Please let me know if anything in this post is confusing or too technical. I'm being trained as a geneticist and am not always sure how to write for a non-technical audience.
Having delved into the background of twin studies yesterday, today we will review the latest and most informative twin studies of homosexuality.
Archives of General Psychiatry, 1991, Volume 48, pages 1089-1096
For identical twins: 29/56 of the twins were also homosexual (52%)
For fraternal twins: 12/52 of the twins were also homosexual (22%)
For related, non-twin brothers: 13/142 of the brothers were also homosexual (9%)
For adopted brothers: 6/57 of the brothers were also homosexual (11%)
From our discussion yesterday, these data suggest a genetic contribution to homosexuality because the identical twins are more likely than fraternal twins to both be homosexual. We also see evidence for a shared uterine environment because the fraternal twins are more likely to both be homosexual than the related, non-twin siblings. We also see evidence for a shared environmental effect because the adopted brothers and non-twin brothers are similarly likely to be homosexual and this is above the population rate of homosexuality of ~5%. The study ended up estimating that the genetic contribution to homosexuality ranged between 31%-74%, with 17%-69% coming from unique environmental effects, and 0%-23% coming from shared environmental effects.
The biggest criticism of this study is that the participation rate may be influenced by whether or not the homosexual has a homosexual sibling and so that could affect the estimated rates of homosexuality in the different groups, however as long as there was no difference between whether someone decided to participate based on whether they had an identical twin vs a fraternal twin, the basic conclusions still stand. Another criticism is that only male twins were analyzed.
Journal of Personality and Social Psychology, 2000, Volume 78, pages 524-536
39/299 male identical twin pairs had at least one homosexual twin, with 9/39 having both twins being homosexual (37.5%). 31/177 male fraternal twin pairs had at least one homosexual twin with 1/31 having both twins being homosexual (6.3%). 79/618 female identical twin pairs had at least one homosexual twin, with 14/75 having both twins being homosexual (30.1%). 45/338 female fraternal twin pairs had at least one homosexual twin, with 8/37 having both twins being homosexual (30.2%).
This data again supports a genetic basis for male homosexuality with identical twins being more likely to both be homosexual than fraternal twins, but not for female homosexuality where both identical and fraternal twins are equally likely to both be homosexual. The study estimated for male homosexuality it is 45% genetic, 0% shared environment, and 55% unique environment. For female homosexuality it is 8% genetic, 41% shared environment, and 50% unique environment. Confidence intervals contained 0 in all of the estimates, except for unique environment estimates.
This study overcame the prior criticism of selective recruitment by using a national database of twins, however the number of homosexuals is low in the population, so most of the twin groups were actually smaller than in the first study, which leads to less precision in the estimates of genetic and environmental effects.
Archives of Sexual Behavior, 2010, Volume 39, pages 75-80
71/807 male identical twin pairs had at least one homosexual twin, with 7/71 having both twins being homosexual (10%). 53/517 male fraternal twin pairs had at least one homosexual twin with 3/53 having both twins being homosexual (6%). 214/1513 female identical twin pairs had at least one homosexual twin, with 26/214 having both twins being homosexual (12%). 140/989 female fraternal twin pairs had at least one homosexual twin, with 13/140 having both twins being homosexual (9%).
This data again supports a genetic basis for male homosexuality with identical twins being more likely to both be homosexual than fraternal twins, and some support for female homosexuality being genetic with identical twins have a slightly higher rate of shared homosexuality than fraternal twins. The study estimated for male homosexuality it is 39% genetic, 0% shared environment, and 61% unique environment. For female homosexuality it is 19% genetic, 17% shared environment, and 64% unique environment. Confidence intervals contained 0 in all of the estimates, except for unique environment estimates.
This study also avoided the criticism of selective recruitment by using a national database of twins, however even using the largest population registry of twins in the entire world, most of the twin groups were not much larger than in the first study due to the small numbers of homosexuals in the population, so estimates of genetic and environmental effects on homosexuality are still less precise than we might hope. This study also used sexual behavior rather than sexual orientation. As previously discussed on my blog, this is an imprecise measure due to straight men that have sex with other men and gay men who have not had sex with men (like me). This may also have contributed to the lower estimates of homosexuality in this study.
Conclusions
I love stuff like this. I would also be interested to see how these twin studies compare to other studies of traits with unclear origins, such as handedness.
ReplyDeleteThanks for doing all the hard work for us!
