Twins Studies and Homosexualty

[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. 

38 thoughts on “Twins Studies and Homosexualty

  1. Subscribing as well and thanks to Consi and Shelley for deciding to discuss it here rather than privately as was considered.  smile

  2. I just wanted to acknowledge your post, Consig and let you know that I will have a few brief introductory comments and that I will prepare these as soon as I have a few moments to do so.


  3. If anyone could link me to a chart that gives conversion rates for heritability based on raw concordance rates I would be appreciative.

  4. chart that gives conversion rates for heritability based on raw concordance rates

    I tried to find one but my luck seems to be as good as yours has been.

  5. I’m pleading with you, please, please, please, leave this thread for the discussion of the studies.  Let the discussion proceed as set forth in the introduction.  Feel free to e-mail me with questions that you have, your opinions anything.  Just please leave the thread alone.

    I promise that there will be a follow up thread where we can all discuss the implications or lack of implications that we may reasonably derive from the studies.  I’m looking forward to hearing what impact, if any, the discussion had on your thoughts on the origins of homosexuality.  Until then, I beseech you, leave the thread alone.

  6. Long time reader, first time poster. Subscribing to this thread because it seems like a really interesting approach to a difficult subject. Looking forward to seeing what the conclusions are … if it’s even possible to reach any.

  7. To try and help contain thread drift I’m going to transfer any comment from someone that is much more than “I’m subscribing” to the Homosexuality free-for-all thread. This means some of you may lose your subscription as EE ties it to the comment itself.

    So, when I’m done, if you don’t have ANY comments in this thread and want to subscribe then leave a short “I’m subscribing” comment. If you still have at least one comment in this thread when I’m done then you don’t have to resubscribe. Please limit any comments to subscriptions only until Consi and Shelley have run their course with this thread as I’ll be moving or closing any other comments that show up. Consider this an SEB experiment.

    Also, I did modify a couple of comments (DOF, Gudio) to remove any extraneous comments that might provoke thread drift.

  8. Shelley will be posting within the next 48 hours.  In the meantime, I wanted to take a moment to discuss some of the general problems with twin studies.  Some academics believe that the problems with twin studies are so numerous and so unreported by the media that the value of twin studies is being misrepresented and misused.  See Kaplan On this point I would agree.

    What are some of the criticisms?

    One criticism centers on an a number of problems inherent in studying human beings, and therefore, twins.  Most prominent is the equal environment assumption.  As I mentioned above, when a twins study is done, it assumes as a fundamental proposition that the environment for identical twins is no more similar than that of fraternal twins.  If this proposition is violated, the twin study has no validity at all.

    With animals or plants, researchers can create a strictly controlled environment and the results regarding the heriability of a trait will have value.  That is simply impossible to do with human beings.  At best, the researchers will attempt to estimate this through the use of 4-5 questions.  At worst they ignore it.

    Let us think about this for a moment and look at what is being done in the best studies. In one of them the following four questions are asked: whether twins shared the same room, had the same playmates, were dressed alike, and were in the same classes at school.  The answers to each question were assigned a value and a scale was created.  The scales for the answers for identical twins was compared to that of fraternal twins. 

    Now, ask yourself: Can those four questions quantify the life experience of being raised as a twin to such an extent that we can then use those answers to extrapolate over 18 years of child-rearing?

    I can’t say they do.  I’m not aware of anyone, including the researchers that can say that.  It’s obvious that it takes much more depth than four questions to capture enough information to be meaningful.  Furthermore, to be of value, the question asked should have a bearing on the trait being studied.  We don’t know whether those 4 questions have any bearing at all on the trait being studied: homosexuality.   

    I’m not faulting the researchers.  Asking some sort of background questions is at least an attempt to do something in this regard.  However, this limitation is well-known in the field of twin studies.  It’s even discussed within the some of the studies themselves as a cautionary note regarding the statistical power interpretation of the findings.  The problem is that cautionary note issued by the researchers never makes it to the general population. Therein lies the rub. 

    There are also concerns about the use of heritabilty rates that will have to wait for another post.

  9. This is the follow-up post on problems with using heritability rates.

    I wanted to first clear up a possible misconception that exists about concordance rates and heritability rates that I saw in one of the comments earlier.  Concordance rates are not heritability rates.  When a study states that the concordance rate is X, a reader can’t read the concordance rate to be a heritability rate.  The concordance rates are part of the calculation to estimating heritability ranges.  The formula for determining heritability ranges is called Falconer’s Formula.  In twin studies is as follows:

    h2=2(r(mz)-r(dz)) where r is the intraclass correlation figure. Analytic equations to twin data using structural equations

    If you are a math geek, click on the link and have fun.  If you are like me, that stuff is not only heavy sledding, it is boring. I don’t want to get bogged down in the math because the math isn’t the real issue here.  What is important to understand is that heritability is not dependent on a higher or lower raw concordance number.  Heritability is dependent on the spread between the concordance rate for the identical twins and the concordance rate for the fraternal twins.  The larger that spread, the larger the theoretical likelihood that the trait has a genetic component.

