Genetics of Homosexuality: Twin Studies, Part III

By Mister Curie

Chris commented on my previous post about twin studies and the genetics of homosexuality that he would like to see a comparison to "other studies of traits with unclear origins, such as handedness."  So, Chris, this post's for you.

Neuropsychologia, Volume 47, January 2009, Pages 330-337

Twins from the Australian Twin Registry and the Netherland's twins registries were asked about what hand they used to write with.  54,270 individuals from 25,732 families participated in the study.  Identical twins both wrote with the same hand about 24% of the time, while fraternal twins wrote with the same hand about 14% of the time.  The study estimated a 23.6% genetic influence on handedness with 0% shared environmental influence, and 76.4% unique environmental influence on handedness.

And apparently handedness is first demonstrated around 9-10 weeks gestation, as embryos begin to have single arm movements.  Interesting . . .

As for a trait I'm a little more familiar with research on, cholesterol levels:

New England Journal of Medicine, 1993, Volume 328, pages 1150-1156

Twins from the Swedish Twin Registry were approached to have a physical exam where cholesterol levels were measured.  302 pairs of twins participated in the study and included sets of twins that grew up together and sets of twins that were separated at birth.  Identical twins that grew up together had similar total cholesterol levels 66% of the time, while fraternal twins that grew up together had similar total cholesterol levels 41% of the time.  When the twins were separated at birth, the identical twins had similar total cholesterol levels 37% of the time and fraternal twins had similar total cholesterol levels 14% of the time.  This study estimated that cholesterol levels are influenced genetically 63%, influenced by a shared environment 18%, and by non shared environment 19%.

Handedness is interesting because it's estimated genetic influence is quite a bit less than the estimates for the genetic influence on homosexuality, like homosexuality does not appear to have a shared environmental influence (meaning the rest of the influence are unique environmental factors), and yet I don't know of anyone who believes you choose to be right or left handed.

Cholesterol levels appear to have a somewhat higher genetic influence than homosexuality.  I am familiar with the genetic research on cholesterol levels and many genes are now being identified with influencing cholesterol levels.  There are also several severe genetic defects that are known to cause huge changes in cholesterol levels, many of these are familial diseases that often result in heart disease.

Genetics of Homosexuality: Twin studies, part II

By Mister Curie

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

Homosexuals were recruited through advertisements in gay magazines asking for the participation of homosexuals with twins or adopted brothers.  161 homosexuals responded, 115 with twin brothers and 46 with adoptive brothers.  Homosexuals were also asked about the existence of non-twin, related brothers.  Relatives were contacted to determine their sexual orientation.  Relatives and homosexuals were asked if they considered themselves "homosexual/gay, bisexual, or heterosexual" and were given a Kinsey assessment of sexual fantasy and behavior.

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

The study contacted twins who joined the volunteer Australian Twin Registry and asked for information on sexual orientation and Kinsey assessment.  9,112 twin pairs were contacted and 4,901 (54%) completed the questionnaires.  In the end, there were 312 male identical twin pairs, 182 male fraternal twin pairs, 668 female identical twin pairs, 376 female fraternal twin pairs, and 353 opposite-sex fraternal twin pairs, where both twins in the pair had completed the questionnaire.  Pairs were analyzed based on Kinsey scores.

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

The Swedish Twin registry records all twin births in Sweden and is the largest population register of twins in the world.  21,481 men and 21,607 women were contacted to participate in the study with 11,229 men and 14,096 women actually taking the survey.  The survey had no question about self-defined sexual orientation, instead it asked about whether one had ever been sexually together with a person of the same gender and how many different partners the person had been with of the same gender over the course of their lifetime.  In the end, there were 807 male identical twin pairs, 517 male fraternal twin pairs, 1,513 female identical twin pairs, and 989 female fraternal twin pairs, where both twins in the pair completed the survey.  Pairs were analyzed based on whether they had ever had a same-sex partner over the course of their entire lives.

