This study found that male-to-female transsexuals* (MTF) had thicker cortexes than control males did in certain areas of the brain. It is not clear what the differences mean.
We don’t know what caused the difference, we don’t know how the difference affects people, and we don’t know if the difference is related to gender identity, sexual orientation, or some other factor.
That’s the essence of the study; we don’t know.
Back to the study. The cerebral cortex is the outer layer of your brain. It is made up of gray matter which contains mostly neuronal cell bodies,
As with many studies, the difference they found might be related to gender identity or to sexual orientation. The MtF group included 6 people who were male-oriented and 18 who were female-oriented, so 25% of them were attracted to men. It is likely that 95% of the control males were attracted to women.
The authors suggest that:
“future studies need to explore the possibilities that brain anatomy in MTF transsexuals varies depending on whether they are attracted to men, attracted to women, or attracted to both. Ideally, those studies will also include heterosexual / homosexual control men, matched to MTF transsexuals with respect to their sexual orientation.“
As with other studies, we don’t know what the practical effects of these differences might be. What does it do to you to have a thicker cortex? What does it mean if your cortex is thicker in the front versus the back of your brain? Does it matter if it is only thicker on the right side?
or as the authors put it:
“further research characterizing the relationships between cerebral micro-structure and macro-structure as well as brain function is clearly necessary before these regional structural differences (and any inherent hemispheric effects) can be precisely interpreted.”
The authors suggest that the thicker cortexes in the trans women (MtFs, i.e. born male) “resemble the direction of previously reported gender-typical pattern among non-transsexuals, such as thicker cortices in women than in men.”
In other words, they are similar to females, at least in terms of having thicker cortexes than males.
Maybe, maybe not.
This study did not look at any female brains, so we can’t know how the brains of the males or trans women would have compared to female controls.
Furthermore, studies have also linked thicker cortexes to obsessive-compulsive disorder (OCD), autism, social anxiety disorder (here and here), and, in children, to generalized anxiety disorder.**
There is even a study that found a small area*** of increased cortical thickness in people with xenomelia (the feeling that one of your limbs is foreign, also known as Body Identity Integretive Disorder).
Cortical thickness has also been linked to a positive trait; valuing religion or spirituality.
Finally, a couple of studies have found that meditation and sports training are associated with increased cortical thickness.
If we just go be the direction of the difference, the thicker cortex found in trans women could be linked to OCD, social anxiety disorder, autism, gender identity, xenomelia, spirituality, or life experiences.
These are not crazy possibilities; this Spanish study found that trans people had more social phobias than the general population. This Dutch study found that children and teens with gender dysphoria had a higher rate of autism spectrum disorder than the general population.
The important question is not is the cortex thicker, it’s where is the cortex thicker?
So where is the cortex thicker and what does it mean?
First of all, although some studies have found that females have thicker cortexes than males, the differences are not always in the same place in the brain.
This study found that females had thicker cortexes in the right inferior parietal and posterior temporal regions even without correcting for total brain volume (i.e. the males had bigger brains, but the females cortexes were thicker in these areas anyway). When they corrected for age and brain volume, females had thicker cortexes in the temporal and parietal lobes.
This study, on the other hand, found that females had thicker cortexes in the frontal, parietal, and occipital lobes.
And this study, found that females had thicker cortexes in the frontal, parietal, and occipital lobes on the left, and mostly the parietal lobe on the right. In the temporal lobe “small regions of the left and right caudal superior temporal gyrus (STG) and the left temporal pole showed significantly greater cortical thickness in women.”
Finally, this study found that females had thicker cortexes in all four lobes of the brain and on both sides, unlike the studies listed above.
This is like doing four studies comparing all the bones in men’s and women’s bodies.
One study finds that women have thicker arm and leg bones.
The next finds that they have thicker spines, ribs, and leg bones.
A third study finds thicker ribs, skulls, and elbows.
A fourth finds that women have thicker bones everywhere.
You wouldn’t be sure what this means about women’s bones. You wouldn’t even be sure if women’s arm bones were thicker than men’s without more studies.
The effects of having thicker bones would be very different if women had thicker ribs as opposed to thicker elbows. Knowing exactly where the bones were thicker would also be important if you were trying to figure out what exactly caused women to have thicker bones.
Going back to our brains, the frontal lobe is in the front, the occipital lobe is in the back.
The frontal lobe is involved in evaluating consequences and choosing the best action to take.
The occipital lobe is the visual processing center of the brain.
The parietal lobe integrates sensory information; it is also where we have our internal map of our own body.