    Further heritability is not the same as inherited.  The term inherit refers to the inheritance of genes and those genes will determine certain traits. Beckwith That is not heritability.  Heritability is is defined as the proportion of the variation of that trait within a population living in a specific range of environments that is due to genetic differences among the people in that population. Id.

    Since we don’t know the environment, and have no real means to do so within the human population, estimating heritability becomes a lot less like carbon dating and much more like shaking the magic 8 ball for answers. 

    The idea of detetermining heritability rates really is a good idea.  It has translated into practice in agriculture rather well.  It doesn’t translate well in practice when attempting to study human behaviors.  In part because we are much more complex chemically, and in part because we are much more complex environmentally.

    The result is that countless things have found to be heritability: becoming a doctor when your father is a doctor, watching tv, and getting a divorce, also included are such things as eye color, hair color, and height.  Since we can’t properly segregate out the environment, what is really being measured is whether a trait is familial, not whether it is genetic. Which should be expected.  Such studies are a first step though, not a step from which conclusions may be drawn.

    Kaplan captures the heart of the criticism for estimating heritabilty rates for human behavior. 

    For some techniques, critiquing the reliability of the data obtained is less important than understanding the limits of the data’s legitimate uses and interpretations. In these cases, the techniques in question, even if applied perfectly, answer very specific questions in very specific domains. As such, the results of these studies cannot simply be extended to other domains, nor can they be used to answer other kinds of questions.

    Kaplan, supra pg. 56.

  10. First a disclaimer: I am not involved in homosexuality or genetic research at any level. My area of research is strictly in an unrelated applied area and therefore I am not an expert on the subject matter. I do understand the fundamentals of science, research methods, and statistics (as do several others who post here on a regular basis), and I can only give my best impressions of what the research tells us. 

    Second disclaimer: From time to time I will refer to disease states and mutations as examples of how genes and genetic influences may be expressed. This is for illustrative purposes only and not because I believe that homosexuality is a disease. I do not believe that the evidence supports the idea that homosexuality is a disease any more than I believe it supports the idea that left-handedness is a disease state.

    It is Consig’s position that the evidence shows that homosexuality has no genetic basis whatsoever. It is my position is that the evidence, across a variety of studies, testing different questions, using a variety of methodologies, demonstrates that biology (including genetic influences) underlie the emergence of homosexuality – in all likelihood, there is some interaction between biological and environmental factors: Homosexuality is not a ‘lifestyle choice’ and cannot be traced to things like sexual abuse or bad parenting.

    I also want to point out an important caution here as we look at the literature. When scientists use the term “environmental factors,” that should not be interpreted to mean “psychosocial factors.” Environmental factors in scientific terms includes absolutely everything *except* the genes, including hormones, nutrition, stress, viruses and infections, medications, education, family configuration, social environment, parenting, and so on and so on. This is an important point because some people have looked at twin studies and concluded that because homosexuality has an environmental element, it is cannot be innate. This is not so.

    People want simple black and white answers: Is there a gene for it or not? Which gene is it? Unfortunately, the answers are not at all simple and there are an infinite number of ways in which the environment affects biology and the way that genes are expressed. What is absolutely clear is that single gene explanations are exceedingly rare for any human trait or behaviour. We’re complex creatures, and it is likely that we are shaped by the interaction of multiple genes both with one another and with the environment. The fact that the answers are complex doesn’t mean that it isn’t worth studying them nor does it mean that we can’t find the answers.

    Consig provided an interesting and brief summary on concordance rates and twin studies. I would want to add a couple of additional pieces of information:

    Most of us are familiar with the principles of basic genetic inheritance: If a gene is dominant, a child has a 50% chance of inheriting the gene and the characteristic. If a gene is recessive, the child has a 50% chance of carrying the gene and a 25% chance of inheriting the characteristic. In addition, if you were born a girl, you inherited two X chromosomes, one from each of your parents, and if you were born a boy, you inherited one X chromosome (from your mother) and one Y chromosome (from your father). Males are at particular risk for any mutation in the X chromosome because they inherit only one X. If there is a problem in the X chromosome, each male child will have a 50% chance of inheriting the characteristic and each female child will have a 50% chance of carrying it and passing it on.

    These straight-forward ideas sometimes lead people to think about genes and inheritance in very simple terms. Few things are like eye-color, however. Many different kinds of gene mutations can result in an identical outcome.(For example, there are thought to be about 700 different mutations in the CFTR gene that can lead to Cystic Fibrosis but we can only test for about 70 of them). I suspect that there are multiple genetic pathways to homosexuality.
    In addition, we need to consider gene expressivity when we think about what genetic influences mean. Expressivity refers to the quantitative differences in the expression of any mutation (the degree to which a gene is expressed). Expressivity can be affected by environment, modifier genes, and random chance.

