The differences between heterosexual and homosexual people are as much the subject of fascinating science as they are a source of social debate. And in many cases, the former can help to inform the latter. There is now plenty of research which shows that a person's sexual orientation, far from being a phase or a lifestyle choice, is a reflection of fixed properties of their brain that develop at an early age.
A new study adds new weight to this evidence by using brain-scanning technology to look at the differences between the brains of gay and straight people. Ivanka Savic and Per Lindstrom at the Karolinska Institute in Stockholm scanned the brains of 90 men and women of different sexual orientations. Their images show that in the brains of gay people, certain features including symmetry and connections to the brain's emotional centre are more closely matched to the brains of straight people from the opposite sex.
Symmetry
Using magnetic resonance imaging (MRI), Savic and Lindstrom showed that the brain's two halves are almost exactly the same size in straight women and gay men. However, both straight men and lesbians had slightly asymmetrical brains, with the right hemisphere being 1-2% larger on the left. These differences only applied to the large cerebrum, which makes up most of our brains. The two halves of the cerebellum, which sits at the brain's base, were symmetrical in all of the volunteers, regardless of sex or sexual preference.
Earlier studies have found similar results for patterns of brain activity. For example, parts of the brain involved in reward and emotion are more strongly activated when straight men and lesbian women look at female faces, and when straight women and gay men see male faces. The same patterns apply when people smell chemicals that probably act as human pheromones. But attractive faces and enticing pheromones are both related to sex, and responses to them could be learned over time.
But Savic's and Lindstrom's new study shows that these differences extend to fundamental aspects of the brain that aren't directly linked to sex or behaviour, and that are probably fixed from birth.
The idea that straight men have more asymmetrical brains than gay men fits with previous research. When listening to sounds, straight men tend to have a bias for their right ear, which both gay men and straight women lack. They also tend to outperform gay men and straight women in tests of spatial awareness, where success depends on a part of the brain - the parietal cortex - which is usually larger in men than in women.
Connections
Savic and Lindstrom also used another brain-scanning technique called PET to measure the flow of blood into the amygdala, a part of the brain involved in processing emotions. These visuals revealed the connections that link the amygdala to other parts of the brain. From previous studies, we know that these connections usually link to different areas in the brains of men and women, and sprout from different hemispheres -the right in men, and the left in women.
That was the pattern that Savic and Lindstrom saw the straight volunteers from their study, but the homosexuals showed the reverse pattern. For example, the amygdalas of gay men had more in common with those of straight women - the two halves were well-connected, they had more neurons projecting from the left half (as opposed to the right in straight men) and these neurons connected to the same parts of the brain that those of straight women do.
These connections provide some tantalising hints about how gay and straight people differ in their behaviour. In straight men and lesbians, the amygdala (which influences our emotional reactions to stress) connects to the sensorimotor cortex and the striatum, parts of the brain involved in the "fight or flight" response. But in straight women and gay men, the amygdala's connections feed into the anterior cingulate cortex and the subcallosum. These areas influence our moods and have been implicated in mood-related disorders such as anxiety and depression.
That certainly seems to fit with what we know about these "affective disorders". Women are 2-3 times more likely than men to suffer from them, while gay men are also more vulnerable to depression. However, Savic and Lindstrom caution that their results don't tell them how far these conditions are affected by the architecture of the brain. Certainly, in gay men, social stigma is an equally likely explanation for higher rates of depression.
It's very likely that the relative size of the brain's two halves are set very early in development. Other groups have certainly detected asymmetries before in children, and some have even done so in the brains of foetuses. Nonetheless, the evidence isn't entirely consistent and while Savic and Lindstrom think that it's likely that these traits are fixed from birth, they say that it's still an "open question".
Likewise, it's not clear why some brains are asymmetrical and others are not, although animal studies provide some clues. In animals, homosexuality in females is often attributed to an overabundance of male hormones - androgens - in the womb, while male homosexuality results from a lack of these. In male monkeys and rats, the right side of the brain has higher concentrations of receptors for these male hormones to lock onto; in females, they are distributed equally among the two halves. Whether the same applies to humans, and how that might eventually affect our behaviour, are questions for future studies.
Reference:Savic, I., Lindstrom, P. (2008). PET and MRI show differences in cerebral asymmetry and functional connectivity between homo- and heterosexual subjects. Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.0801566105
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Brain asymmetry isn't something you either have or don't - there are degrees of it. So does it follow that men with very asymmetrical brains are in some way "very heterosexual"? What would that even mean?