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

The definitive twin study has not been performed and the data can be interpreted to support those who think there is and those who think there is not a genetic basis for homosexuality.  Although limited by size, all three studies support a genetic influence to male heterosexuality (39%-74%) with a smaller genetic contribution for female homosexuality (8%-19%). Shared environmental effects appear to be nearly non-existent in contributing to male homosexuality, but are more pronounced for female homosexuality (17%-41%).  However, the confidence interval includes 0 in the estimates of the genetic contribution to homosexuality.  Both male and female homosexuality appear to be strongly influenced by unique environmental effects, and it is only the unique environmental effects that can be definitively identified as contributing to homosexuality in the large, population based studies.  Due to the small number of homosexuals in the general population, even larger studies are needed to more precisely define the genetic and environmental contributions to homosexuality.  It would also be nice to have a twin study with twins separated at birth to better tease apart the genetic and environmental contributions to homosexuality.

Genetics of Homosexuality: Twin Studies, part I

By Mister Curie

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.

I found the results of my poll on whether scientists should study the genetics of homosexuality to be quite interesting.  While I received several comments on the post cautioning against the study of homosexuality genetics and describing the ways in which such research could be misused, when forced to choose one or the other, 90% (28 votes) thought that scientists should study the genetics of homosexuality, while only 10% (3 votes) believed that scientists should not.  This tells me that even though people are nervous about the implications of such research, most of my readers that voted are interested in learning more about the genetics of homosexuality, and that many would also be interested in a discussion of the ethical concerns raised by such research.  I am looking forward to working on several series of future posts to address these different interests.

Before delving into the details, a bit of background is probably required.  Homosexuality is complex and influenced by both genes and the environment.  When scientists first want to understand the relative contributions of genes and environment to the outcome of interest, they often turn to families and in particular, families with twins.  The first thing to investigate is whether the outcome of interest (such as homosexuality) is more likely to occur in families than in the general population.  If it is, that may come from shared genetics or shared environment.  Twins are useful in teasing these two apart.  Siblings share the same environment (except perhaps the uterine environment) but only half of the same DNA.  Identical twins share the same environment (including uterine environment) and have the exact same DNA.  Fraternal twins share the same environment (including uterine environment) but only half of the same DNA, they are no more genetically alike than siblings.  Really fancy twin studies will further tease the effects apart by studying adopted siblings (who share the environment but none of the same genes) and identical twins separated at birth (who share the same genes, but not the same environment - except the uterine environment).

 

Scientists then look at the different types of twin and sibling pairs and calculate how often the pairs have the same outcome of interest (here homosexuality).  Scientists can then estimate how much of the outcome comes from shared genes, shared environment, and unique environment.  If the identical twins have the same outcome of interest more often than fraternal twins, that is evidence of a genetic effect because the twins all shared the same common environment.  If fraternal twins have the same outcome of interest more often than siblings, that is evidence that the uterine environment may be important.  If related siblings have the same outcome more often than adopted siblings that is evidence of a genetic effect. If adopted siblings have the same outcome more often than population rates of the outcome, that is evidence of shared environment effects.  If identical twins that grow up together have the same outcome more often than identical twins separated at birth, that is evidence of a shared environment effect. Non-shared outcomes are attributed to unique environmental effects.

A few words additional words, identical twins do not need to always share the same outcome in order for it to be genetic, they only need to share the same outcome more often than fraternal twins (who share the same environment but not the same genes).  Twin studies are typically small and the results are typically presented as point estimates (given the data, what is the best estimate for the genetic contribution, shared environment contribution, and unique environment contribution) and confidence intervals (given the size of the sample, how much might the point estimate vary if we re-did the study with different individuals).  The point estimate is considered significant if the confidence interval does not include zero.  However, just because a confidence interval includes zero, that does not mean that there is no effect.  Larger studies are more likely to have confidence intervals that do not include zero if the point estimate is not zero.

Tomorrow we will review the latest and most informative twin studies for homosexuality.