    Finally, we also need to consider gene penetrance. Some mutations are entirely penetrant: The person with the mutation will always get the disease. On the other hand, some mutations are incomplete or are only partially penetrant, in which case, the person may never get the disease. In this case, the mutation works along with other mutations or environmental factors (e.g., the ApoE gene and Alzheimer’s disease.). All of these factors can result in misleading concordance rates in twins.

    It is also important to recognize that twins come in more flavors than just MZ and DZ. There are also half-identical twins (where the egg splits before fertilization), twins with different fathers (yes, it happens), and sometimes twins share an amniotic sac and or placenta and sometimes they don’t. Each of these conditions changes the unique environment of the individual twins and consequently compromises straight-forward concordance rates.

    So, while Consig has correctly pointed that identical twins share 100% of their genes and fraternals share 50%, it would be a great over-simplification to assume that genes are expressed in exactly the same way all the time for everyone. (So for example, we find that even identical twins are discordant (unalike) for handedness about 18% of the time.)

    This is not simple stuff and the answers are not simple.

    It is easy to forget that the first completed draft of the human genome was only produced in 2000 and the full human genome sequenced in 2003. Ten years ago, researchers studying human behaviour could access tests for only a few genes and the information available would have been extremely limited (see Benson, 2004).

    Between 1960 and 1990, the only way to look for genetic markers for complex behavioral traits was through twin and family studies. Usually, researchers began with an observation that a particular characteristic appeared to ‘run’ in families and then conducted a family study to determine whether the hunch had any merit. If a family study suggested that the characteristic runs in families, a twin study would be contemplated and then researchers got down to the business of trying to falsify various explanations for the findings (and have done so for homosexuality for a number of social explanations). Consequently, Consig is quite incorrect when he says:

    Since we can’t properly segregate out the environment, what is really being measured is whether a trait is familial, not whether it is genetic.

    Study by study, we are able to examine and eliminate potential familial and social explanations.

    Yes, twin studies certainly are plagued with difficulties, but until the advent of the age of the genome, they were by far the best available information to point us in the direction of biological origins. I expect that molecular biology will take up where family studies and twin studies point and we’ll see what the emerging studies bring.

    Consig takes issue with Kinsey’s (1948) research and considers it deeply flawed because it found that 10% of the males sampled were predominantly homosexual between the ages of 16 and 55. I would argue that it was the best (only) available information for many years, and that it should be noted that Kinsey actually believed that it was impossible to know how many people were homosexual or heterosexual and that it was only possible to determine behaviour at any particular time (I’d also point out that no one gets too excited about Kinsey’s other findings drawn from the same sample. I’ve yet to see a paper critiquing Kinsey’s estimates on women’s masturbation rates.)

    Although the 10% estimate was popular for a number of years due both to well-intentioned activists and to the fact that there was a complete lack of anyone doing this kind of research), recent estimates put worldwide rates of exclusive homosexuality at around 2 % (see the National Health and Social Life Survey) and rates of all those who have ever had a same-sex sexual experience at somewhere between 6 and 8%.

    Consig has simply but not entirely completely explained how and why concordance rates are calculated the way they are. Pairwise and proband rates can answer different kinds of questions: The pairwise rate is usually used when we want to determine whether MZ and DZ twins differ in terms of their concordance. The probandwise rate is the proportion of affected co-twins of probands and is intended to tell us the risk of being affected if your co-twin is affected. Probandwise concordance rates are frequently used because they are considered robust against incomplete ascertainment when compared to pairwise concordance. It may give slightly higher concordance rates but doesn’t invent them. It is in very common usage and has been so for many years. (I’ve got the formulas for them around here somewhere and will dig them up if anyone is interested.)

    Finally, it isn’t enough for Consig to argue that the twin studies are flawed. He must show both that they are fatally flawed and offer an alternate explanation for existing findings. I’ll look forward to reading that.

    One last thing—no, two—in my opinion, no one should look at straight-forward heritability for human traits or behaviour. The impact of genes can be extrmely important in some environments and not at all in others, and in some populations and not in others.

    Last, I’ve been out of town for a few days and I’m running on 3 hours sleep in the last 35. My apologies if this is rambling and a bit incoherent.

  11. Shelley:

    Thank you for taking the time to respond. 

    You slightly misstated my burden.  You accurately stated my position:

    It is Consig’s position that the evidence shows that homosexuality has no genetic basis whatsoever.

    That entails showing that there is no evidence.  No more, no less.

    Study by study, we are able to examine and eliminate potential familial and social explanations.

    I look forward to seeing any twin study that is able to account for environment in a meaningful way other than through a scaled estimation based on a few scant questions.

    I agree whole-heartedly that

    This is not simple stuff and the answers are not simple.

    To the readers out there, please keep in mind that:

    The pairwise rate is usually used when we want to determine whether MZ and DZ twins differ in terms of their concordance.

    Shelley, the formulas are linked above in the Analytic equations link.

    no one should look at straight-forward heritability for human traits or behaviour. The impact of genes can be extrmely important in some environments and not at all in others, and in some populations and not in others.


    I’ll follow with a review of the the Bailey, Pillard 91 study.

