Leading scientists who research human sexuality strongly believe that genes play a key role in the development of sexual orientation.
“There is plenty of evidence that genes play a role in the development of sexual orientation,” said Dr. Marc Breedlove, a Rosenberg Professor of Neuroscience at Michigan State University. “I don’t know of any reputable scientists who are working on this who don’t agree on that. There is not even any controversy on this among scientists. Of course, genes influence sexual orientation.”
The best evidence comes from several twin brother studies, where identical twins are shown to have a greater concordance for homosexuality than fraternal twins. This is true even if the twins are raised apart. Dr. Alan Sanders, a Clinical Associate Professor of Psychiatry at Northwestern University, is currently conducting the largest twin brother study to date. The researcher has interviewed and analyzed 400 pairs of gay brothers and almost 400 families, which makes it three times as large as any previous study.
“We are looking for a series of markers that show linkage on the various chromosomes,” said Dr. Sanders. “Probably the main thing is that the Xq28 [Dean Hamer's 1993 genetic study which found the Xq28 marker for homosexuality on the X chromosome] finding seems to be supported in a pretty large sample. And there probably is some gene or genes in that region contributing to variation of the trait.”
Unfortunately, there are anti-gay activists who don’t understand or distort the research. They point to twin studies and say, “if homosexuality was truly genetic, the concordance rate for identical twins would be 100-percent.” Scientists say that this is a gross misunderstanding of the research. According to Dr. Eric Vilain, Professor of Human Genetics, Pediatrics and Urology at UCLA, and the Director of the Institute for Society and Genetics:
All sorts of traits that are clearly genetic do not show 100% concordance [in twins]. I’ll give you examples, if people look at obesity. Or autism, looking at twins. There is undeniably, no one will even challenge that there is a genetic cause for both obesity and autism.
The twin studies do not show that it is 100% genetic. They just demonstrate that there is a genetic influence. There could be other influences on sexual orientation. Some of them might be environmental. For instance, the older brother effect [studies showing the more older brothers a boy has, the more likely he is to be gay] is an environmental influence. That’s what has happened in the past, in the same womb seems to influence the sexual orientation of future pregnancies. So, it is certainly not an all or nothing phenomenon.
Dr. Breedlove says that sexual orientation is likely to be influenced by several genes, although more research needs to be done to identify them:
The example I like to use is, to talk about height. The estimates are there are 150 different genes that influence height. So, is there a gene for being tall? A gene, if you get that gene you will be tall no matter what? No, there probably isn’t a gene for being tall. But, does that mean there is no genetic influence on height? Of course not, that would be silly. Of course, there is a genetic influence on height. Of course there is a genetic influence on sexual orientation. The data are really completely firm.
Finally, there are scientists that hypothesize that homosexuality is explained by looking at epi-genetics. In other words, they think that the trait may occur because of alteration in the regulation of genes in the womb, and not the genes themselves, and could be passed down from parent to child. At the present time, however, there is only limited research to support this new theory.
Key research on the role of genes on the development of sexual orientation will continue, which will lead to further understanding. However, we currently do possess enough knowledge to say that there are genetic contributions to sexual orientation.