Gay twin studies?

February 20, 2005 6:11 PM

Patty's coming out on tonight's episode of The Simpsons made me wonder how often it happens that one sibling in a pair of identical twins is gay and the other straight. Does anyone know of any twin studies about homosexuality?
posted by painquale to human relations (21 comments total)

Here's a reference for you: "Exotic Becomes Erotic: Interpreting the Biological Correlates of Sexual Orientation."
posted by googly at 6:19 PM on February 20, 2005

painquale: I belive if one male identical twin is gay, the other has a 50% chance of also being gay.
posted by delmoi at 6:31 PM on February 20, 2005

WAY TO BLOW IT!!!
posted by Quartermass at 6:31 PM on February 20, 2005

err,
Due to timezone differences, some of us havn't seen it yet. Not that it really matters that much...
posted by Quartermass at 6:38 PM on February 20, 2005

Here is a review article on TWIN STUDIES OF HOMOSEXUALITY.

There are many studies, but the question remains contentious. I've always been curious if the rate of homosexuality was higher among twins. This is what I've noted anecdotally.
posted by gesamtkunstwerk at 6:49 PM on February 20, 2005

Did you try Google at all, painquale, before posting here? You get relatively scholarly summaries (like I see in gesamtkunstwerk's comment on preview), skeptical takes from religious organizations with a political stake in disproving a genetic component, and thoughtful attempts to cover both camps fairly. It's not that hard to Google, read and *then* post to AskMe if you have questions.

On preview,
posted by mediareport at 6:59 PM on February 20, 2005

painquale: I belive if one male identical twin is gay, the other has a 50% chance of also being gay.

That it's not 100% in identicals suprises me. Doesn't that point to nurture?
posted by jikel_morten at 7:00 PM on February 20, 2005

Google Scholar turns up 439 studies about homosexuality in twins.

Among the more popularly cited studies otherwise are the Bailey & Pillard ones:

Bailey and Pillard (1991): occurrence of homosexuality among brothers

* 52% of identical (monozygotic) twins of homosexual men were likewise homosexual
* 22% of fraternal (dizygotic) twins were likewise homosexual
* 11% of adoptive brothers of homosexual men were likewise homosexual

J.M. Bailey and R.C. Pillard, “A genetic study of male sexual orientation,” Archives of General Psychiatry, vol. 48:1089-1096, December 1991.

Bailey and Pillard (1993): occurrence of homosexuality among sisters

* 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 homosexual

Bailey, J. M. and D. S. Benishay (1993), “Familial Aggregation of Female Sexual Orientation,” American Journal of Psychiatry 150(2): 272-277.

posted by Lush at 7:03 PM on February 20, 2005

Doesn't that point to nurture?

What it points to pretty obviously is a *mix* of nurture and nature - a mix that is in all likelihood widely varying in different people who consider themselves homosexual. I'll never understand why it's so hard for folks looking at this issue to grasp the concept that homosexual behavior may be >90% genetic in some people, <5 % genetic (and thus mostly social/psychological) in others, and somewhere in-between for most of us who favor erotic/emotional attachments to the same sex.br>
It's called variety. It's what humans are good at.
posted by mediareport at 7:06 PM on February 20, 2005

That it's not 100% in identicals suprises me. Doesn't that point to nurture?

Developmental biology concerns itself with how some things happen as a direct result of DNA and others by chance. So identical twins for example, do not have the same fingerprints but that doesnt point to fingerprints as a byproduct of nurture.

Likewise, if the figure really is 50%, thats much higher than the estimated percentage of homosexuals in general, so it would still imply a biological "tendency"
posted by vacapinta at 7:07 PM on February 20, 2005

No, jikel_morton, it actually is a pretty good argument for nature. This is because the average, randomly-chosen man in the general population has a chance of being gay that is considerably less than 50%. The fact that the rate of homosexuality in the monozygotic twins of homosexuals is so much higher than in dizyogotic shores this up more.
posted by LittleMissCranky at 7:11 PM on February 20, 2005

Thanks for spoiling the show, not that I'm very surprised.

It's funny to see this question- just LAST NIGHT I ran into a gay friend and, upon seeing that there were two of him, realized he was an identical twin. However, his twin bro was straight. I have met two pairs of gay twins, though, and two pairs of fraternal twins where one is gay and one is straight.
posted by BuddhaInABucket at 7:20 PM on February 20, 2005

Doesn't that point to nurture?

Not really. There are many points for divergence between one's genes and how one turns out that don't involve 'nurture,' i.e. environment in the womb, prenatal exposure to hormones, epigenetic factors, even the bacteria one picks up as a newborn! (which gets me thinking if anyone's bothered studying patterns of commensal flora in twins...hmmm..)
posted by greatgefilte at 8:07 PM on February 20, 2005

Whoops! Sorry guys. I didn't realize that Patty's coming out was a big spoiler. I thought it was just... a Simpsons plot. On googling, it turns out they were building up to this. I had no idea.