  12. I look forward to seeing any twin study that is able to account for environment in a meaningful way other than through a scaled estimation based on a few scant questions.

    It is not necessary to use twin studies to do so. There is no reason to believe that environmental effects will have any more impact on homosexual twins than they do on the general population of homosexuals.

    Whereever we find consistent significant differences (or similarities) between homosexuals and heterosexuals, it tells us something about the role that environmental factors play in the development of homosexuality.

    So for example, when we see that homosexual men demonstrate a robust cross-sexual shift in spatial navigation (men typically use an orientation stratgey and women a landmark strategy), we can assume that there is some environmental agent that causes a shift in homosexual men.

    no one should look at straight-forward heritability for human traits or behaviour. The impact of genes can be extrmely important in some environments and not at all in others, and in some populations and not in others

    That’s a personal opinion and not a statistical argument.

  13. Sorry, I should have given you the reference for that example:

    A Specific Sexual Orientation-Related Difference in Navigation Strategy.
    Rahman, Q., Andersson, D., Govier, E. (2005)Behavioral Neuroscience. 119, 311-316.

  14. It is not necessary to use twin studies to do so. There is no reason to believe that environmental effects will have any more impact on homosexual twins than they do on the general population of homosexuals.

    My point was that none of the twin studies are able to account for environmental factors in a meaningful manner as part of the study.  Almost all of the studies, up to Bailey’s just simply assume it’s all equal and then based on that assumption proceed. 

    I’m not sure I’m clear on what your saying. Would you do me the favor of clarifying whether you believe that an equal environment assumption is problematic with respect to twin studies on human behavior?  If not, why not?

  15. I’m not so much interested in citations for the example, as I’m not clear how the example plays into what your attempting to convey.  Citations for your position about the equal environment assumption would be appreciated.

  16. Just to recap:

    Shelley said:

    Study by study, we are able to examine and eliminate potential familial and social explanations.

    Consig said:

    I look forward to seeing any twin study that is able to account for environment in a meaningful way other than through a scaled estimation based on a few scant questions.

    My follow-up point to this was that we do not need twin studies in order to examine environmental influences on the development of homosexuality. Study by study researchers can consider environmental and social influences individually. There’s absolutely no reason to assume that homosexuality develops differently in twins than in non-twins. When environmental influences are either found or dismissed in other studies, the results can be assumed to apply similarly to twin homosexuals. I gave you an example of one of these types of studies.

    You said:

    My point was that none of the twin studies are able to account for environmental factors in a meaningful manner as part of the study.  Almost all of the studies, up to Bailey’s just simply assume it’s all equal and then based on that assumption proceed.

    The point that twin studies are unable to control for all of the environmental factors is irrelevant to the question of the degree to which environment can be studied and considered. Moreover, no twin studies (with the exception of studies of MZ twin adoption studies) can control for virtually all environmental factors (and even then, they may not be able to account for all prenatal environmental factors nor can they account for the fact that people are treated differently on the basis of their appearance—if you have identicals reared apart,  people will treat them at lease somewhat similarly based on how they look. Consequently, we can’t ever control for absolutely all environmental factors.)

    If you’re concerned about whether the equal environments assumption is valid in twin studies generally, I’d point you to the MISTRA project (Minnesota Twin Study of Twins Reared Apart), and specifically to:

    Sources of human psychological differences: the Minnesota Study of Twins Reared Apart
    TJ Bouchard Jr, DT Lykken, M McGue, NL Segal, and A Tellegen in Science, 1990, Vol 250, pp 223-228.

    However, if you want to dismiss the equal environments assumption as a fatal flaw of all twin studies that invalidates them, we have nothing more to discuss on twins. Of course, there are many other areas of research that point to biology in the origins of homosexuality and I’d be happy to continue with those.

  17. Moreover, no twin studies (with the exception of studies of MZ twin adoption studies) can control for virtually all environmental factors

    Which is why I have said that is problematic because it brings into question the assumption that the environments are equal.  For those unfamiliar with the background, the criticisms of the equal environment assumption dates back to the 1960s.  A sampling of those who have published caution and criticism are: Don Jackson, (no link available) Jay Joseph,, Collin A. Ross, ,Sun-Wei Guo PhD, Jonathon Kaplan, George Mandler, notably Jonathon Beckwith and Joseph S. Alper.  (E-mail me for direct links to the articles)

    The point that twin studies are unable to control for all of the environmental factors is irrelevant to the question of the degree to which environment can be studied and considered.

      In a hyper-technical sense that is correct.  It is true that independent studies about environmental factors can shed light, after the fact, on prior twin studies.  However, when evaluating a twin study, the degree to which environmental factors can be excluded goes to the very heart of the validity of the twin studies.  “Without the equal environment assumption, the twin method is transformed into a measure of the differing natures of MZ and DZ environments.”  Twin Studies in Psychiatry: Science or Pseudoscience

    Furthermore, in 1996, there was a study that reviewed those studies that were said to support the equal environment assumption, including the claims arising from the Minnesota twin studies.  The study is entitled: The “equal environments assumption” in MZ-DZ twin comparisons: an untenable premise of psychiatric genetics? It concluded that “many MZ-DZ pedigree studies have dubious scientific value, given the non-viable premise of the EEA, as well as the misleading operational definition of what has been called “heritability”.’

