# identical twin sexuality - always the same? **URL:** \ **Category:** Factual Questions\ **Created:** [November 9, 2010, 8:02pm UTC](https://boards.straightdope.com/t/identical-twin-sexuality-always-the-same/560079 "2010-11-09T20:02:04Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![crypto](https://avatars.discourse-cdn.com/v4/letter/c/b3f665/32.png) [@crypto](https://boards.straightdope.com/u/crypto)\ **Post date:** [November 9, 2010, 8:02pm UTC](https://boards.straightdope.com/t/identical-twin-sexuality-always-the-same/560079/1 "2010-11-09T20:02:04Z")
Have there been any studies done to show the outcome of twin sexuality? I’m thinking of identical twins, since they should be identical biologically, but if anyone has any info on fraternal twins, that would be interesting also. My question seeks percentages of the following: 1. both twins heterosexual 2. one twin homosexual For the spilt sexual set, has there been any studies as to differences in the two people (i.e. is there any discernible difference between the two that are tangible and measurable?) ---
**Author:** ![Acsenray](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/acsenray/32/4519_2.png) [@Acsenray](https://boards.straightdope.com/u/Acsenray)\ **Post date:** [November 9, 2010, 8:08pm UTC](https://boards.straightdope.com/t/identical-twin-sexuality-always-the-same/560079/2 "2010-11-09T20:08:37Z")
Among my oldest acquaintances is a pair of twins who have opposite sexual preferences. They look like identical twins to me, but they claim to be fraternal. They look so much alike, I think there must be some mixup involved. ---
**Author:** ![Shodan](https://avatars.discourse-cdn.com/v4/letter/s/9f8e36/32.png) [@Shodan](https://boards.straightdope.com/u/Shodan)\ **Post date:** [November 9, 2010, 8:12pm UTC](https://boards.straightdope.com/t/identical-twin-sexuality-always-the-same/560079/3 "2010-11-09T20:12:05Z")
> [@](#): > > …a Northwestern University study by J. Michael Bailey and Richard Pillard of 56 sets of identical twins in which at least one twin was a homosexual, discovered that in 52% of the cases, the other twin was homosexual also. Among 54 sets of fraternal twins in which one was homosexual, the other was homosexual 22% of the time. They also studied adoptive brothers and there found an 11 percent correlation. [Cite.](http://home.messiah.edu/~chase/h/articles/regenera/twins.htm) > [@](#): > > 48% of identical (monozygotic) twins of homosexual women were likewise > homosexual (lesbian) > 􀁺 16% of fraternal (dizygotic) twins were likewise homosexual > 􀁺 6% of adoptive sisters of homosexual women were likewise homosexuall [Cite.](http://abouthomosexuality.com/policy-org-twins.pdf) > [@](#): > > Twin studies have received a number of criticisms including self-selection bias where homosexuals with gay siblings are more likely to volunteer for studies. Nonetheless, it is possible to conclude that, given the difference in sexuality in so many sets of identical twins (who are genetically identical), sexual orientation cannot be purely caused by genetics.[9] > > Another issue is the recent finding that even monozygotic twins can be different and there is a mechanism which might account for monozygotic twins being discordant for homosexuality. Gringas and Chen (2001) describe a number of mechanisms which can lead to differences between monozygotic twins, the most relevant here being chorionicity and amniocity.[10] Dichorionic twins potentially have different hormonal environments and receive maternal blood from separate placenta. Monoamniotic twins share a hormonal environment, but can suffer from the ‘twin to twin transfusion syndrome’ in which one twin is “relatively stuffed with blood and the other exsanguinated”.[11] If one twin receives less testosterone and the other more, this could result in different levels of brain masculinisation. [Cite](http://en.wikipedia.org/wiki/Biology_and_sexual_orientation). Regards, Shodan ---
**Author:** ![panamajack](https://avatars.discourse-cdn.com/v4/letter/p/47e85d/32.png) [@panamajack](https://boards.straightdope.com/u/panamajack)\ **Post date:** [November 9, 2010, 8:31pm UTC](https://boards.straightdope.com/t/identical-twin-sexuality-always-the-same/560079/4 "2010-11-09T20:31:47Z")
> [@acsenray](#): > > Among my oldest acquaintances is a pair of twins who have opposite sexual preferences. They look like identical twins to me, but they claim to be fraternal. They look so much alike, I think there must be some mixup involved. The Olsen twins are fraternal, yet look so similar that they played the same exact child for years. ---