Mediareport, I did Google, but the information is overwhelming and all over the place. (I didn't Google Scholar though... didn't think of that.) I wanted recommendations and advice from people who might be studying this kind of thing. Maybe I didn't ask my question precisely enough. I didn't just want to be pointed to some psychological papers; I wanted someone to tell me what kind of stuff is going on now, to tell me if recent research (the paper you pointed to cites its last study as being from 1991) has said anything new, and to tell me what that information means. I don't think its a misuse of AskMe to ask for community experts to edit down and interpret the huge mass of information available online.

(When posting, I wondered if this question would ruffle some feathers. I didn't think it would be ruffling these particular ones.)
posted by painquale at 9:20 PM on February 20, 2005

Maybe I didn't ask my question precisely enough.

Yeah, I'd agree. Mentioning that you'd Googled and offering a more carefully phrased question incorporating that fact would have been better.
posted by mediareport at 10:58 PM on February 20, 2005

Peruse the Minnesota Twin Registry. As a gay man - and a twin - I am intrigued by the likely genetic origin of my sexual orientation.
posted by ericb at 11:28 PM on February 20, 2005

Er ... well ... now that it's spoiled anyway. Anyone wanna go ahead and fill in the gaps about the episode?
posted by RavinDave at 1:13 AM on February 21, 2005

The origin of (male) homosexuality is most likely biological. Genetic? Not so much. Actually the heritability is much less for homosexuality than for most measured traits/behaviors (i.e. religosity, voting preference, personality, IQ, smoking, criminality). Early studies by Bailey found around 50% , but there was a potential problem with the method for finding these twins - ads in gay newspapers, which would be more likely to find concordant twins than nonconcordant twins (because concordant twins would be twice as likely to find the ads). So Bailey and company tested to see if this was effecting the numbers by doing a study using the Australian Twin Registry (which wouldnt have a "biased" sample) and found uncommonly low heritability:

http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=10743878&dopt=Abstract

Journal of Personality and Social Psychology
2000, Vol 78, No. 3, 524-536

Genetic and Environmental Influences on Sexual Orientation and Its Correlates in an Australian Twin Sample

Mike Bailey, Michael Dunne, Nicholas Martin


The MZ probandwise concordance for men was 20% (*very* low for these kind of studies; IQ, for instance, is usually 80%+, and many times close to 90%). For women 24%. The pairwise concordance was 11% for MZ male pairs and 0% for DZ pairs.

Genes are not really consistent with rational evolutionary models (which is why the scientists were skeptical of their own [much celebrated!] early results). I would put my money on something in utero or in early childhood. Perhaps something viral or some sort of not well understood environmental disturbance related to the womb.
posted by dgaicun at 1:29 AM on February 21, 2005

I would put my money on something in utero or in early childhood. Perhaps something viral or some sort of not well understood environmental disturbance related to the womb.

Thanks for the informative links. I'll second your conclusion, and also pose this: twin studies don't usually examine if the twins share the same amniotic sac or not, or share the same placenta or not. Those differences could theoretically relate to the probability of identical twins both being homosexual, or having other similar traits.
posted by greatgefilte at 7:55 AM on February 21, 2005

There's also a study of oppposite-sex twins..Bearman and Bruckner "Opposite Sex Twins and Same-Sex Attraction" American Journal of Sociology, 2002 . The abstract reads:

The etiology of human same-sex romantic attraction is generally framed in terms of (1) social influences, (2) genetic influences, or (3) hormonal influences. In this article, we show that adolescent males who are opposite-sex twins are twice as likely as expected to report same-sex attraction; and that the pattern of concordance (similarity across pairs) of same-sex preference for sibling pairs does not suggest genetic influence independent of social context. Our data falsify the hormone transfer hypothesis by isolating a single condition that eliminates the opposite-sex twin effect we observethe presence of an older same-sex sibling. We also consider and reject a speculative evolutionary theory that rests on observing birth-order effects on same-sex orientation. In contrast, our results support the hypothesis that less gendered socialization in early childhood and preadolescence shapes subsequent same-sex romantic preferences.

The data used is the Add Health dataset (i.e. good, well-collected, reliabile data, generally). Though the right has grabbed hold of this study to use in some sort of anti-gay agenda, it is not written or intended (based on talking with Bearman) as such.
posted by duck at 10:00 AM on February 21, 2005

Thanks, everyone. Great answer, dgaicun; that was exactly what I was looking for!

Again, apologies for wording my question poorly. Next time I'll be sure to mention what sort of research I've already done, and will shy away from mentioning recent Simpsons episodes.
posted by painquale at 1:44 PM on February 21, 2005