Sexual Dimorphism in the Brain
|Duck cross-dressing involves a lot of dye.|
Not all sexual dimorphism is so obvious; some critical differences are internal. While early scientists were able to prove that boys are not actually made of frogs and snails, and girls aren't made of sugar and spice, some discoveries had to wait on more sophisticated equipment and analytical techniques. In this case: a type of MRI scanning called Diffusion Tensor Imaging, or DTI.
DTI allows neuroscientists to examine fine white matter structures of living brains. This is as opposed to the "gray matter" where the neurons live. (Gray matter is shown as orange in the illustration here.) The effects of white matter on cognition and memory is a relatively new area of research.
In 2004, a study came out that showed differences in the white matter structure of left handed vs. right handed people (Büchel et al., 2004). For this reason, studies looking for other patterns tend to exclude left-handed people (typical, right?).
In 2011, the journal NeuroImage published a paper called "Men and women are different: Diffusion tensor imaging reveals sexual dimorphism in the microstructure of the thalamus, corpus callosum and cingulum" (Menzler, 2011).
The green pixels in the image on the right show a "white matter skeleton" (not literal bones) that men and women share in common. The yellow pixels show places where men in the study had "significantly increased fractional anisotropy" compared to the women. In similar areas, the men showed "significantly decreased radial diffusivity" compared to the women. These are mathematical terms that you can read about elsewhere if you really want! The important thing to know is that these terms reflect differences in what the water molecules in our brains are doing. The same process is used to detect lesions and trauma-induced injuries, as well as to monitor degeneration over time from Multiple Sclerosis.
Click below for the full illustration from the paper:
Yellow highlights show increased fractional anisotropy in men, while the blue highlights show decreased radial diffusivity in men.
Transgender Men (Female-to-Male)
In the same month, the Journal of Psychiatric Research published a study (Rametti, February 2011) comparing:
- 18 transgender men
- 24 cisgender men
- 19 cisgender women
The results are complicated, but here is a revealing graph that compares fractional anisotropy in four key locations where cisgender men and women differ:
The line-with-a-dot between some pairs of bars indicates a high statistical confidence of a distinction. Trans men are measurably distinct from cisgender women in all four areas! Furthermore, trans men are only measurably distinct from cisgender men in one of these areas. According to expectations developed from a separate study of men and women (outlined above), transgender men are neurologically closer to cisgender men than cisgender women.
Transgender Women (Male-To-Female)
The same research group shortly carried out another study with:
- 18 transgender women
- 19 cisgender women
- 19 cisgender men
Again, the line-and-star comparisons indicate a statistically significant difference. In five of these six areas, transgender women who haven't started hormone therapy are distinct from both cisgender women and cisgender men. In one, a distinction from cisgender men was not demonstrated. According to the study authors, "The direction of the differences suggests that some fasciculi do not complete the masculinization process during brain development before the individual seeks treatments" (Rametti, July 2011).
These studies have shown that transgender men and women don't fit their assigned genders at the neurological level, and the natural direction of difference is toward their own gender identity.
Büchel, C., Raedler, T., Sommer, M., et al. (Sept. 2004). White matter asymmetry in the human brain: A diffusion tensor MRI study. Cerebral Cortex, 14(9). Retrieved from http://dx.doi.org/10.1093/cercor/bhh055
Menzler, K., Belke, M., Wehrmann, E., et al. (February 14, 2011). Men and women are different: Diffusion tensor imaging reveals sexual dimorphism in the microstructure of the thalamus, corpus callosum and cingulum. NeuroImage, 54(4). Retrieved from http://dx.doi.org/10.1016/j.neuroimage.2010.11.029
Rametti, G., Carillo, B., Gómez-Gil, E., et al. (February, 2011). White matter microstructure in female to male transsexuals before cross-sex hormonal treatment: A diffusion tensor imaging study. Journal of Psychiatric Research, 45(2). Retrieved from http://dx.doi.org/10.1016/j.jpsychires.2010.05.006
Rametti, G., Carillo, B., Gómez-Gil, E., et al. (July, 2011). The microstructure of white matter in male to female transsexuals before cross-sex hormonal treatment: A DTI study. Journal of Psychiatric Research, 45(7). Retrieved from http://dx.doi.org/10.1016/j.jpsychires.2010.11.007