    When you say:

    There’s absolutely no reason to assume that homosexuality develops differently in twins than in non-twins. When environmental influences are either found or dismissed in other studies, the results can be assumed to apply similarly to twin homosexuals.

    That would seem to indicate that a study shouldn’t be considered flawed unless those environmental factors that might influence homosexuality are not equally experienced between identical twins and fraternal twins.  That is problematic in two respects. 

    First, nobody knows what exposure to or lack of exposure to specific environmental factors may be influencing homosexuality.  It does not follow from that lack of knowledge that environmental factors aren’t influencing the studies. In fact, attributing the difference in concordance rates to genetic factors because the environment can’t be quantified is error.  The reason that it is error is because there is no way to rule out that the entire difference in the concordance rate is due to environmental factors. 

    Second, it is well known that familial studies are confounded by shared environments.  “In the same
    way, the twin method is confounded by the greater environmental similarity of MZ twins, regardless of whether the specific trait-relevant environmental factors are known.” Id at Pg. 76  The fact that familial studies and twin studies are confounded in the same manner would require that the trait-specific EEA be applied in a like manner to familial studies (Nobody claims familial studies indicate genetic causation). Id.  Yet, the scientific community is not arguing that. Id.  So, there appears to be a special application for trait-specific EEA, namely twin studies.  Id.  This appears to be “special pleading” by those advocating trait-specific EEA. Id.

    I do believe that the equal environment assumption underlying the twin studies does fatally flaw them.  Nevertheless, I believe I’m obligated to proceed on with the discussion of the twin studies in the event that there may be a soul or two that is still grasping to those studies.  It’s important to show that even if the studies were not fatally flawed they do not reflect genetic causation as has been bandied about at different times.

  18. Oh please, not Jay Joseph!

    Jay Joseph hasn’t any use for genetic studies of any kind, and even believes that “molecular genetic research in psychiatry and psychology may well prove to be a waste of time, energy and money” (See The Gene Illusion: Genetic Research in Psychiatry and Psychology Under the Microscope, p. 345).

    He also believes that the high correlations between identical twins for certain mental illnesses is the result of [u]“ego fusion” (p.57) and that schizophrenia is the consequence of having grown up in deeply disturbed homes (p. 273). Somehow he manages to completely ignore an entire literature from neuroimaging scans that shows abnormalities in the brains of people with schizophrenia (Hanson, 2004).)

    His thinking is completely black and white and 20 years behind—he sees no role for genetic/biological influences at all in anything. He’s got an agenda, Consig: He worries that genetic studies will be used for eugenics (and he completely ignores the medical/healing potential).

    In any scientific field, you can find scientists who will make criticisms of the research methodology. Tobacco companies had their own scientists who quite legitimately argued that you can’t infer causation from correlation—just because smoking and lung cancer are related, there’s no reason to believe that one causes the other! Why, it could be that there’s a third variable (like stress) involved!!!

    When people don’t like research findings, they argue methodology, and usually, there’s some level of merit to the criticism—whenever you deal with human subjects, you’re precluded from randomly assigning people to social conditions and there are inevitable compromises. Does this invalidate the research? No. As I’ve said repeatedly, science does not draw conclusions from single studies. Conclusions are only drawn based on converging evidence across multiple domains, using multiple methodologies and measures.

    Twins raised together, whether MZ or DZ have similar environments—it is about as close as we can get without actually separating twins at birth (which you’ve also dismissed as invalid). Are the environments identical? Not precisely. But it isn’t a bad approximation, and most scientists would agree.

    So for example, if we look at the IQ of unrelated persons who are reared together, the correlation between them on IQ is about .20—for identical twins reared apart, the correlation is .70 and for MZ reared together, .90. Bothers and sisters reared together are at about .45 but when reared apart, they’re at about .25. Do you think that tells us anything about IQ, environment and genetics?

    Well, it should—further research has shown us that enriching the environment increases IQ. (You can thank a twin study for things like food programs, enrichment programs, and so on).

    I do believe that the equal environment assumption underlying the twin studies does fatally flaw them. Nevertheless, I believe I’m obligated to proceed on with the discussion of the twin studies in the event that there may be a soul or two that is still grasping to those studies.

    Sorry, Consig, I’m not about to stick around to be a foil for a polemic.

  19. Polemic is defined as: a person who argues in opposition to another.  I was clear, on the other thread, in my e-mails, and in the introduction of this thread, that I would argue against the twin studies.  I have. I can only say that of course I was going to be a polemic.  I said I was going to be.

    Now that I’ve begun in earnest, in the third tantrum I’ve seen from you in response to my criticisms of the twin studies, you are withdrawing from the discussion.  I’m disappointed to say the least.  I’ve spent considerable amounts of time to prepare for this thread so that I could present a coherent picture of why the twin studies are flawed and how they are often misused when individuals reference them.

