For more than 50 years researchers have studied twins in an effort to understand the contributions of nature and nurture to the development of homosexuality, with mixed conclusions.
The rationale behind such studies is that if there is a difference between the same-sex attraction rate for homosexuality in identical twins and nonidentical twins it would represent strong evidence of some genetic component in the origins of homosexuality.
Early studies suggested homosexuality was genetically mediated but these studies were criticized for selecting from nonrepresentative samples and offering only nonrepresentative results.
In 1991 a study of gay twins found 52 percent of identical brothers and 22 percent of nonidentical twins were homosexual. This and other similar studies have been criticized for recruiting through queer media, potentially resulting in a higher response rate from twins who were both gay.
In 2002 a large-scale study lead by Columbia University’s Peter Bearman and Yale’s Hannah Brückner found that among identical twins 6.5 percent expressed same-sex romantic attraction versus 7.2 percent of nonidentical twins. It also concluded none of their comparisons between identical twins and others were even remotely significant. “If same-sex romantic attraction has a genetic component, it is massively overwhelmed by other factors,” states the study.