There is an article today in the Chicago Sun-Times that profiles an effort by Researchers from the Evanston Northwestern Healthcare Research Institute, Northwestern University, University of Chicago, and University of Illinois at Chicago, headed by Dr. Alan Sanders, to determine whether one or more genes can be identified that may be instrumental in the development of male sexual orientation.

The project is funded by the National Institutes of Health and involves research on a 1,000 sets of gay brothers. I encourage those gay men (or ex-gay men) who read here and who have a gay brother to review the study’s website to determine whether to participate.

There was one quote in the story which I found intriguing:

Scientists have rejected earlier notions that homosexuality is a mental illness. The thinking now is that sexual orientation is determined by roughly 40 percent genetic factors and 60 percent environmental factors.

I am not particularly knowledgeable on heritability and how it is measured in terms of determining attributes. I emailed Dr. Sanders about this 40/60 allocation and he was gracious enough to respond:

I appreciate your writing in. There is not a hard and fast number, but rather, twin studies allow one to make estimates (such as that 40%, also expressed as 0.40), and I certainly provided that estimate to Jim Ritter [the reporter]. The technical term is “heritability” which means the proportion of the variance in a trait that genetics/heredity accounts for. There are a number of twin studies out there on male sexual orientation, and all of them find that similarity for sexual orientation among identical twins (who are genetically identical) is significantly higher than that for same sex fraternal twin pairs (who are as similar genetically as any other full siblings, i.e., born of the same biological mom and dad). We are passing on the estimate (remember that it is an estimate with a range of possibilities — this ~40% is just the mid-range number if you will, it could be more or it could be less genetic) from what we judge to be one of the largest and best done methodologically studies. Here is the fuller quote from our study web site’s FAQ section (

Researchers found that in a study of brothers, if one male sibling was homosexual, the chance his identical (monozygotic) twin would be homosexual was 52%; the chance that his fraternal (dizygotic) twin would be homosexual was 22%; the chance that his adopted brother would be homosexual was 11% [21].

[21] Bailey JM, Pillard RC (1991): A genetic study of male sexual orientation. Archives of General Psychiatry 48:1089-1096. PMID: 1845227

There were actually several estimates of heritability even within that paper: “Heritability estimates ranged from 0.31 to 0.71. Thus, estimated heritability remained substantial under a wide variety of assumptions.”

So, if one says variation in sexual orientation is exactly 40% due to genes (and hence exactly 60% due to other contributions, i.e., the environment), this is not accurate since we are really talking about an estimate here. The main point is that genetics/heredity make very significant contributions to the trait. I hope this helps.

If you have difficulty following the above, please review the website to see if your questions are answered there.

As Dr. Sanders’ research will be the most comprehensive to date and across the largest number of gay brothers, perhaps his results will give us some sound answers as to the extent that genes contribute to a man’s orientation and which genes have an impact. I wish him the best and hope that something conclusive comes from his efforts.

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