The temporal lobe is involved in the retention of visual memories, processing sensory input, comprehending language, storing new memories, emotion, and deriving meaning.
So it matters a great deal where exactly women have thicker cortexes than men. In fact, it matters where exactly the difference in cortical thickness is within the lobes of the brain.
In short, we are still figuring out the differences between men and women when it comes to cortical thickness. What exactly are they and what do they mean? This makes it hard to draw any conclusions about whether or not trans women’s brains are like cis women’s.
To further complicate matters, this study found that age affected the sex differences in cortical thickness.
Going back to the original study, where were the trans women’s cortexes thicker than the control males?
In the left hemisphere, their cortexes were thicker in “the orbito-frontal cortex, the middle frontal gyrus, in the vicinity of the central sulcus (near midline), in perisylvian regions (close to the post central gyrus), as well as within the paracentral gyrus and orbito-frontal gyrus (medial surface).”
These are areas within the frontal lobe and the parietal lobe.
In the right hemisphere, their cortexes were thicker “along the post and pre central gyrus (expanding into middle frontal regions), the parietal cortex (near midline), the superior temporal sulcus, the inferior temporal gyrus, as well as within the orbito-frontal, fusiform, and lingual gyrus, and the precuneus (medial surface).”
These areas are within the frontal lobe, the parietal lobe, the temporal lobe, and the orbital lobe.
Although the areas where trans women have thicker cortexes are in all four lobes of the brain, it looks like the total area where they have thicker cortexes is not large (see Figure 1).
It is very hard to compare, but it does not look like the areas where trans women have thicker cortexes than men in this study are the same as the areas where females have thicker cortexes than males in other studies (Figure from the first study of males and females, above).
It doesn’t look like the areas where the trans women’s cortexes were thicker are the same as in the studies that linked increased cortical thickness to OCD, although there might be some overlap with the patients who had social anxiety disorder.**** Perhaps one of the trans women had social anxiety disorder and it affected part of the results.
On the other hand what if the areas of increased cortical thickness in trans women have to do with something specifically about trans women?
I want to stress that we do not know what the areas of increased cortical thickness mean functionally for trans women or cis women or people with OCD or people who meditate. Many areas of the brain are involved in more than one process. In addition, it may be that what we are looking for is a network of areas, not just one area.
We really can’t tell what it means that trans women have thicker cortexes where they do.
Nevertheless, I want to offer up a hypothesis that I hope someone will test.
Looking at the right side of the brain, the areas where trans women’s cortexes are thicker seem to have something to do with the body and perception.
According to my trusty Wikipedia,
“The postcentral gyrus is the location of the primary somatosensory cortex, the main sensory receptive area for the sense of touch. Like other sensory areas, there is a map of sensory space in this location, called the sensory homunculus.”
The precentral gyrus includes the primary motor cortex which works with other areas of the brain to plan and execute movements. It also includes a map of motor areas corresponding to body parts.
The parietal lobe integrates sensory information.
The superior temporal sulcus is involved in the perception of where others are gazing and the perception of biological motion. It may have multisensory processing capabilities.
The inferior temporal gyrus is associated with visual processing and possibly face perception.
The orbitofrontal gyrus is part of the orbitofrontal cortex which is involved in the cognitive processing of decision-making; it doesn’t seem to fit the pattern here.
The fusiform gyrus is involved in face and body recognition. Furthermore, “Increased neurophysiological activity in the fusiform face area may produce hallucinations of faces.”
The lingual gyrus is linked to processing vision.
The precuneus is part of the parietal lobe. It includes three subdivisions: a sensorimotor region, a cognitive/associative region, and a visual posterior region. It is involved in sense of self, memory, and motor imagery and coordination.
On the left side of the brain, most of the areas of relative cortical thickness do not seem to be related to the body. However, the paracentral lobule includes the supplementary motor area in the frontal lobe and part of the parietal lobe.
Could it be that the increased cortical thickness on the right side is somehow related to trans women’s negative feelings about their bodies?
Could something in their body maps or perception be causing their dysphoria?
Could their gender dysphoria have changed their brains?
This might be an area for future research.
Future research into gender identity and the brain should definitely include some gay men and lesbian controls so that we can separate out the effects of gender identity and sexual orientation.
It would also be a good idea for future studies to include more information on other conditions that might affect their results such as depression, anxiety, Aspergers syndrome, etc. Although this study says that the subjects were free of psychosis, they do not discuss issues like OCD, etc.
I don’t think the best approach would be to exclude trans people with other conditions because part of the reality of gender dysphoria is that some people have more than one condition. If we really want to understand what is happening, we need to include that data. However, it might make sense to include some controls who had social anxiety disorder, etc., if only to find where the differences are.