    I will address but one point in your parting post. I find it puzzling that you don’t find fault with Beckwith or Alpers.  The credentials of both men are impeccable.  That is likely why you have ignored them and the linked article above.  Joseph’s words were simply representative of the body of criticism.  That is why I took the time to link to a sampling of the critics and their credentials.  The very point of linking to those individual was to reflect the cross-section of backgrounds for those who have written criticism of twin studies.  Joseph isn’t standing alone. Each of those individuals cited, and many more, has written that twin studies are flawed when they are used to represent heritability of behavioral traits. 

    In response, you have compared them as a group to tobacco scientists.  They represent a cross-section of doctors, psychiatrists, scientists, and researchers.  Given the body of work that they represent, that analogy is lacking in all respects.  Moreover, the comparison of the aforementioned individuals to tobacco scientists is insulting to them, and wholly lacking, if one only looks at their respective body of work. 

    To the readers, and most especially to Les given the efforts that you have undertaken, I apologize to you for Shelley’s withdrawal from the discussion.  I blame myself for not having been more judicious in choosing a counterpart. 

    I would like to finish the discussion of the twin studies.  In light of Shelley’s withdrawal, I will endeavor to present a summary of the twin studies.  That will entail two or three more posts.  After those posts, consider the thread open.

  20. The polemicist , on the other hand, proceeds encased in privileges that he possesses in advance and will never agree to question. On principle, he possesses rights authorizing him to wage war and making that struggle a just undertaking; the person he confronts is not a partner in search for the truth but an adversary, an enemy who is wrong, who is armful, and whose very existence constitutes a threat. For him, then the game consists not of recognizing this person as a subject having the right to speak but of abolishing him as interlocutor, from any possible dialogue; and his final objective will be not to come as close as possible to a difficult truth but to bring about the triumph of the just cause he has been manifestly upholding from the beginning. The polemicist relies on a legitimacy that his adversary is by definition denied.

    Michel Foucault

    In light of Shelley’s withdrawal, I will endeavor to present a summary of the twin studies.  That will entail two or three more posts.

    I had no doubt that you would do so.

  21. Moving on to the studies themselves.  J. Michael Bailey and Richard Pillard authored a study in 1991 that has garnered the most attention. This is the extent of what I was able to locate in the way of a homepage for Pillard.  He does have a Wikipedia entry.  Bailey’s home page is here.

    As a preliminary note, Bailey has been recently embroiled in a controversy over a book entitled: “The Man Who Would Be Queen.”  There were allegations that he obtained intimate details from men seeking sex change operations without informing them that they were part of a study.  Allegedly, those details were then included in his book.  His account of the incident may be found through his home page.  An account of those critical of Bailey may be found here. My only comment, since I haven’t read the book and don’t know enough about the incident, is that I find it fascinating how Bailey has managed to come under fire from groups that once championed him.

    A copy of the 1991 study is not available online.  Most of the material from that study is widely known and discussed.  As a result, I will be referring to articles that report on the study for references, rather than the study proper.

    The 1991 study is the study that reported 52% proband concordance rates within the group of identical twins. See here.  The proband concordance rates for fraternal twins was 22%. Id. From discordance between the identical and fraternal twins, a heritability rate was estimated.  The result was that homosexuality was widely reported to have a genetic component based on this study.  Such claims have also been made here.

    Bailey was concerned, and mentions his concerns in the study itself, that there might be an ascertainment bias driving the concordance rates higher.  Id.  “Ascertainment bias refers to a systematic distortion in measuring the true frequency of a phenomenon due to the way in which the data are collected.”  See Example.  The ascertainment bias Bailey was concerned about had to do with how he and Pillard put together the study.

    When Bailey and Pillard set out to recruit a study population they did this by advertising for participants in homosexual literature.  Bailey was concerned that twins deciding whether to participate in a study clearly related to homosexuality may have considered the sexual orientation of their co-twins before agreeing to participate.  I’ll come back to this later, when I go through another study Bailey did more recently.

    Another criticism of the study was how the results were reported.  To determine the heritability findings, the authors were using proband concordance rates.  This was puzzling because as was agreed earlier, the pairwise rate is usually used when we want to determine whether MZ and DZ twins differ in terms of their concordance.  The result of using a proband concordance rate is that it will reflect an increase in the reported percentage.  This will be illustrated and the calculations from the raw figures from the 2000 study will be provided.

    Two other criticisms of the 91 study should be mentioned.  The study relied on the participants to report the sexual orientation of their relatives instead of verifying this.  The last criticism is one that is unlikely to have much impact on any twin study outcomes, nevertheless, zygosity of the twins should be determined by blood tests rather than self-reporting. Definitive Limitations

    Now, on the basis of the criticisms that exist it was possible for reasonable people to disagree, (assuming that one accepts the validity of the EEA) about what could be inferred.  It would have been reasonable to say that the study reflected a genetic component, albeit, a somewhat inflated one.  That would have been solid ground.

