The widespread popular belief of a biological or genetic origin to homosexuality can be traced to three highly publicized studies:
- Simon LeVay and the INAH-3 (1991): post-mortem examinations of the brains of cadavers.
- (J. Michael) Bailey and (Richard C.) Pillard Twin Studies (1991): study comparing identical and fraternal twins, and adoptive brothers.
- Dean Hamer and the X Chromosome (1993): study found a specific “chromosomal region” containing “a gene that contributes to homosexual orientation in males”.
Perhaps what is less widely known is that each of these studies suffered from serious methodological weaknesses, such as small sample sizes, non-random samples and even possible misclassification of their subjects, which bring the validity of their conclusions into question. Other scientists have also been unable to replicate the same dramatic findings of these studies.1, 2
Michael Bailey conducted a subsequent study3 on a larger sample size of 27 Australian male identical twin pairs, where at least one of the twin brothers was homosexual. Since identical/monozygotic twins have identical genes, we should expect that whenever one twin is homosexual, the other twin would be as well (100% concordance rate). Instead, “in only three of the pairs were the second twin brother gay as well” (11% concordance rate).
Researchers Peter Bearman and Hannah Brückner, from Columbia and Yale Universities respectively, studied data from the National Longitudinal Study of Adolescent Health, and found even lower concordance rates for homosexuality in identical twins (6.7% for male; 5.3% for female), neatly refuting several of the biological theories for the origin of homosexuality. On the contrary, social experiences in childhood are found to be a far more significant contributor.4