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Chapter 2 (continued)
Biologic Theories
The research on biological contributors to homosexuality can be divided into four basic areas of inquiry: indirect genetic studies (such as studies of similarities and dissimilarities among twins), hormonal studies, neuroanatomic studies, and direct studies of genes. The significant research in each of these areas is reviewed below.
Twin Studies
One of the more significant twin studies to date is that by Bailey and Pillard published in 1991. These researchers interviewed 110 homosexual males who had twin brothers. Roughly half of these men were from identical twin (called monozygotic, or MZ twin) pairs while the other half were from non-identical twin (called dizygotic, or DZ twin) pairs. The researchers found that in 52% of the identical (MZ) twin pairs both brothers were homosexual while only 22% of the non-identical (DZ) twin pairs contained two homosexual brothers. The sexual orientation of other (non-twin) biological and adoptive brothers in these men’s families was also investigated. The researchers found that 9.2% of the non-twin biologic brothers were homosexual and that 11% of adoptive brothers were homosexual (see table below).2(p1089)
Relationship of Brothers |
Percent Sharing Homosexuality |
| Identical (MZ) twins | 52% |
| Non-identical (DZ) twins | 22% |
| Non-twins | 9.2% |
| Adoptive | 11% |
In a review of this and other biologic studies, psychiatrists William Byne and Bruce Parsons of Columbia University point out several important factors. The most obvious observation regarding this study is that only half (52%) of the identical twin brothers—who share identical genetic material as well as prenatal environments—also shared homosexuality. If sexual orientation were fully genetically determined one would expect the rate of identical twin brothers sharing homosexuality to be closer to 100%.
Furthermore, Byne and Parsons point out that non-identical (DZ) twins and non-twin biological brothers share the same amount of genetic material, which is essentially a random sample of the genetic material of their parents. So, if homosexuality were simply genetic, the rates of homosexuality among such brothers would be similar. Yet the study shows a substantial difference (22% for non-identical twins and 9.2% for other brothers). Doctors Byne and Parsons suggest that the greater environmental similarity for non-identical twins verses non-twin brothers (twins experience their environment more similarly than non-twins) must be considered as an explanation for this difference.
Since adoptive brothers share no genetic material, one would expect the rate of homosexuality among such brothers to be far lower than that among biologic brothers. Yet this study shows a rate of 11% for adoptive brothers, which is clearly not lower than the 9.2% rate among the non-twin biologic brothers.3(p229-230)
Bailey and Pillard printed a comment in their report regarding bias of the sample. Their method of recruiting subjects for the study was by placing ads in gay publications. They suggest it is possible that, because the sampling was not done randomly, certain types of individuals may have been more likely than others to respond, which would create a biased sample and skewed results.2(p1094) In other words, men with homosexual twins might be more interested in participating in a study about homosexuality among twins than twins with heterosexual brothers. In conclusion, this study seems to show a correlation between twinship and sexual orientation, but only a partial correlation between genetics and homosexuality. Other studies show similar results.
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