    It is not now though.  The primary author of the study, Bailey, in his subsequent 2000 study, has explicitly stated:

    concordances from prior studies were inflated because of concordance-dependent ascertainment bias

    Bailey 2000

    I’ll next go through the 2000 report.  There are 2 other studies that should be mentioned as well.  Those will follow in the next two posts.

  22. Bailey’s 2000 study is linked above.

    The 2000 study population was put together in a way that attempted to eliminate the ascertainment bias the authors were concerned about in the 1991 study.  The latter study sent sex surveys to twins who participated in prior survey and were derived from the Australian twin registry.  In all, there were 4,901 individual twins that completed surveys. Unfortunately, zygosity remained self-reported.

    The findings of the study revealed that the rate of exclusive heterosexuality was roughly the same between men and women around 92%.  As it relates to discussions elsewhere on the site, women were more likely to have slight to moderate homosexual feelings, while men tended to be more exclusively homosexual.

    The study found 3 pairs of identical twins in which both twins were homosexual, and 24 pairs of identical twins in which 1 twin was homosexual and the other was not.  There were no pairs of fraternal twins in which both twins were homosexual and 16 pairs of fraternal twins in which 1 twin was homosexual. 

    To calculate the proband concordance rate, it is important to know that each individual twin that is homosexual is considered a proband.  What this means is that the twin pairs in which both are homosexual are counted twice.  So, the 3 is doubled to 6 to determine the proband concordance rate.  Since there is only 1 homosexual twin in each pair of identical twins where there is only 1 homosexual, the 24 stays static.  The now doubled number of identical twins in which both twins are homosexual, 6, is added to the static number of 24 to come up with total number of pairs, 30, and then the 6 is divided by 30 which equals 20%.


    Pairwise concordance rates don’t double the number of twins because to figure this rate the numbers stay static.  So, it is 3 divided by, 3 and 24, 27 and that equals 11.1% 


    As is evident, the concordance rates for identical twins is nearly doubled by reporting a proband concordance rate.  The same doubling does not take effect for the fraternal twins which impacts the heritability rates that are reported.

    There were no pairs of fraternal twins in which both were homosexual.  So, there is nothing to double.  The number of twins in which 1 twin is homosexual, 16, stays static and 0 is divided by 16 which equals 0


    As was indicated earlier, it is not the raw concordance scores that are important.  It is the spread between the identical and fraternal twin scores that is important.  When a proband reporting method is employed here, it nearly doubles that spread and accordingly the heritability estimates.

    That said, the 2000 Bailey study, even with the nearly doubling of the spread, concluded that:

    Consistent with several studies of siblings we found that sexual orientation is familial. In contrast to most prior twin studies of sexual orientation, however, ours did not provide statistically significant support for the importance of genetic factors for that trait.

    Bailey 2000 at 20. 

    The Bailey study also finds moderate heritability ranges for sexual orientation.  Id.  Bailey has stated that is because heritability does not translate into genetic factors. 

    It should be noted that Bailey has cautioned that:

    Despite the large number of participants, there was an insufficient number of nonheterosexual (and especially homosexual) participants to guarantee a high degree of statistical power in the genetic and environmental analyses.


    It is clear now that the prior study from 1991 was fatally flawed.  Accordingly, citation to that study as support that there is a genetic basis for homosexuality is error.

  23. There are 2 other studies that should be mentioned.  The first is King & McDonald.  The study was completed in 1992.  It found a concordance rate for identical twins of 10% and concordance rate fraternal twins of 8%.  The study concluded that a 2% difference between the two groups does not implicate genetic factors. 

    The second study is Bearman & Bruckner It should be noted that the researchers involved, Peter Bearman and Hannah Bruckner are well known and have been cited repeatedly here by Les, zilch and Sexy Sadie for the research they have done on abstinence pledges.  They are clearly not the proverbial “tobacco” social scientists, yet they have similar observations about twin studies.

    Here is what they had to say about separating out environment from genetic factors:

    Outside an experimental context, separating additive genetic variance of a trait from non-additive variance is difficult, if not impossible

    That is what was being pointed out above regarding the EEA. 

    A review of the protocols for the Bearman & Bruckner reveal that the protocols are vastly superior to those utilized by prior twin studies.  The study was done at 3 different points in time.  The subjects and their siblings had DNA samples taken.  The genetic sample consists of 289 pairs of MZ twins, 495 pairs of DZ twins, 1251 pairs of full siblings, 442 pairs of half siblings, and 662 pairs of nonrelated siblings. Id at 11. The question utilized was broader than a study of homosexual behavior and instead focused on attraction.  Id at 12.

    Among identical twins there was an 6.7% concordance rate.  Fraternal twins were concordant at 7.2& and full siblings were 5.5% concordant. To quote the study:

    Clearly, the observed concordance rates do not correspond to degrees of genetic similarity. None of the comparisons between MZ twins and others in table 5 are even remotely significant.

