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Wednesday, 15 April 2015

Male vs Female Brains

Women are from Venus and men are from Mars, or so we have long been told. There are obvious physical differences between the sexes, but do these disparities extend to our brains? And if there are sex differences to be found in the brain, are they there from before birth, or are they a product of our upbringing? As well as being interesting areas for scientific study, these questions open up some ethical conundrums - if we did find robust, biological sex differences in the brains of men and women, what would this mean for how we should treat the sexes, and how we should raise our children?

Artist's impression of the cerebrum, with the temporal lobe coloured
We all have one of these - but are men's and women's brains different?
Image credit: Anatomography, via Wikimedia Commons [CC BY-SA 2.1 jp]

The first, and probably easiest, question to answer is whether there are physical differences in men's and women's brains. We know that males tend to have larger brains than females, and this has been confirmed by a recent meta-analysis[1]. But do these physical disparities correspond to a difference in ability, or function? Some have argued that larger brain volume suggests greater intelligence, but it is now widely accepted that total brain volume is not a very good indicator of intelligence - Einstein’s brain was actually found to be slightly smaller than average[2]. A criticism of many studies on brain volume is that they fail to take into account that women, on average, have smaller bodies than men - so it seems reasonable to expect their brains to also be smaller. However brain to body size ratio can’t account for the dissimilarities completely - the correlation between the two is not strong in humans, and boys’ brains remain bigger even at age 11-13, where their bodies are, on average, smaller[3].

As well as looking at the brain as a whole, researchers look at specific structures inside the brain to see if there is divergence there. The same meta-analysis found size differences in a huge number of structures in the brain, including the amygdala, which is involved in emotional processing and the hippocampus, which is important for memory. Again, these differences weren’t adjusted for the overall distinctions in size between men & women, but as the variations in size and connectivity differed by region it seems it is not just as simple as every area being bigger in men. Discrepancies have also been found in the percentage of grey matter and white matter in the brains of men & women[4].

So it seems there are variations in the physical size of the brain and its various regions, but what about the way our brains are wired up? Researchers have begun trying to unravel the complicated connections that allow the brain to transmit information. FMRI studies allow researchers to look inside the functioning brain, and map patterns of activity. Studies using this method have found that women’s brains are more highly interconnected[5] - by up to 14%[6]. Others looked at more specific connections and found differences in how symmetrical brain connections are in different brain regions for men and women[7]. Men, it seems, have less symmetrical brains when you look at short range connections - these tend to be on the right side of the brain. When looking at longer range connectivity, some regions were more lateralised in women, and tended towards the left side of the brain, while others were more asymmetrical in men, and tended towards the right. There have been suggestions[7] that people with autism have brains with more short-range and fewer long-range connections, so this could explain why men are more likely to suffer with the condition than women.

While these studies suggest various differences in the brains of men & women, their findings are still widely debated. FMRI is a notoriously tricky method, and even very small changes in the statistical methods used can give wildly varying results[8]. While these studies suggest some interesting discrepancies between the brains of large groups of men & women on average, it isn’t clear what conclusions can really be drawn from them. The differences that are found are usually small, and only seen when you look at the average of a large group. The variation within each group is often large in comparison to the differences between the two groups[9]. This means that taking a man and a woman off the street at random, we can’t claim to know anything about variations in their specific brains - they may actually be more similar than if you had picked two women. In fact, a review paper on sex differences in teenage brains concluded that, aside from the overall size difference, “Male and female brains are overwhelmingly more alike than different”[3].

So does it really matter? While it might be interesting to know how or whether our brains differ, it could be argued it is only important if it changes the way we behave. Again, there have been many claims as to how abilities and ambitions differ between men & women, but even here, the evidence is less than clear. While some argue that there are real differences between girls’ and boys’ abilities in spatial processing[10] and language, others say that these differences are so small as to be meaningless[13]. The real difficulty here is picking apart those differences that may be intrinsic to sex, and those that are caused by the influence of culture.

Sex vs Culture

It is a huge challenge to pick apart whether any differences that are found, either in behaviour or in the brain, are caused by the differences in our genes, or the effects of different sex hormones, or whether they are simply down to differences in lifestyle. Interestingly, the gender gap in science and maths is larger in economically prosperous countries, suggesting that upbringing must be having some effect, at least in subject choices[12]. But could differences in upbringing alone change the structure and connectivity of the brain?

While it may seem surprising, we are realising more and more how flexible our brains really are. Rather than being fixed once we reach adulthood, our brains are constantly changing; being “re-wired” by everything we do and experience. And as children, they are even more flexible. This means that many differences in male and female brains could, potentially, be due to differences in their experience.

We may not know it, but the way we treat girls and boys differs from the earliest age. When told a child is a girl, people are more likely to suggest playing with dolls[13] and will claim that its behaviours are feminine, even when the child is really a boy[14]. So even very young children have experienced differences in their interactions with other people, because of their gender. The expectation that little girls will prefer books to sums may be a self-fulfilling prophecy, and it is well documented that people do badly on tasks they are expecting to struggle with. This so called ‘Stereotype threat’ can affect anyone, on any type of task - from women expecting to be bad at spatial tasks[15] to long distance runners performing badly because they expect East Africans to dominate[16]. To really put this matter to rest, we would need to examine the brains of men and women raised in a gender-less (and race-less) society, but of course, this is impossible. For now, at least, it is incredibly challenging to unpick whether any differences that are seen in male and female brains are down to genetics, the effects of sex hormones - prenatally or at puberty, or simply shaped by experience.