**Author:** ![sqweels](https://avatars.discourse-cdn.com/v4/letter/s/f19dbf/32.png) [@sqweels](https://boards.straightdope.com/u/sqweels)\ **Post date:** [November 9, 2010, 8:50pm UTC](https://boards.straightdope.com/t/identical-twin-sexuality-always-the-same/560079/5 "2010-11-09T20:50:46Z")
> [@panamajack](#): > > The Olsen twins are fraternal… That’s funny, I thought they were sisters. ---
**Author:** ![Tim\_T-Bonham.net](https://avatars.discourse-cdn.com/v4/letter/t/46a35a/32.png) [@Tim\_T-Bonham.net](https://boards.straightdope.com/u/Tim_T-Bonham.net)\ **Post date:** [November 9, 2010, 8:52pm UTC](https://boards.straightdope.com/t/identical-twin-sexuality-always-the-same/560079/6 "2010-11-09T20:52:04Z")
There have been a few studies (Shodan cited several of them). To summarize, most seem to show: - identical twins – about 50% likelihood of same sexuality (gay, straight, bi) - fraternal twins – about 25% likelihood. - pairs of brothers or sisters – about 15% likelihood - adoptive brothers or sisters – about 11-12% likelihood. All these are higher than the base figure, which is roughly 10% gay, 90% straight. The percentages for males seem to be higher than for females (though this may just be that females are less publicly open about sexuality). And the percentages for twins seem to be true even if they are separated at birth and raised in different families. Somewhat confusing, but seems to show that sexuality is determined by both heredity AND environment. The twins findings would indicate the influence of heredity, but the adoptive findings indicates some influence from environment. Sexuality is a very complex thing! ---
**Author:** ![Chessic\_Sense](https://avatars.discourse-cdn.com/v4/letter/c/7c8e57/32.png) [@Chessic\_Sense](https://boards.straightdope.com/u/Chessic_Sense)\ **Post date:** [November 9, 2010, 9:02pm UTC](https://boards.straightdope.com/t/identical-twin-sexuality-always-the-same/560079/7 "2010-11-09T21:02:38Z")
> [@t-bonham@scc.net](#): > > There have been a few studies (Shodan cited several of them). > > To summarize, most seem to show: > > - identical twins – about 50% likelihood of same sexuality (gay, straight, bi) > - fraternal twins – about 25% likelihood. > - pairs of brothers or sisters – about 15% likelihood > - adoptive brothers or sisters – about 11-12% likelihood. > All these are higher than the base figure, which is roughly 10% gay, 90% straight. The percentages for males seem to be higher than for females (though this may just be that females are less publicly open about sexuality). And the percentages for twins seem to be true even if they are separated at birth and raised in different families. > > Somewhat confusing, but seems to show that sexuality is determined by both heredity AND environment. The twins findings would indicate the influence of heredity, but the adoptive findings indicates some influence from environment. Sexuality is a very complex thing! Whoa, whoa, whoa. Those numbers are for pairs in which at least one member is gay. There’s no way that if I’m straight, there’s only a 50% chance that my identical twin is straight, too. I know you know that. I’m just being a good Doper and being precise/pedantic. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [November 9, 2010, 9:15pm UTC](https://boards.straightdope.com/t/identical-twin-sexuality-always-the-same/560079/8 "2010-11-09T21:15:03Z")
> [@](#): > > - adoptive brothers or sisters – about 11-12% likelihood. > All these are higher than the base figure, which is roughly 10% gay, 90% straight. I wouldn’t say that that adoptive figure is higher. Yeah, 11 or 12 is higher than 10, but there’s enough uncertainty in all of those numbers that you can’t really tell the difference. The big difficulty in sexuality studies is the need for self-reporting, and there are a significant number of people who would lie about their sexuality to a researcher, or even to themselves. To the extent that adopted siblings have a higher-than-normal correlation, it could probably be explained entirely by this “closet effect”, that they would both or neither be raised in an environment where they’d be allowed to admit what they are. ---