    They conclude, with good reason that:

    The findings presented in this paper confirm some findings from previous research and stand in marked contrast to most previous research in a number of respects. First, we find no evidence for intrauterine transfer of hormone effects on social behavior. Second, we find no support for genetic influences on same-sex preference net of social structural constraints. Third, we find no evidence for a speculative evolutionary model of homosexual preference. Finally, we find substantial indirect evidence in support of a socialization model at the individual level.

    Why do they believe their research netted different results?

    Substantially higher concordance for homosexual orientation has been reported in previous research. We believe that previous work is largely incorrect as a result of reliance on nonrepresentative samples, for example, readers of gay publications, and reliance on indirect
    evidence. Specifically, while some studies obtained reports on sexual orientation from both
    siblings, others relied on one individual’s report on his or her sibling’s sexual orientation. These
    data structures are clearly associated with potential bias on the dependent variable….the inflation of concordance may be a product of an interaction between small sample
    size and subtle selection dynamics. Specifically, their sibling and twin response rates were low.18
    If individuals jointly participate in a study, and self-selection dynamics are present, as they likely are in this case, then concordance on traits other than willingness to participate in a study is to be expected. Consequently, we consider their concordance rates for same-sex orientation to be higher than would be expected under study designs less susceptible to self-selection.

    For those that have taken the time to read through the thread up to this point will recognize that the exact methodologies that were being criticized earlier are those being pinpointed by Bearman and Bruckner for skewing the results in the prior studies.

    It should be noted that the study found no correlation between an increase in same-sex attraction and birth order.  That is, the study directly calls into question the biological arguments that are not directly addressed in this thread but are hinted at elsewhere.

    What is most surprising about this study is not the results.  It is that there was no media circus about the conclusions from researchers who are so familiar with the media from the abstinence pledges. 

    Again, to those that have read this far, thank you for your patience with me. Please consider the thread open for comments.

  24. itdontmatter United States on 11/13/06 at 08:44 PM wrote the following…

    I have a curiosity in knowing what causes a person to be gay, although I seriously doubt that a discussion such as this will bear any fruit.  I personally know that being gay not a choice, I also do not believe that it is caused by ‘nurture’. 

    I still feel this way and I still do not know what this Twin Studies discussion proved or was intended to prove.

  25. Twin studies provide good clues to the etiology of various conditions as to whether they are genetic or environmental. 

    I also feel that being gay is no more of a choice than my heterosexuality was.  There was no time at which I wondered; “Hmm, men or women?”  And most human characteristics are subject to wide variation.  It seems likely to me that a great number of people are naturally more flexible in their sexuality and when they choose to go one way or the other, the religious right will say that is normative for everyone, that everyone has a choice.

  26. Twin studies provide good clues to the etiology of various conditions as to whether they are genetic or environmental.

    I fully agree with that. 

    I had expressed my serious doubts that this Twin Study discussion would bear fruit.

  27. I still do not know what this Twin Studies discussion proved or was intended to prove.

    The purpose of the thread from my perspective was rather limited.  The known evidence for genetic causation of homosexuality was limited.  Hamer’s studies have not been replicated.  Levay’s work doesn’t suggest a genetic basis, if one gets past the problems with the study itself.  That left reliance on twin studies as the basis for any claims of genetic influence. 

    With respect to the twin studies, I had two points I wanted to make in this thread.  First, to clarify exactly why twin studies for human behavior are problematic themselves. Second, even if one believes twin studies are good indicators of genetic causation, I wanted to document what the twin studies actually reflect.  The reason being, that if one were to accept twin studies as a legitimate means of discerning enironmental/genetic influences, the latest studies, utilizing better methodologies (still flawed but better nonetheless) don’t reflect any type of genetic influence.

    In so doing, I hoped to to end statements like this:

    It seems certain, however, that there is a strong genetic componenent,[sic] based on twin studies:(citation to stats from Bailey & Pillard’s studies in the early 90s)

    Such a statement is simply not true and misrepresents what the twin studies actually show in light of the studies that followed and admissions by the very author of the study that it was flawed.

    The scope of the discussion, on my part at least, did not encompass any attempt to change feelings, yours or others.  Attempts at changing feelings on an issue where so many have so much emotional investment would be an exercise in futility.  My purpose was simply to bring some clarity to the issue of twin studies and reports of genetic causation.  I hope this thread has done that.

    As regards the issues of biological causation or a complex genetic-environmental pathway with multiple interactions, I believe its too early to be drawing any definitive conclusions.

  28. Twin studies confound other traits….The nature/nurture debate is a false one

    You don’t get more polar; if there’s a confoundment, then the debate is real.

    The EBE theory asserts that biological factors, at childhood and before, derive temperments conducive to homosexuality, which are recognized through gender non-confirmity.

    All the hard-core biologists on this issue are working at finding the particular biological factors, rather than asserting they exist. That’s where the twin studies came in.

Leave a Reply

Your email address will not be published. Required fields are marked *

This site uses Akismet to reduce spam. Learn how your comment data is processed.