Photograph of Feyisa Lilesa on his way to winning the Dublin Marathon in 2009
If people are reminded of a stereotype in which they are inferior, their performance decreases[16].
Photograph by William Murphy [CC BY-SA]

So why do scientists continue to pursue this question, when it is so difficult to answer, and when an answer may not even be particularly useful, as it can’t tell us anything about a specific man or woman? There are some very important cases where a greater understanding of the differences between male and female brains could be hugely helpful. One such case is in mental health research. It is widely known that the genders suffer different mental health problems at different rates[17]. Women are more likely to suffer anxiety disorders and schizophrenia, while men are at higher risk for autism and alcohol or drug addiction. Men are also far more likely than women to commit suicide, despite women being more likely to be diagnosed with depression. Understanding why these differences occur could help us develop treatments and support tailored to each gender, to improve recovery times.

There is also evidence that men and women respond differently to some medications[18], and that this difference is often missed during clinical trials. While some of these differences are down to hormones or other bodily differences, there are some medications where it isn’t known exactly why men and women respond differently[16]. Understanding differences in the brain may help shed light on this problem.

So what can we say about the brains of men & women? We know that everyone’s brain is as individual as their personality, and is shaped by a mixture of genetics, hormones and experience. So it shouldn’t come as too much of a surprise if there are differences between men and women, after all, we do tend to have different experiences over the course of our lives. But whether these differences are solely down to experience, or whether some of them are biologically programmed is harder to say.

While doctors are just starting to realise they should take into account gender differences when looking at drug dosing, or treatments for mental health conditions, one day perhaps these issues will be dealt with on a more personalised basis, and gender as a shortcut will become less important. In the mean time it is vital to remember that what may be found for a group of people on average can’t tell us anything about a specific person and their abilities, so there is no excuse for neurosexism.

This article was written by our Natural Sciences editor, Ginny Smith.

why don't all references have links?

[1] Ruigrok, Amber NV et al. "A meta-analysis of sex differences in human brain structure." Neuroscience & Biobehavioral Reviews 39 (2014): 34-50. doi:10.1016/j.neubiorev.2013.12.004
[2] "Was Einstein's Brain Different?" The Center for History of Physics. Institute of Physics, Nov 1996. Web.
[3] Giedd, Jay N et al. "Review: magnetic resonance imaging of male/female differences in human adolescent brain anatomy." Biol Sex Differ 3.1 (2012): 19.
[4] Gur, Ruben C et al. "Sex differences in brain gray and white matter in healthy young adults: correlations with cognitive performance." The Journal of neuroscience 19.10 (1999): 4065-4072. doi:19(10): 4065-4072
[5] Gong, Gaolang et al. "Age-and gender-related differences in the cortical anatomical network." The Journal of neuroscience 29.50 (2009): 15684-15693. doi:10.1523/JNEUROSCI.2308-09.2009
[6] Tomasi, Dardo, and Nora D Volkow. "Gender differences in brain functional connectivity density." Human brain mapping 33.4 (2012): 849-860.doi:10.1002/hbm.21252
[7] Tomasi, Dardo, and Nora D Volkow. "Laterality patterns of brain functional connectivity: gender effects." Cerebral Cortex 22.6 (2012): 1455-1462. doi:10.1093/cercor/bhr230
[8] Bennett et al. "Neural correlates of interspecies perspective taking in the post-mortem Atlantic Salmon: An argument for multiple comparisons correction." Conference Posters - 2009.
[9] Hyde, Janet Shibley. "The gender similarities hypothesis." American psychologist 60.6 (2005): 581. doi: 10.1037/0003-066X.60.6.581
[10] Ingalhalikar, Madhura et al. "Sex differences in the structural connectome of the human brain." Proceedings of the National Academy of Sciences 111.2 (2014): 823-828. doi: 10.1073/pnas.1316909110
[11] "Men and Women: No Big Difference." American Psychological Association 2005. Web.
[12] Reilly D. "Gender, Culture, and Sex-Typed Cognitive Abilities." PLoS ONE 7(7): e39904. (2012) doi: 10.1371/journal.pone.0039904
[13] Seavey, CA. et al. "Baby X The Effect of Gender Labels on Adult Responses to Infants." Sex Roles, Vol 1., No 2, 1975. doi:10.1007/BF00288004
[14] Delk, John L et al. "Adult perceptions of the infant as a function of gender labeling and observer gender." Sex Roles 15.9-10 (1986): 527-534.
[15] Wraga, Maryjane et al. “Neural Basis of Stereotype-Induced Shifts in Women’s Mental Rotation Performance.” Social cognitive and affective neuroscience 2.1 (2007): 12–19. PMC. doi: 10.1093/scan/nsl041
[16] Baker, J, and S Horton. "East African running dominance revisited: a role for stereotype threat?." British journal of sports medicine 37.6 (2003): 553-555.doi:10.1136/bjsm.37.6.553
[17] "Mental Health Statistics: Men & Women." Mental Health Foundation. 2011. Web.
[18] Whitley, Heather P. et al. "Sex-Based Differences in Drug Activity" American Family Physician. 2009 Dec 1;80(11):1254-1258. Web.