Back, one last time to the study.
The authors’ discussion of their results includes one odd statement: “In addition, our current findings of significantly thicker cortices in MTF transsexuals than in control men correspond with previous in vivo outcomes revealing that MTF transsexuals show significantly larger gray matter volumes than control men.”
This is an exaggeration of the results of an earlier study that they did on gray matter volume in trans women’s brains.
This earlier study found that trans women’s brains were more like men’s brains in 18 out of 20 areas of the brain. In two areas, the left and right putamen, trans women had more gray matter than both males and females, although within the average range of the females.
Their previous findings could be better summarized as trans women have gray matter volumes that match men’s except in the putamen.
This study did not look at the putamen as it is not in the cortex. The increased gray matter volume in the putamen found in the earlier study is irrelevant to this study.
Actually, at first glance the findings of this study don’t fit well with the findings of the previous study. The cortex is made up of gray matter, so you might expect its thickness to be related to the volume of gray matter. Why do trans women have gray matter volumes that are equal to or less than cis men’s but cortexes that are thicker?
The authors go on to address this question:
Nevertheless, it appears rather surprising that the previous whole-brain approach analysing gray matter did not detect any group differences in cortical regions (e.g., the ones revealed in the current study directed at cortical thickness). It is possible, however, that these two anatomical measurements reflect slightly different aspects on a micro-anatomical level as also suggested by studies investigating direct correlations between cortical thickness and gray matter concentration. Importantly, the current approach also provides an additional dimension of cortical morphology such as its thickness in millimeters, which is not directly captured by voxel-wise analyses of signal intensity changes throughout the brain.
In other words measurements of cortical thickness and gray matter volume may not be the same thing. It is possible that trans women’s gray matter volume is like men’s but their cerebral cortex is thicker in certain areas.
It is still not clear what it means that trans women’s cerebral cortexes are thicker than control males in certain areas.
We don’t know what the difference means functionally.
We don’t know if the difference is related to gender identity, sexual orientation, or some other factor.
We don’t know how the trans women’s cortical thickness would have compared to females since there were no female controls in the study.
We can’t say that trans women’s brains are like females’ brains since both have thicker cortexes than males because a) other factors like social anxiety disorder can make people have thicker cortexes; b) we aren’t sure exactly where females’ cortexes are thicker than males; and c) it looks like trans women and females’ cortexes thicker than males in different places.
And we don’t know if the difference in cortical thickness is related to body perception and sensory integration.
Increased Cortical Thickness in Male-to-Female Transsexualism by Eileen Luders, Francisco J. Sánchez, Duygu Tosun, David W. Shattuck, Christian Gaser, Eric Vilain, and Arthur W. Toga in J Behav Brain Sci. Aug 2012: 2(3): 357-362.
* I am using the language of the study.
** In general, a thicker cortex is probably a good thing; it is associated with meditation and learning. In addition, your cortex thins with age. The volume of the gray matter in your brain also goes down with age or due to illnesses such as depression.
*** the right central sulcus. I thought you’d never ask. And yes, trans women had a thicker central sulcus than control males, but it was on the left side and the overall pattern is different; patients with xenomelia mostly had areas where their cortex was thinner, not thicker. Then again, it might be worth looking further at the central sulcus in trans people’s brains.
**** The study of patients with OCD found that their cortexes were thicker in the right inferior frontal cortex and the right middle temporal gyrus.
This study suggests that the areas of cortical thickness in autism change with age. I don’t see how to compare the data on autism to the data on trans women.
One study of patients with social anxiety disorder found that they had increased cortical thickness in the left inferior temporal cortex. Trans women had increased cortical thickness in the right inferior temporal gyrus – as well as many other areas not found with social anxiety disorder.
Another study of people with social anxiety disorder found that they had increased cortical thickness in two clusters: “One was located in the right middle frontal gyrus extending into superior frontal sulcus, belonging to the dorsolateral prefrontal cortex (DLPFC, Brodmann areas 6/8/9/46). The other covered right superior parietal lobule and angular gyrus and extended in part into right precuneus and inferior parietal lobule (Brodmann areas 7/39/19).”
Trans women also had increased cortical thickness in the right middle frontal region, the right parietal cortex (near the midline), and the precuneus – but they had other areas of increased thickness as well.
Perhaps one of the trans women in the study had social anxiety disorder and it explained some of the areas of increased cortical thickness, but not all.
Or perhaps both conditions involve similar areas of the brain.