Recently, due to Prop 8 in California, I’ve heard several people argue that homosexuality is a genetic thing or that people are otherwise “born gay”.
This was news to me. Is there a scientific consensus (or at least substantial evidence) supporting this?
People say this as though it were obvious, but I was surprised. Maybe I’ve just been out of the loop for too long. 
It’s not known whether homosexuality is genetic or environmental or both, or caused by some as yet undiscovered factor, but it is generally accepted that whatever does cause it happens quite early on. The most convincing source for this evidence is gay people themselves, most of whom report feeling sexual attraction to the same sex, or lack of attraction to the opposite sex, long before puberty.
Among the evidence for biological gayness:
Another bit of evidence for a biological (though not necessarily genetic) cause is the correlation of finger length to lesbianism, due to exposure to androgens in the womb. What’s particularly fascinating about the results of this study is that homosexual men with multiple older brothers have “unusually masculine” hands, which implies that the mother’s body keeps track of how many sons she’s carried and increases certain hormones as a result.
The twin studies are ambiguous. As I recall, they’ve also found that adopted-and-raised-together brothers have a higher correlation than males not of the same family, and fraternal twins have a higher correlation than other siblings. In both of these situations, the increased “incidence of both-being-gay” can’t be accounted for by genetics. Fraternal twins are no more closely related than any other brothers; and while adoption into a family with a gay brother seems to increase the statistical likelihood of being homosexual, that can’t be genetic.
Bolding mine.
Coupling that with the evidence that men with multiple older brothers are more likely to be gay and the implications of the hand study regarding the mother’s role, this would imply that two male fetuses (despite not coming from the same zygote) increase the chances of the womb conditions responsible for homosexuality. No, it doesn’t point to it being genetic, but it does point to it being biological in nature. There is a difference between the two ideas that must be taken into account.
He did say biological, not genetic: fraternal twins do share conditions in the womb, and birth order, for example.
Not only is this not THE most convincing source, it’s not a convincing source at all. Self-reported descriptions of one’s feelings about their sexual orientation when they were too young to really know anything about sex aren’t evidence of anything.
If that’s true you have to discard any evidence that heterosexual children know their sexual orientation before puberty and I don’t know of anybody who does that. Most children are attracted to members of the opposite sex from a very early age. You can argue that this is just cultural conditioning, of course, but that argument makes the contrary even more powerful.
This is probably the best evidence out there. It would appear that conditions in the womb play a key role in determining sexual orientation. But it’s quite likely that there are other factors at play, or even that there could be different causes for different people. Bottom line, though, we really don’t understand why some people are gay. In fact, we really don’t know if there is a dichotomy or a continuum. However, it does seem extremely unlikely that being gay is a choice, at least for most people.
Seconding this.
When I hear people arguing that same-sex attraction is a choice, it’s usually in the context of contending that it is, in some way, not “natural,” or somehow hetrosexuality should be preferred. However, people making that argument usually dodge the question of whether those arguments also mean that hetrosexual attraction is also a choice. I, at least, never decided I liked those of the opposite gender. I can’t think of any reason why it would be different for someone who likes those of similar gender.
So, I would ask anyone who’s arguing being attracted to those of the same gender is a choice–don’t any arguments about sexual preferences being a choice also imply that being straight is also a choice?
Nonsense. At the age of 5, I had no knowledge whatsoever of sex, yet I had an unmistakable, powerful same-sex fantasy at that age. The fantasy included elements (my taste in men; my preferred activities) that still hold true today, 58 years later. I have no idea how that particular fantasy came to be . . . certainly there was nothing in my experience that fueled it . . . yet I can’t deny its reality.
The OP implies the myth that heterosexuality is a universal default, and that something in our experience has to change (go wrong) in order to deviate from that default. What I’m saying is that I don’t think heterosexuality is ***my ***default.
They’re not talking about attraction, they’re talking about lifestyle. As in, you get to choose who you have sex with. In that context, the argument is usually that one should live with the opposite sex regardless of which gender they feel attracted to.
People are often arguing both. I would say most conflate the two concepts.
As an ex-Ph.D. candidate in the behavioral neuroscience of sexual differentiation, I am pleased with the responses in this thread. People now seem to understand that this is caused by sexual hormones and not genetics directly. Biologically determined <> genetics all of the time and this is the prime case. There are plenty of people that look and act as the opposite sex that chromosomes suggest. It isn’t rare and neither is being gay. Sex hormones determine the vast majority of sexual differentiation and sometimes things go wonky with it leaving us with a person that has some male traits and some female traits either in the brain or otherwise physically (the brain is a physical organ too).
The most direct answer to the OP title is “Everywhere and almost all of it.” It is very easy to produce animal test subjects that exhibit something like gay behavior with the appropriate hormone injections during different stages of development called “critical periods”. Many of those are irreversible in mammals and cause a profound change in one or more later behaviors. I conducted those experiments on many hundreds of animals as did thousands of other researchers and the results are not subtle at all and are obvious even to the most casual observer.
That makes my point stronger. If it’s just a lifestyle choice, irrelevant of attraction or genetics, then explain to me why we ought to prefer opposite-sex lifestyle choices? (as you’ve just thrown away the argument that we ought to respect some “natural” or “innate” preference for opposite-gender sex).
I eminently agree that, if lifestyle is all that’s being talked about, then choosing to have sex/live/etc,etc,etc. with someone of the same gender is on some level a choice.
It’s exactly the same kind of choice to choose to have sex/live/etc,etc,etc. with someone of the opposite gender.
So’s having sex with a sheep (live or inflatable).
So’s becoming a Shaker.
So that argument egregiously begs the question … if lifestyle choices ‘ought’ to be made regardless of attraction, genetics, or innate desires… (as someone might reasonably argue given the premise that sex is a lifestyle choice, just like getting coffee or tea in the morning is), why should we then favor the choice to do so with someone of the opposite gender? Forget gay, given that argument, why should we favor opposite-gender lifestyles over inflatable sheep lifestyles?
For the record, I think that attraction isn’t a choice.
Another thing to think about in this discussion is there’s really no “gay” or “straight” dichotomy, rather sexual preference is more of a continuum.
This is all completely off topic. What one does with the answer to the OP’s question is different than the answer to the question itself.
Think of the children.
Favoring zoophilia/bestiality does not lend itself to creating new offspring to argue in the future about zoophilia – ad infinitum.
It’s an inductive proof using humans as the terms of the equation.
Not to mention same-sex directed sexual and courtship behaviors in a certain percentage of all chordate species. Seagulls presumably are not exercising “choice” or making decisions in the way that humans do, but still exhibit homosexual behaviors.