Nature,
or nurture?
It’s been long established that there are structural differences between the brains of biological males and females, however new evidence is beginning to look at the differences found in the brain structure of transgender individuals. To begin, I’ll define some key terms including sex difference, male, female, transgender and functional significance before examining sex differences observed in grey matter volume (GMV), white matter and the corpus callosum. Physiological changes are discussed as well as the functional significance of the altered brain structure before reviewing behaviour said to be correlated with the adaptations.
I will then explore the sex differences in transgender individuals including the interesting discovery that the brain structure of transgender individuals are not like either the male or female brain which raises questions around whether sex differences in brains occur due to nature or nurture. Although studies have shown physiological differences in the brains of men and women, the causation of these alterations affecting functional significance is unclear raising a question on whether sex differences make much difference at all beyond the aesthetic.
First, let me define some words as they appear in this paper so we’re on the same page. The first is the use of the words sex and gender. Differences between men and women can be discerned by observing biological, physiological and socio-cultural factors. Due to the intertwined nature of these factors, it does not make sense to separate the social environment (gender) from the biological (sex) factors.
For that reason I will use the terms ‘sex differences’ to also mean ‘gender differences’ when referring to any differences found to occur between men and women with a man defined as a human born with male external genitalia (penis) and a woman defined as a human born with female external genitalia (vagina). A transgender individual is defined as a person with a constant feeling of belonging to the opposite biological sex to the one assigned at birth due to a misalignment between a person’s biological sex and their perceived gender.
Functional significance is defined as the role, significance or consequence of any given structure. Gender differences in the brain were assessed by reviewing literature studying fully functioning normal brain structures containing the right and left cerebral hemispheres, the cerebellum and extending to the base of the brain steam ending at the medulla oblongata with a focus on alterations in the structure of the brain between sexes by drawing comparisons between physiological sex differences and observed changes in functional significance correlated with these differences.
The first thing to understand is, sex differences are not unique to humans. There are long standing beliefs that the brains of men and women across species function differently with historical human stereotypes depicting women as better at multi-tasking and men better at abstract reasoning. One way in which men and women have different brains can be found by observing grey matter volume (GMV). While it is agreed that there are sex differences in GMV there is still no agreement on the exact magnitude between the differences.
Gender differences have been found in precentral, cingulate and anterior temporal grey matter areas leading researchers to believe that within these regions, which are responsible for organizing white matter, both sexes are displaying different characteristics.
As grey matter is involved in processing information, including to control our muscles and sensory perception, it has been suggested that a reduction in grey matter volume could be responsible for a decline in cognitive and functional changes as well as a change in attentional focusing. However, while correlation between GMV and cognitive functions have been observed the literature is non-committal, suggesting that further research needs to be conducted to declare any change in functional significance occurs due to sex differences in GMV.
Another region that has been identified to display sex differences in brain structure is the band of white matter known as the corpus callosum. The corpus callosum is the largest volume of white matter found in the body connecting the right and left cerebral hemispheres and located towards the centre of the brain. The corpus callosum is responsible for passing communication between different regions of the brain to aid cognitive functions.
Sex differences have been observed, including acceptance that the male corpus callosum is larger, however the research provides conflicting results. Fractional anisotropy (FA), a measure used for connectivity levels in the brain, has been recorded as higher in the corpus callosum for women although other studies claimed women displayed significantly lower FA in the corpus callosum than men. These studies suggest that the alterations cause a difference in cognitive function, although how exactly is unclear.
While some studies state that the presence of a larger corpus callosum and higher FA correlates with an increase in cognitive function, women displaying lower FA may in fact be an indication of how the corpus callosum functions more efficiently for women meaning they do not require the same volume as men to do the same tasks. Both studies state the alterations that occur cause changes in functional significance however an inverse relationship has also been suggested.
There have been studies to explore whether the observed sex differences appear as a result of differences in cognitive ability; meaning the change in cognitive function occurs first with the changes resulting in the adaptation of the corpus callosum. The results showed that two segments of the corpus callosum involved in mental rotation revealed gender differences. One segment, the rostrum, showed correlation with visuo-spatial processing while the second was affected by both visuo-spatial processing and gender. These conflicting studies demonstrate the inconclusive understanding on how the size of the corpus callosum relates to the function of the brain and whether changes in functional significance are the cause or effect of the adaptation.
Over the past decade, studies have continued to demonstrate how sex differences in the brain extend to both structure and chemistry with key structural changes said to occur in utero and during adolescents. Studies have confirmed that women have slightly larger brain volume as well as a higher percentage of grey matter that involves sensory perception, motor control, speech, emotion and self-control.
This differs from white matter which sends information and controls autonomic bodily functions such as the heartbeat. It is believed these developments in white and grey matter during utero and adolescents contributes to the divergence in cognitive and behavioural functions in men and women.
Additional studies have shown men to possess a greater desire for control and dominance in high-risk situations compared to women, with sex differences in the limbic network suggested to be involved. It is these relationships that researchers are trying to learn more about to better understand correlation and causation. Some of these differences have been observed by assessing behavioural performance using a simple response task. This showed proactive and cautious cognitive processing in women and a more reactive and fast cognitive processing in men.
When discussing brain structure between genders, transgender individuals must also be considered although the research only adds raises more questions. The fascinating discovery about transgender individuals is that they do not display the same structural alterations to their brain as their biological gender or their chosen gender, instead having a completely different pattern of brain structure to both the male and female brain altogether.
This is significant for three reasons. When considered alongside the inconclusive nature of the studies above, the non-committal results may be due to an over-simplification of the genders to include only male and female not allowing for a true sample of the population to be measured. The second assumes that men, women and transgender have different brains which raises the question about how many different brain types exist and whether they are based on biology or choice? And lastly, are all brain structures fixed and do they only change during utero and adolescents or are they fluid?
While previous studies have suggested changes occur during utero and adolescents, differences have also been found between pre-transition surgery and post-transition surgery transgender individuals suggesting that brain structure may be fluid and can be adapted based on choice or forced physiological changes.
If transgender individuals display a brain structure different to both men and women, which again changes after transitional surgery, the adaptations in brain structure may be caused by choice, behaviour and cognition rather than changes in the brain structure occurring first and subsequently affecting behaviour and cognition.
This could mean that changes in brain structure are not dependent on nature, which is to say biological sex, but rather based on nurture, the environment and life choices made by the individual. This, of course, relies on confirmation that there is causation between sex differences and brain function as there are some who believe that sex differences make little to no difference to brain function.
Sex differences in the brain have long been accepted to occur, however there is still debate over whether these differences contribute to the functional significance and whether the structural changes in the brain are influenced by sex differences at all. The exact types and size of sex differences have not yet been agreed upon nor have many certain conclusions been committed to declare clear causation between sex differences and the functionality of the brain.
Women have been displayed to show larger grey matter, affecting emotion, while men have been shown to have a larger corpus callosum which may be related to visuo-spatial processing however the results require further research.
Further questions are raised while observing transgender individuals as they display a brain structure pattern different to both a male and a female, raising questions over whether brain structure, including sex differences, are biological or chosen. There is also still no clear verdict on whether changes in the functional significance within the brain are the cause or effect of sex differences. This means the age-old question of whether structural changes occur due to nature or nurture still requires further study, however the key may lie in the brains of transgender individuals. By giving us the opportunity to better understand how brain structure is affected by chosen gender and biological sex we may gain a better understanding as to what comes first; the change in behaviour or the change in brain structure, including how this impacts functionality.