Have there been any studies examining homosexuality and identical twins?

Most people I know think homosexuality is genetic. Is there any study about the incidence of homosexuality in twins and/or siblings? Do identical twins tend to either both be gay or straight? Thanks in advance.

I don’t have an answer, but I’ve often wondered the same thing. I have a good friend who is a lesbian, but her identical twin sister is straight, married, with several kids. I’ve told my friend that she needs to find some kind of twin study to be a part of, so we could see how prevalent this is. It’s very interesting.

These are the best known (and most comonly cited) Twin studies and homosexuality. There are not a lot of such studies out there. (The link cites the actual studies, but does not provide a further link to any online version of them.)

Another article on homosexuality in brothers and twins with references to studies.

The findings of a more recent registry study by Bailey, Dunne and Martin, motivated by awareness of the potential bias problems of inviting gays to take part in studies, were published in 2000.

See this post in the thread
Gay Dopers, if you could take a pill and be straight, would you? for more details and further links.

If one twin is gay, the other is more likely to be gay than the average person, according to all studies, including the study (warning: PDF) that roger thornhill mentions.

It seems that they all agree on a “familial” influence on sexuality, although not all agree to which extent nature and nurture are each represented.

a son of the circus by irving (guy that wrote world according to garp) explores this in the novel. not a study and i don’t know what studies he quotes from but a great read, especially if interested in this topic

I realize that the plural of anecdote is not data, but me too. Twice.

Although in the case of one set of twins, they are both straight. The unmarried one is not only hetero, but staunchly against homosexuality in all of its forms. She knows that she will one day actually have sex with a man, though in her 48 years she never has. She also has never had sex with a woman (including her live-in lesbian lover). All that getting naked with and genital manipulation of other women was just, you know, a phase.

This may be obvious and repetetive, but since this is a hot issue,

I want to point out that an attribute can be completely physically determined, yet not be genetically determined.

For instance, there’s no way I can choose to be immune to chicken pox or not by pure power of will, prayer or any other mental effort; it’s completely physically determined. Yet it’s not genetic. Likewise, particular allergies are not genetically determined, but there’s no way someone can just choose to be allergic to peanuts or not.

If those examples aren’t enough, consider a Thalidomide baby: there’s no way they can change their physical state, but it’s not genetically determined.

Alternatively, they could go on the talk show circuit. A (straight) friend of mine whose twin sister is a lesbian went on one of them, and then ended up appearing on several others. It was a lark for a couple of years, at least.

Aren’t there also studies of twins or siblings adopted into different families at birth? Those would remove most of the environmental variables from consideration, leaving only genetics and the prenatal environment.

I, too, know a set of identical twins (guys, in this case). One is gay, the other isn’t. The heterosexual one is married, my gay friend is dating (but single).

But would they? And why would it be desirable to thus remove one potential causing factor from the research? Why not, for example, the prenatal environment (the “uterine soup”)?

One could argue that conducting tests/observations with identical twins brought up in the same household provides the scientist with a variable that is the same for both kids, and thus “removes most of the environmental variables from consideration”, assuming, ceteris paribus, that the parents/primary caretakers treated them (more or less) the same. Having them both brought up in the same household thus acts as a kind of control.

And by ruling out environmental variables from consideration, wouldn’t the scientist be ruling out what might be the most significant data, something at any rate that s/he ought to be testing for? Would it not be tantamount to making the colossal assumption that different environmental and social factors play no part in the child’s homosexual orientation? No scientist should rule out one possible causing factor before s/he makes his tests/observations.

That study that you referenced showed why this isn’t really a great control. When asked questions about the environment they grew up in, oftentimes the two twins seemed like they were describing completely different families.

There are plenty of ways of approaching something scientifically, and ruling out possible factors for a particular test is perfectly fine (as long as you’re willing to reintroduce them later as needed). A scientist can perform testing that takes into account only a single factor, then move on to the next single factor, etc. It’s still perfectly valid science (and is even a recommended method in many instances).

I don’t think this type of study has been done with one of the “Twins” databases, such as the Minnesota or Australian versions, which helps to remove some of the potential bias that was discussed earlier. I could be mistaken, but can’t find one.