modest at best.