Last updated on: 6/23/2008 | Author:

Do Studies Conducted on Homosexuality in Twins Show a Genetic Basis of Sexual Orientation?

PRO (yes)


J. Michael Bailey, PhD, Professor of Psychology at Northwestern University, in the Dec. 17, 1991 New York Times article “Gay Men in Twin Study,” discussed the 1991 Archives of General Psychiatry article “A Genetic Study of Male Sexual Orientation”:

“We found 52 percent of identical twin brothers of gay men also were gay, compared with 22 percent of fraternal twins, compared with 11 percent of genetically unrelated brothers [brothers by adoption], which is exactly the kind of pattern you would want to see if something genetic were going on. The genetically most similar brothers were also the ones most likely to be gay, by a large margin.”

Dec. 17, 1991


Kenneth Kendler, MD, Director of the Virginia Institute for Psychiatric and Behavioral Genetics, et. al, reported in the Nov. 2000 American Journal of Psychiatry article “Sexual Orientation in a U.S. National Sample of Twin and Nontwin Sibling Pairs”:

“In accord with findings from prior twin studies, resemblance for sexual orientation was greater in monozygotic [identical] twins than in dizygotic [fraternal] twins or nontwin sibling pairs. These results suggest that genetic factors may provide an important influence on sexual orientation.

The concordance rate for nonheterosexual sexual orientation in monozygotic twins found in this sample (31.6%) is similar to that found in the one previous general population twin study but lower than the approximately 50% concordance rates found in the two previous studies that ascertained twin pairs through homophilic publications.”

Nov. 2000


Ontario Consultants on Religious Tolerance, in the Apr. 21, 2007 article “Homosexuality and Bisexuality: Is Homosexuality Genetically Determined?” published on its website, explained:

“Religious conservatives often point to studies of identical twins who were separated at birth and raised independently. If one is gay, then the other twin is found to be gay only about 55% of the time. They reason that: since identical twins have the same genetic structure, then if homosexual orientation were determined by genes, 100% of the other twins would be gay… [This is] based upon a faulty or inadequate knowledge of the detailed workings of genetics. Genes have a property called penetrance, which is a measure of their effectiveness, or power…

The penetrance of the gene which causes Type 1 (early onset) diabetes is only 30%. So, if one identical twin has the allele that causes diabetes, then the other twin will have the same allele. Both will have a 30% chance of developing the disorder. Both twins will have the same genetic structure. But it may or may not be triggered by something in the environment, and cause diabetes. If one identical twin develops schizophrenia, the other twin has about a 48% chance of also developing the disorder. If one twin develops bipolar affective disorder, (formerly called manic depression) the other twin’s chances are about 60% of having it as well…

We do not wish to imply that homosexuality is a disease. We are merely suggesting that the root cause of many diseases – and traits like left-handedness – are genetic. Most human sexuality researchers who are not religious conservatives regard homosexual orientation as a trait like left-handedness.”

Apr. 21, 2007

CON (no)


A. Dean Byrd, PhD, Clinical Professor of Medicine at the University of Utah School of Medicine, in his Sep. 2, 2004 article in Meridian Magazine titled “Born That Way? Facts and Fiction About Homosexuality,” wrote:

“Michael Bailey and Richard Pillard who focused on identical twins, non-identical twins, non-adopted siblings and adopted siblings… found a 52% concordance rate for the identical twins which means that for every homosexual twin, the chances were about 50% that his twin would also be homosexual…

If there is something in the genetic code that makes an individual homosexual, why did not all of the identical twins become homosexual since they have the exact same genetic endowment?… Some comparative data on twin studies [are] the concordance rate for identical twins on measures of extroversion is 50%, religiosity is 50%, divorce is 52%, racial prejudice and bigotry is 58%. From the Bailey and Pillard study one has to conclude that environmental influences play a strong role in the development of homosexuality.”

Sep. 2, 2004


Neil Whitehead, PhD, Scientific Research Consultant, reported in his Apr. 20, 2006 article “The Importance of Twin Studies,” published on the website of the National Association of Research and Therapy of Homosexuals (NARTH):

“Identical twins have identical genes. If homosexuality was a biological condition produced inescapably by the genes (e.g. eye color), then if one identical twin was homosexual, in 100% of the cases his brother would be too. But we know that only about 38% of the time is the identical twin brother homosexual. Genes are responsible for an indirect influence, but on average, they do not force people into homosexuality…

For perspective, it is valuable to compare genetic contributions to homosexuality with the question – is a girl genetically compelled to become pregnant at 15? Her genes might give her physical characteristics that make her attractive to boys – but whether she gets pregnant will depend greatly on… some influence from the environment (in this case a boy)… The effects of genes on behaviors are very indirect because genes make proteins, not preferences.”

Apr. 20, 2006


William Byne, MD, PhD, Director of the Laboratory of Neuroanatomy and Morphometrics at Mount Sinai School of Medicine, stated in his May 1994 Scientific American article “The Biological Evidence Challenged”:

“Bailey and Pillard found that the incidence of homosexuality in the adopted brothers of homosexuals (11%) was much higher than recent estimates for the rate of homosexuality in the population (1 to 5%). In fact, it was equal to the rate for non-twin biological brothers. This study clearly challenges a simple genetic hypothesis and strongly suggests that environment contributes significantly to sexual orientation…

Indeed, perhaps the major finding of these heritability studies is that despite having all of their genes in common and having prenatal and postnatal environments as close to identical as possible, approximately half of the identical twins were nonetheless discordant for orientation. This finding underscores just how little is known about the origins of sexual orientation.”

May 1994