On the Science of Changing Sex

This is your brain; This is your brain on hormones…

Posted in Brain Sex by Kay Brown on February 19, 2010

shrinking brainYour brain is like a muscle:  Use it or lose it… and it responds to sex hormones, which are really “growth hormones”, but with specific areas of the body that are targets for sexually dimorphic development.

In exploring the possibility that transsexuals may have a “brain sex” that is similar to their preferred gender identity, we are also stepping into the realm of gender politics in general.  What is the difference between men and women?  Is there a significant difference between them?  Once upon a time, in pre-feminist days, the early 19th Century, scientists supported the then prevailing view that women’s’ minds were inferior to men.  Proof of such was found in the measurable fact that women have, on average, smaller brains than men.  Of course, women also, on average, are both shorter and weigh less, so really, was this difference in brain size a significant one?  Then, came a more unbiased age (relatively speaking) in which it was shown that women and men have similar mental abilities, similar intelligence, and most importantly, similar potential for intellectual achievement.  Thus, came an age where to suggest that men and womens’ brains might differ in significant ways, was academically unpopular.  But nagging differences were still found… in animals.

In animal research, one could side step sexual politics, as well as ethical problems of experimenting on humans.  In animals, in the early to mid-20th Century, it was found that sex hormones, were responsible for brain changes during critical periods in development which led to sexually dimorphic behaviors in adulthood, especially those related to sexual behavior, sexual preference, and rearing of young.  One could castrate a young male rat and that rat would fail to behave in the typical masculine fashion as an adult.  Similarly, one could inject a young female rat with testosterone, and that female rat would later mount other female rats as an adult.  Dissection of these rats showed that certain regions of their brains showed sexually dimorphic structures that were changed to be more like the opposite sex to the presence or lack of testosterone.  Since these behaviors are similar in nature to behaviors found in humans this led to the belief that these changes should be present in human brains as well,  and that humans should have sexually dimorphic brain structures as well.  In the beginning of the research into possible sexually dimorphic brain structures, it was thought that there were be only a few areas involved.  But, as techniques for brain research improved, it was found that more and more areas were involved.  Eventually, it became standard practice to consider each part of the brain sexually dimorphic until proven otherwise!

From the early work, it was recognized that some areas of the brain must be sexually dimorphic from early in embryonic development, while others only become sexually dimorphic later.  It has been found that some effects are ‘locked-in’ by exposure to sex hormones at different times in development.  There are associated behavioral consequences to these locked in changes.  These are referred to as “organizational effects” of the sex hormones.  Other effects were found to occur later, and even be reversible, for example, testosterone will increase both libido and aggressive behaviors, and even improve one’s spatial navigation and mental object rotation skills, while estrogen will increase verbal language skills.  But, stop taking these hormones, and the effects will reverse back to previous levels.  These kinds of effects are called “activation effects”.  Both of these kinds of effects involve changes in both relative size, morphology, and density of neural connections in different areas of the brain.  It turns out, that using cross-sex hormones, even in adulthood, really does make one’s brain look more like the other sex!

Much is made of the research on the sexually dimorphic brain differences between transsexuals and non-transsexuals by transsexuals who find comfort in them and believe that they “prove” that they in fact have the neurological organization of their preferred target sex (gender identity).  But what is this evidence and how significant is it?  Further, are these observed differences from early exposure to anomalous sex hormones (or differences in receptor density or sensitivity, which has the same effect) or are they the result of later, exogenous hormones, from Hormone Replacement Therapy (HRT)?

The most well-known study by Zhou, et. al.  involved a very small number of subjects.  The team specifically searched for areas of the brain that were sexually dimorphic but were not known to also be associated with sexual orientation.  This is potentially most of the brain!  They were specifically looking for an area of the brain which would provide a unitary theory of transsexuality, an area of the brain which would be effected in the same way in both HSTS and AGP transsexuals and thought that they had found it in the BSTc.  The study failed in two ways, first, they failed to include any HSTS subjects, mistakenly using self-reports of sexual orientation, they were all in fact AGP; second, the BSTc turns out to be sexually dimorphic only in adulthood.  That is to say, that this area is very plastic, responding to sex hormones, the sexually dimorphic structure being an “activation effect”, casting serious doubt on the value of the entire study.  Anne Lawrence has a very well thorough discussion of this study, which you may want to read:


For more essays on trans-brains see Brain Sex.


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