**Author:** ![Quercus](https://avatars.discourse-cdn.com/v4/letter/q/7ab992/32.png) [@Quercus](https://boards.straightdope.com/u/Quercus)\ **Post date:** [November 9, 2010, 9:32pm UTC](https://boards.straightdope.com/t/identical-twin-sexuality-always-the-same/560079/9 "2010-11-09T21:32:28Z")
> [@t-bonham@scc.net](#): > > … the base figure, which is roughly 10% gay, 90% straight. I was under the impression that most recent estimates are lower than 10%. Do you have a cite for that? ---
**Author:** ![Musicat](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/musicat/32/20189_2.png) [@Musicat](https://boards.straightdope.com/u/Musicat)\ **Post date:** [November 9, 2010, 9:43pm UTC](https://boards.straightdope.com/t/identical-twin-sexuality-always-the-same/560079/10 "2010-11-09T21:43:13Z")
> [@Quercus](#): > > I was under the impression that most recent estimates are lower than 10%. Do you have a cite for that? Just another caution besides the statistics one Chronos brought up…just what is “gay”? This can be a subjective assignment and can fluctuate with social mores. If being gay is a capital offense, you will get less of the population declaring themselves gay than if it is the hip thing to be. And if you experimented with homosexuality once, what category will you fall into? I’m not trying to define it, just pointing out the statistical uncertainty that exists here. ---
**Author:** ![alphaboi867](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/alphaboi867/32/3898_2.png) [@alphaboi867](https://boards.straightdope.com/u/alphaboi867)\ **Post date:** [November 9, 2010, 9:59pm UTC](https://boards.straightdope.com/t/identical-twin-sexuality-always-the-same/560079/11 "2010-11-09T21:59:07Z")
I personally know two sets of indentical twins where one is gay and the other straight. ---
**Author:** ![MandaJo](https://avatars.discourse-cdn.com/v4/letter/m/ac91a4/32.png) [@MandaJo](https://boards.straightdope.com/u/MandaJo)\ **Post date:** [November 9, 2010, 11:30pm UTC](https://boards.straightdope.com/t/identical-twin-sexuality-always-the-same/560079/12 "2010-11-09T23:30:08Z")
> [@Stink Fish Pot](#): > > I’m thinking of identical twins, since they should be identical biologically, but if anyone has any info on fraternal twins, that would be interesting also. Identical _genetically_ does not mean identical _biologically_. Developmental events can fundamentally alter your biology without effecting your genetics. Cerebral palsy, for example, is the result of brain damage in the womb or during/around birth, but it’s still a biological condition. ---
**Author:** ![MPB\_in\_Salt\_Lake](https://avatars.discourse-cdn.com/v4/letter/m/c0e974/32.png) [@MPB\_in\_Salt\_Lake](https://boards.straightdope.com/u/MPB_in_Salt_Lake)\ **Post date:** [November 10, 2010, 12:03am UTC](https://boards.straightdope.com/t/identical-twin-sexuality-always-the-same/560079/13 "2010-11-10T00:03:45Z")
> [@alphaboi867](#): > > I personally know two sets of indentical twins where one is gay and the other straight. Me too; two brothers who I knew since they were just kids… ---
**Author:** ![Little\_Nemo](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/little_nemo/32/3120_2.png) [@Little\_Nemo](https://boards.straightdope.com/u/Little_Nemo)\ **Post date:** [November 10, 2010, 1:52am UTC](https://boards.straightdope.com/t/identical-twin-sexuality-always-the-same/560079/14 "2010-11-10T01:52:07Z")
There’s one celebrity example. Alexandra Paul, the actress, is straight. She has an identical twin sister, Caroline, who is gay. ---
**Author:** ![Markxxx](https://avatars.discourse-cdn.com/v4/letter/m/5daacb/32.png) [@Markxxx](https://boards.straightdope.com/u/Markxxx)\ **Post date:** [November 10, 2010, 2:04am UTC](https://boards.straightdope.com/t/identical-twin-sexuality-always-the-same/560079/15 "2010-11-10T02:04:05Z")
> [@Chronos](#): > > IThe big difficulty in sexuality studies is the need for self-reporting, and there are a significant number of people who would lie about their sexuality to a researcher, or even to themselves. This is a EXTREMELY important point, I was hired by a local gay health center in conjunction with a Chicago university to administer questions for a study, and it was VERY hard to do this. I don’t believe the people were lying but were not understanding the questions. I had set up a 100 question survey and use key questions to see if answers given could invalidate others. In otherwords we only wanted 22 questions and the rest were phrased differently to see if the core 22 question were being answerd consistantly. For instance, we never would say gay, or straight or bi. We used direct questions. You a male, received oral sex from another male. We alled it “males who have had sex with males” You had to be extremely explict about it, to get an accurate feedback. I mean you would have to ask “During the course of the event, did you mouth ever come into any contact with his genitalia.” You had to be that specific to get the answers you needed. Actually it was more specific, but that is too much information for GQ 🙂 ---
