I always used to cringe when I heard someone try to explain being transgender in terms of "male brains" or "female brains."

It has been my belief that gender is more complicated than that. But then, it has also long been my belief that it shouldn't really matter why we ar transgender...we're all human beings and should all be according the respect and dignity due all human beings.

Be that as it may...science marches on.

A team of researchers led by Daphna Joel, a behavioral neuroscientist at Tel Aviv University in Israel has found that human brains do not separate into distinctly male and female compartments.

In the mid-19th century, researchers claimed they could tell the sex of an individual just by looking at their disembodied brain. But a new study finds that human brains do not fit neatly into “male” and “female” categories. Indeed, all of our brains seem to share a patchwork of forms; some that are more common in males, others that are more common in females, and some that are common to both. The findings could change how scientists study the brain and even how society defines gender.

The team published its findings in the Proceedings of the Academy of Sciences: Sex beyond the genitalia: The human brain mosaic

Using existing sets of MRI brain images, they measured the volume of gray matter (the dark, knobby tissue that contains the core of nerve cells) and white matter (the bundles of nerve fibers that transmit signals around the nervous system) in the brains of more than 1400 individuals. They also studied data from diffusion tensor imaging, which shows how tracts of white matter extend throughout the brain, connecting different regions.

Analyses of internal consistency reveal that brains with features that are consistently at one end of the “maleness-femaleness” continuum are rare. Rather, most brains are comprised of unique “mosaics” of features, some more common in females compared with males, some more common in males compared with females, and some common in both females and males. Our findings are robust across sample, age, type of MRI, and method of analysis. These findings are corroborated by a similar analysis of personality traits, attitudes, interests, and behaviors of more than 5,500 individuals, which reveals that internal consistency is extremely rare. Our study demonstrates that, although there are sex/gender differences in the brain, human brains do not belong to one of two distinct categories: male brain/female brain.

On the other hand, those are not the most recent voices heard from the science world. Francine Russo has a new article in Scientific American Mind, Is There Something Unique about the Transgender Brain?

There is an interesting subtitle:

Now there is a thought that will severely disturb some people.

Russo reports:

Spanish investigators—led by psychobiologist Antonio Guillamon of the National Distance Education University in Madrid and neuropsychologist Carme Junqué Plaja of the University of Barcelona—used MRI to examine the brains of 24 female-to-males and 18 male-to-females—both before and after treatment with cross-sex hormones. Their results, published in 2013, showed that even before treatment the brain structures of the trans people were more similar in some respects to the brains of their experienced gender than those of their natal gender. For example, the female-to-male subjects had relatively thin subcortical areas (these areas tend to be thinner in men than in women). Male-to-female subjects tended to have thinner cortical regions in the right hemisphere, which is characteristic of a female brain. (Such differences became more pronounced after treatment.)

More on the Spanish results is available at New Scientist.

Trans people have brains that are different from males and females, a unique kind of brain. It is simplistic to say that a female-to-male transgender person is a female trapped in a male body. It's not because they have a male brain but a transsexual brain.

--Guillamon

They found significant differences between male and female brains in four regions of white matter – and the female-to-male transsexual people had white matter in these regions that resembled a male brain (Journal of Psychiatric Research, DOI: 10.1016/j.jpsychires.2010.05.006). “It’s the first time it has been shown that the brains of female-to-male transsexual people are masculinised.

In a separate study, the team used the same technique to compare white matter in 18 male-to-female transsexual people with that in 19 males and 19 females. Surprisingly, in each transsexual person’s brain the structure of the white matter in the four regions was halfway between that of the males and females (Journal of Psychiatric Research, DOI: 10.1016/j.jpsychires.2010.11.007). “Their brains are not completely masculinised and not completely feminised, but they still feel female

--Guillamon

Guillamon isn’t sure whether the four regions are at all associated with notions of gender, but Ivanka Savic-Berglund at the Karolinska Institute in Stockholm, Sweden, thinks they might be. One of the four regions – the superior longitudinal fascicle – is particularly interesting, she says. “It connects the parietal lobe [involved in sensory processing] and frontal lobe [involved in planning movement] and may have implications in body perception.”

Of course, behavior and experience shape brain anatomy, so it is impossible to say if these subtle differences are inborn.

--Russo

In a study published in 2014, psychologist Sarah M. Burke of VU University Medical Center in Amsterdam and biologist Julie Bakker of the Netherlands Institute for Neuroscience used functional MRI to examine how 39 prepubertal and 41 adolescent boys and girls with gender dysphoria responded to androstadienone, an odorous steroid with pheromonelike properties that is known to cause a different response in the hypothalamus of men versus women. They found that the adolescent boys and girls with gender dysphoria responded much like peers of their experienced gender. The results were less clear with the prepubertal children.

This kind of study is important because sex differences in responding to odors cannot be influenced by training or environment.

--Baudewijntje Kreukels, an expert on gender dysphoria at VU University Medical Center

In another 2014 experiment by Burke, et. al. the team measured responses of boys and girls with gender dysphoria to echolike sounds produced by the inner ear in response to a clicking noise.

Boys with gender dysphoria responded more like typical females, who have a stronger response to these sounds. But girls with gender dysphoria also responded like typical females.

Complicated shit.

A study by Sean Deoni‘s team at King’s College London suggests it may soon be possible to look for these differences in such children. Deoni’s team adapted an MRI scanner to be as quiet as possible so it could be used to monitor the development of white matter in sleeping infants. Using new image analysis software they could track when and where myelin – the neuron covering that makes white matter white – was laid down (Journal of Neuroscience, vol 31, p 784). Although the sample was too small to identify any gender differences in development, Deoni expects to see differences developing in the brain “by 2 or 3 years of age”.

Overall the weight of these studies and others points strongly toward a biological basis for gender dysphoria. But given the variety of transgender people and the variation in the brains of men and women generally, it will be a long time, if ever, before a doctor can do a brain scan on a child and say, “Yes, this child is trans.”

--Russo