When most lay people say "genetic" I think they mean "biologically determined at birth." This is faulty intuition for two reasons: 1) it erroneously labels effects of the uterine environment as genetic and 2) it neglects the fact that genetic causes result in an observable increase in the trait's appearance in a population but do not predict with certainty the presence or absence of a trait in a given individual.
ReplyDeleteI think we need a new term that is both scientifically accurate and captures the intuition of "genetically and/or prenatally influenced." Does such a term exist?
Another question I have is this: how would you test a hypothesis that the uterine environment contributes to a trait in the offspring but that the aspect of the uterine environment that is responsible for this is itself genetically determined? Does this ever arise in studying other traits beside homosexuality?
Birth order is probably one of the best documented correlations with homosexuality in men (but, interestingly, not women). Have you looked at those studies?
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ReplyDelete@GTBF - that's an excellent suggestion. I'll see what I can dig up.
ReplyDelete@MoHoHawaii - I am not aware of a specific term for "genetically and/or prenatally influenced." Interesting question regarding the genetics influencing the uterine environment, I am not currently aware of any known human offspring trait resulting from a uterine environment (unless you count death - where some deaths come from a genetic maternal inability to provide the proper uterine environment for the fetus). It seems to look for such a trait you would need to look at the maternal and offspring genetics and find when a trait is influenced by the maternal genetics without regard to the offspring genotype. With such a trait, you would expect identical and fraternal twins to have the same rate of the trait, but should be increased above the population rate. I am aware of some such traits in the genetic research of fruit flies which exhibit a phenomenon known as "maternal effects". In fruit flies the first couple of days of development are driven by maternal proteins, so if the mother fruit fly has mutation in one of those fruit flies, the baby fruit flies develop abnormally. The genetic approach to prove that a trait is a maternal effect is quite complex and similar studies would be impossible in humans.
I am aware of the birth order phenomenon for homosexuality, but I have not yet looked at the original research for the correlation. I am interested in it, however, as I am the oldest child and wonder what that means about my 4 younger brothers. I am planning on reviewing that research on my blog at a future point.
@me - While I would agree that science knows more than the prophet knows, your social worker obviously doesn't understand the science. There are many ways in which being gay is not a choice where your identical twin is not gay and you are, genetics is only one of them. Your sexual orientation can be genetically influenced without an identical twin also being gay.
I am supportive of studies of twins and genetic/environmental research.
ReplyDeleteAs the mom of twins, however, I cannot imagine the ethics of splitting up twin pairs (for any reason). I read a book "Identical Strangers" about identical twin sisters separated at birth. Both adoptive parents were not told that the girls had a twin sister - neither of them knew. The parents probably would have taken both daughters if they had known. Psychologists were studying twin pairs (for nature or nurture) without proper consent of the parents.
I believe that there is some psychological connection for twins in the womb, and I disagree with splitting twins unless absolutely necessary. I would question what we would gain, as a society with this research, by purposefully separating twins.
I can agree to disagree here, it's not that I don't think the research is important. I just believe there are some lines that need to be drawn.
@Aerin - sorry, I must have come across as an evil scientist if you felt I was advocating for splitting up twins in the name of science. I do not believe in doing research without proper consent, and I think purposefully splitting twins for the sake of science is unethical and that it is difficult to obtain proper consent from parents and impossible to obtain proper consent from the twins.
ReplyDeleteRather, the study I proposed at the end of this post was to contact adult twins who had already been separated at birth, to ask them about their sexuality. Such a study would probably be difficult to perform due to the inability to find adequate numbers of twins to give groups large enough to be informative, as even the largest twin databases result in identifying very few homosexuals.
I think we can agree that twins should not be split up to perform any such experiments.
Thanks Mister Curie for clearing that up!
ReplyDeleteI agree about contacting adult twins - I think that's great. I didn't think of you as an evil scientist, I was just hoping for that clarification. Thanks again.
The study draws a conclusion that is biased. Fact - if genetic, the trait of homosexual would be at or near 100% in identical twins. There are lots of variables but they all point to non-conclusive evidences. That in turn points to the fact that homosexual behavior is just that, a behavior, deviant of the norm, a choice. Issues such as; how many individuals who lived a homosexual lifestyle but later halted that lifestyle in favor of "normal" sex with someone of the opposite sex is rarely discussed. Why? Points to a choice. The LGBT community for the most part is trying to force a lie on us. I personally know two individuals who lived the "gay" lifestyle for many years but now are happily married to someone of the opposite sex & have families. Again - not possible or probable if this were an inherent behavior. Evolution certainly does not support it, though I do not support macro evolution theory.
ReplyDelete