Bailey & Pillard: Twins and Other Brothers
Bailey and Pillard studied pairs of brothers -- identical twins, non-identical twins, other biological brothers, and adoptive brothers -- where at least one was gay. At first glance, their findings looked like a pattern for homosexuality being genetically influenced. Identical twins were both homosexual 52% of the time; non-identical twins, 22%; other biological brothers, 9.2%; and adoptive brothers, 10.5%. A closer look reveals significant problems with a "born gay" conclusion to this study:
• "In order for such a study to be meaningful, you'd have to look at twins raised apart," says Anne Fausto Sterling, a biologist. The brothers in this study were raised together in their families.
• All the results were different from what one would expect if homosexuality was directly genetic:
◦ Because identical twin brothers share 100% of their genes overall, we would expect that if one was homosexual, the other would also be homosexual, 100% of the time. Instead, this study found that they were both homosexual only 52% of the time.
◦ Although completely unrelated genetically, adoptive brothers were more likely to both be gay than the biological brothers, who share half their genes! This piece of data prompted the journal Science to respond: "this . . . suggests that there is no genetic component, but rather an environmental component shared in families" (Vol. 262 Dec.24, 1993).
◦ If homosexuality were genetic, one would expect each number in the column "Results from the B & P study" to be identical to the corresponding number in the "Expectation if genetic" column. Each one is significantly different!
Both are Homosexual:
Identical twin brothers
Non-ident. twin brothers
Other biological brothers
• Finally, Bailey & Pillard did not use a random sample. The men in the study were recruited through advertisements in gay newspapers and magazines. OOOOPS!http://www2.nau.edu/~bio372-c/class/behavior/apbg.htm