[Ed’s Note: This one’s a bit lengthy so I’ve put some of it after the jump. I’ve also edited the links to be a tad more manageable. Click Read More to see all of it.]
Homosexuality has been the topic of much discussion here at SEB. I’ve tried to avoid the discussion on whether homosexuality is a result of genetics or a lifestyle choice, and have focused primarily on providing my positions as to: 1) Why the legal strategy for the promotion of same-sex marriage was flawed; and 2) A legitimate refutation of Les’s challenge that opposition to same-sex marriage is always grounded in religion. Those discussions always get intentionally sidetracked. So, I have engaged on the subject. That is the purpose of this thread. Shelley has been kind enough to serve as my counterpart in this discussion.
I don’t intend to respond to all comments. The idea is that the thread will be a coherent discussion between myself and Shelley. With few exceptions as to Les, Elwed, Geekmom, zilch, patness, and Ufreker, other than Shelley, I won’t be addressing comments. That is not because I think that the comments are not worth addressing. It is simply because I’ve spent a considerable amount of time e-mailing back and forth with Shelley to set ground rules for the discussion so that there is a coherency to the thread as far as the order of discussion. If you must have an response, e-mail me.
We have agreed to review in this thread the twin studies starting with J.M. Bailey’s 2000 study found here.PDF file We will then discuss the Bearmam study found here. I will likely discuss one other study that I will link later.
It is my position that there is no evidence to support the proposition that homosexuality is genetic. In fact, the evidence indicates that homosexuality does not have a genetic basis. In fairness, Shelley’s position is a bit broader than my position. She believes that there is an interaction between the biological and environmental that results in homosexuality. The difference in our focus being that Shelley’s biological basis encompasses events that possibly happen while a baby is in the uterus. At Shelley’s request, we will move to other areas after we have finished with the twin studies.
As a backdrop to this discussion, I’m disclosing that a large part of the reason for my participation in this thread is that I have read so many statements alleged as scientific fact that are simply not true. Before we start with the first study, I want to go through a few of those alleged facts.
It is often times stated that 10% of the population is gay and “everybody has that gay uncle.” Everybody does not have a gay uncle, and 10% of the population is not gay. It is widely regarded within the scientific community that the number of gay individuals is approximately 2-3%. Shelley has kindly stated that:
I’ll stiuplate that the population of men who identify as homosexual is around 2.5-3%.
The reason for the misconception about the size of the population is the result of an old study from 1948 done by Alfred Kinsey. Kinsey’s study was severally flawed, but for a long time there has been no competing data. So for decades people cited the Kinsey study. Today we know better. The more recent studies do not reflect a 10% figure for homosexuality and a 33% figure for homosexual sex.
There is no gay gene that we know of at this time. It is possible that one could be discovered. This would be the best evidence that homosexuality is innate. The best evidence for an innate genetic basis for homosexuality in a single genetic marker has not been found. Shelley has kindly stated:
I’ll stiuplate that the evidence for a single genetic marker for homosexuality is inconclusive.
What really prompted my inquiry into the twin studies was everybody kept quoting a concordance rate in a study of homosexual twins. They kept telling me that there was a 52% concordance rate for homosexuality for identical male twins. This was compelling evidence, or so they said. Specifically, if I recall, zilch first said it to me here.
For those reading now, if you don’t know anything about concordance rates or twin studies, here is some background on these studies that might be helpful in understanding the discussion that will follow.
Identical twins (referred to in the studies as monozygotic) have the exact same genes. Fraternal twins (referred to in the studies as dizygotic) have roughly 50% of the same genes. Other siblings also share roughly 50% of the same genes. What is being referred to as a “concordance rate” in the studies is the rate for a trait that is shared by the twins. In the studies on homosexuality, the studies are attempting to identify the percentage of identical twins that are both homosexual. The studies also look at the percentage of fraternal twins that are homosexual and the number of other siblings that are homosexual.
When a twin study is done, it assumes that the environment in which twins are raised is nearly identical. The assumption is a big one, but it certainly makes sense to attempt to control for the environment to try and parse out what is genetic. The thinking goes that if there is a difference between the concordance rate for homosexuality in identical twins and fraternal twins, then this is evidence that there is some genetic component to homosexuality. Although attempting to control for the environment between the identical twins, one of the difficulties is that the studies aren’t able to account for the fact that families, friends, and society at large may be treating identical twins more alike than fraternal twins. We will do comparisons with some of the other study groups to evaluate what role that may play in concordance rates and the conclusions that can be fairly drawn.
It is important to note that if the concordance rate is less than 100%, then environmental factors must be exerting some influence. When Shelley says that we don’t see Mendialian heritability for homosexuality she is saying that there are other factors than genetics involved with homosexuality. In other words, before we even begin the twin studies, if the 52% was an accurate gauge to use (which it is not), we know that homosexuality is deeply rooted in environmental factors. In defense of Shelley’s position, in utero biological factors would be considered as a contributing environmental factor for causation. However, I believe that we will cover some material that raises some grave questions about the biological hypothesis. I’ll raise that issue when appropriate.
Another thing that the reader needs to know is how the concordance rates are being reported. There is pairwise concordance and there is proband concordance. It is possible for both members of a twin pair in a twin study to be probands, in which case that pair would appear twice in the study results. An example—if there is a sample of 3 pairs of twins and in 1 of those pairs both members are diagnosed with condition X then according to the pairwise method the concordance rate would be 1/3 or 33%, but according to the proband-wise method the rate would be 2/4 or 50%. It is safe to assume that proband reporting will net larger %s than pairband reporting.
We will also discuss sample populations used. Whole textbook could be written on, and several have, sample groups. In simple words, what you put into the pie determines whether you get apple, pecan or pumpkin. So, we will look at how the sample groups for the studies were put together.
My specific criticism of the Bailey and Pillard study, from which the 52% number is drawn, will be posted after I have heard from Shelley that she has reviewed this post and had the opportunity to comment should she want to. We have agreed to spend time on this thread as time permits, which may mean that there is a long delay between posts. Nothing other than the participants have competing demands upon their time is to be inferred from a failure to respond to the last comment within 24-48 hours.