**Author:** ![John\_DiFool](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/john_difool/32/19543_2.png) [@John\_DiFool](https://boards.straightdope.com/u/John_DiFool)\ **Post date:** [November 10, 2010, 2:08am UTC](https://boards.straightdope.com/t/identical-twin-sexuality-always-the-same/560079/16 "2010-11-10T02:08:05Z")
> [@sqweels](#): > > That’s funny, I thought they were sisters. Sororial twins? ---
**Author:** ![dzero](https://avatars.discourse-cdn.com/v4/letter/d/5f9b8f/32.png) [@dzero](https://boards.straightdope.com/u/dzero)\ **Post date:** [November 10, 2010, 3:09am UTC](https://boards.straightdope.com/t/identical-twin-sexuality-always-the-same/560079/17 "2010-11-10T03:09:18Z")
> [@MandaJo](#): > > Identical _genetically_ does not mean identical _biologically_. Developmental events can fundamentally alter your biology without effecting your genetics. Cerebral palsy, for example, is the result of brain damage in the womb or during/around birth, but it’s still a biological condition. There are also epigenetic changes that occur. [Here is the wiki entry but I don’t think it is very good.](http://en.wikipedia.org/wiki/Epigenetics) Epigenetic changes consist of alterations to the chromosomes that inhibit or promote specific genes without actually altering the DNA sequence. So for example, you can have the correct sequence to manufacture alcohol dehydrogenase, but if that gene is methylated (a methyl group is attached to the initiator - I think), then that gene won’t be expressed (or at the very least will be down regulated). Phosphorylation has the opposite effect but I’m not very familiar with that process. If I’ve gotten anything grossly wrong I hope someone will correct me. [Wiki entry on DNA methylation](http://en.wikipedia.org/wiki/DNA_methylation) ---
**Author:** ![SCSimmons](https://avatars.discourse-cdn.com/v4/letter/s/e495f1/32.png) [@SCSimmons](https://boards.straightdope.com/u/SCSimmons)\ **Post date:** [November 10, 2010, 4:24am UTC](https://boards.straightdope.com/t/identical-twin-sexuality-always-the-same/560079/18 "2010-11-10T04:24:10Z")
> [@Little\_Nemo](#): > > There’s one celebrity example. Alexandra Paul, the actress, is straight. She has an identical twin sister, Caroline, who is gay. :eek: Um … I’ll be in my bunk … ---
**Author:** ![panache45](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/panache45/32/64_2.png) [@panache45](https://boards.straightdope.com/u/panache45)\ **Post date:** [November 10, 2010, 10:58am UTC](https://boards.straightdope.com/t/identical-twin-sexuality-always-the-same/560079/19 "2010-11-10T10:58:27Z")
I’ve known three pairs of twins with different orientations (two male pairs and one female). In one case I knew the gay one for several years before meeting his brother. It took a lot to convince myself that the brother was straight; I was so used to seeing the “gay version.” ---
**Author:** ![Pandoranoid](https://avatars.discourse-cdn.com/v4/letter/p/ce73a5/32.png) [@Pandoranoid](https://boards.straightdope.com/u/Pandoranoid)\ **Post date:** [November 10, 2010, 9:45pm UTC](https://boards.straightdope.com/t/identical-twin-sexuality-always-the-same/560079/20 "2010-11-10T21:45:36Z")
I know one pair of identical twins with one homosexual sister and one purportedly bisexual. The bisexual one is my friend—she mainly dates men, but I have known her to be with women. And she’s kinda flirty with me. But really, she seems more hypersexual than anything-she always reminds me of this old Madonna interview where she told Johnny Carson that she flirted with everyone from garbage collectors to grandmothers. So I dunno if that counts as an example of divergent sexual preference or not. [Next page](https://boards.straightdope.com/t/identical-twin-sexuality-always-the-same/560079.md?page=2)