As we're dealing with amounts of 1-2%, it's not difficult for researchers to mismeasure. Did they know which scans belonged to which gender and sexuality? If they did, that could bias the result. Remember Robert Bean measuring the brains of black people.
The sample sizes were quite small - 45 subjects in all. Are there plans for a larger study that would increase the signal to noise ratio?
And finally, to quote the results from Language Log:
Rightward hemispheric asymmetry was found in the brains of 14 of 25 heterosexual males and 11 of 20 homosexual females, but in only 13 of 25 heterosexual females and 10 of 20 homosexual males.
...so actually about half gay people have straight brains, and about half straight people have gay brains. Which is like saying half of all left handed people prefer their right hand.
Correct me if I'm wrong, but it looks like yet another "gay people are biologically different" headline has nothing behind it.
I don't know if you've read this yet or not, but if you haven't, you really ought to read Chandler Burr's A Separate Creation: The Search for the Biological Origins of Sexual Orientation. It talks about a lot of the sexual-orientation-brain-structure that has been done before this, and does an excellent job of highlighting the problematic aspects (as well as illuminating some of the people who work in the field, just for fun). It isn't a book that particularly answers questions, more raises them, but it's an invaluable background to have when looking at a piece of research like this.
Thanks for the comments both. I was tossing up whether to write up this paper and part of the reason I wanted to do it was to see what you lot thought. Luna - the book you recommended sounds fascinating. Care to offer a few nutshell points from it?
What about bisexual brains?
The problem with all such studies, I think, is that they seem to overlook bisexuality. That's a glaring omission, to my mind.
It could be called an omission but I consider it a positive one. If the scientists are attempting to find a correlation between biological aspects (the brain) and subjects' sexual orientation, an important first step is to look at opposite ends of the sexuality spectrum. This can provide the scientists with a road map for further investigation if there are traits that correlate with only one side of the spectrum.
This is the approach I would take if I was a scientist (I'm a trained engineer) so I'd love to hear other people's thoughts on this.
I have no doubt the biological picture is complicated when it comes to sexual orientation but I am greatly encouraged by scientists trying to figure things out, even if there is more work to do.
hey luna, chandler is a good friend of mine :-) he sends me perfume samples all the time. perhaps you shouldn't be recommending that book since i'm evil so by association chandler must be evil >8-0)
One suspects that the roots of homosexuality are various; combinations of biological, experiental, etc. etc. Perhaps some homeosexuals can be "cured", others cannot.
I just found your blog today. It's A++ work.
razib, even sexists can occasionally have interesting friends. I try to judge everyone on their own merits; it's only when people claim total-hard-core, totally unredeemable idiots like "Vox Day" as friends that I condemn by association. You have to potential to be an intelligent human being, if only you were capable of treating women as being fully human and deserving of the rights thereof.
Jim Thomerson -- what evidence is there, that you know of and we don't, that any homosexual can be "cured"? About the closest I've ever seen is the web adverts for fundamentalist programs who claim success on the basis of one person who declares himself "cured" and a number who have been trained never to act on their inclinations. And all too often the man who declares himself cured commits suicide within the next few years (cf. too many ex-clients of Cohen at PFOX).
The official position of the APA is that "reparative/'reorientation' therapy" aimed at "curing" gays has no evidence behind it, has never demonstrated effectiveness in peer-reviewed trials, but has been shown to be damaging to individuals undertaking it; they recommend avoidance as a useless and potentially very damaging undertaking. And although the evangelical Christian groups who offer this "therapy" regularly advertise that they have helped "thousands", when Dr. Robert Spitzer asked these groups to refer people on to him in order that he be able to speak to them for a study in 2001, he was able to find only 200, of which "sixteen (11%) of the men and 21 (37%) claimed to now have a heterosexual orientation." That's a fairly crap piece of support, and that is the best there is.
So really, that seems like grasping at straws.
Hi,
in my eyes, the whole is just bullshit.
It is known that STRAIGHT men are sexually MORE aroused than gay men by pictures of nude men having sex. That contradicts this article in all and everything.
In my eyes men are just SEXUAL, and their so-called "orientation" is a result of personal circumstances, NOT of physiological differences.
andrew, do you have any source of this information at all other than "it is known"?
I mean, "it is known" has, in the past, meant a wide variety of things misremembered and/or invented out of whole cloth. Reference to actual data and experiment are a real plus, ibn terms of any credibility.