Sex Differences in the Brain - a Myth?

‘Male brain’ and ‘female brain’ are terms often used without much explanation. This thinking seeps into our workforce, the way we raise our children, our education system, until it can be hard to imagine a society based on anything else. How often have you heard phrases like ‘women are naturally more empathetic’ or ‘men have more mathematical brains’? Does science support this? 

While earlier scientists found what seemed to be revolutionary physical differences between ‘male brains’ and ‘female brains’, later research explores a different theory - that it is the influences around us that cause sex differences, rather than the brain itself. Gina Rippon, the scientist at the forefront of disproving common misconceptions about gender and neuroscience, said: “a gendered world creates a gendered brain” [1]. Let us delve deep into the world of neuroscience, psychology, and biology and see if the ‘gendered brain’ is in fact an unfortunate side effect of evolution or something a little more complicated.

Key Definitions

Before we explore the complexity of gender, sex, and neuroscience, it is important to fully understand the scientific terminology used in these discussions. Much of the misunderstanding around what gender is comes from terms such as sex and gender being used interchangeably, when in fact they are completely different - linked, but not the same. The terms we will be using in this article and their definitions are as follows [2]:

Sex - usually a biological term used to refer to a person’s chromosomal makeup at birth. In fact, it is actually a lot more complicated than simply biological - your legal sex is something assigned at birth, but can be changed throughout your lifetime. Sex is not a binary term, although it has been used as such in the past. Biological research suggests humans may operate on a spectrum of six or even more biological sexes. XX and XY are the most commonly known, but intersex people are often born with a different karyotype, for example XXX - genital sex also carries a lot of variation, which is why legal sex sometimes has to be decided at birth for intersex people (a controversial process which we will discuss later). We will use the words ‘biologically male’/’male’ or ‘biologically female’/’female’ to refer to sex, unless referring to intersex people, in which we will clarify.

Gender - used to describe the set of behaviours and expectations associated with being a man, woman, or outside this gender binary. Once more, gender is not a binary term - there are many more than just men and women. An important thing to note about gender, as is the main theme of this article, is that it is more a construct than anything else, and depends primarily on the social context of whatever time period and location we are looking at. ‘Man’, ‘woman’, and ‘non-binary’ will be the terms used to describe gender, rather than sex.

Gender identity - this is the gender someone knows themself to be. Since we know gender to be an incredibly flexible concept, it is no surprise that every single person will experience gender differently, and therefore there is a range of gender identities that someone can hold. Neurodivergent people have been shown to experience the idea of gender and gender identity very differently to neurotypical people [3]. Even the descriptions associated with ‘male’ and ‘female’ behaviours are far more complex than feminine or masculine, especially as it is paired with feminist movements and the fact that in more progressive society, we are challenging gendered stereotypes, such as humanities being for women and STEM being for men. In terms of gender roles, we will use ‘masculine behaviour’ and ‘feminine behaviour’ to refer to these historically gendered roles and characteristics, while acknowledging that what it means to be feminine and masculine is a constantly changing concept, and should not be used to restrict anyone’s ability to pursue the things they want to.

Transgender - this is an adjective used to describe someone whose gender is different from the gender they were assigned at birth (it can generally be used as an umbrella term to include non-binary people).

Cisgender - this is an adjective used to describe someone whose gender is the same as the gender they were assigned at birth.

The Anatomy of the Brain

On average, biologically male and biologically female brains do show anatomical differences. One such example is a study showing that male brains, on average, had 71% more somatostatin neurons than female brains - these are neurons responsible for producing an inhibitory growth hormone in the endocrine system [4]. The most obvious difference that seems to be ingrained into our lives is that male brains are larger than female brains. However, something to keep in mind throughout this article is the way we visualise these differences. Some research puts the male brain at approximately 10% bigger than the female brain [5], but research also shows that different parts of the brain are differently weighted in the male and female brains, like the hippocampus. There is a lot of overlap between the distributions of sex based measurements of the brain, and as stated by Ritchie et al. in a study using Biobank participants, there is both a statistically significant difference and also much variation between male and female brain sizes [6]. Brain size and sex differences are not like height - they exist on a spectrum, but not a linear one. We could imagine it as an infinitely varied colour wheel - the research we have could be used to split the wheel into two halves, male and female, but there is a huge amount of blending and intermediate colours between each half.

One study that focuses on this especially is a 2014 paper into the anatomical sex differences between male and female brains [7]. Six years is a long time for brain research techniques and behavioural neuroscience theories to have evolved, which will be touched upon later, but this paper represents a common thread in the argument that males and females are fundamentally different. It finds that the connections between hemispheres of the brain are stronger in female brains, while male brains are more lateralised, in that the stronger connection pathways are focused within areas of the brain. Here, the phrase ‘complementarity’ is used: this is the idea that ‘male behaviour’ and ‘female behaviour’ are designed (to use the term lightly) to support and balance each other out in society. Another study, conflictingly, finds a relationship between the size of the corpus callosum (which connects the brain hemispheres) and cognitive abilities in women, but not in men [8].

It is incredibly easy to form connections where there are little to none, most notably here between brain anatomy and brain function. The truth is, neuroscientists are now well aware that a single area of the brain does not control a specific action, but rather many pathways working together all at once. Male brains may indeed be more lateralised. However, this cannot mean that the anatomy of the brain directly affects behaviour and skill set, if so, females with more lateralised brains should show more ‘masculine’ behaviours, but this correlation has not been found. Every single person on this planet has a slightly different brain to the next.

For a moment, think of the brain not as a living, pulsing mass of tissue and electricity, but as a set of algorithms. It is constantly taking in information from the environment, associating this information with other pathways, both old and new, and altering its pathways to get a better grip on the world around it. ‘Establishing a prior’ is the term in neuroscience given to the brain’s ability to predict what is about to happen based on previous information, and thus work out what it is supposed to do [9]. This predictive algorithmic ability is key to our survival, both socially and physically. What it does mean, however, is that the rules our brain thinks it has to play by can be full of stereotypes and incorrect assumptions. If our brains were truly hard-wired, this changeability and prediction would not be the case. Could this mean something much more complex than anatomy itself is at play? 

Mood Swings

With that, we move to the next biological section of the explanation - hormonal differences between the sexes. At just 8 weeks into pregnancy, testosterone (Fig. 1) concentrations differ between the sexes [10]. The effect of hormones on the brain is well-documented with a higher exposure of testosterone generally causing areas of the brain linked to ‘masculine’ behaviour to grow larger. This is in line with the fact that male brains are on average larger than female brains [11].

Figure 1 - Structure of the hormone testosterone. Reprinted from Ref [12].

Experimenting with hormonal variation is difficult from an ethical perspective, since most animal experiments have involved medical or chemical castration to observe the effects of changing a mammal’s hormones abruptly. As modern medicine has become more advanced, hormone therapy and its effects on transgender people has been studied; research usually concludes that it is a safe method of reducing dysphoria and is fairly effective in bringing about biological changes [13, 14]. Based on the relative newness of hormone therapy, though, the studies do not provide a huge amount of detail, especially in terms of psychology and brain changes. When thinking about gender, it becomes even more useful to look at hormone changes from a young age, to observe how they react when exposed to our very gendered world without previous experience of being perceived as a certain sex. Case studies of naturally occurring hormonal variation have therefore become a great subject of interest. 

Congenital Adrenal Hyperplasia, abbreviated as CAH, is a genetic mutation causing an excess of androgens (male sex hormones) to be produced [15]. Males with CAH generally still fall within the normal testosterone levels for males, but females with CAH provide a good research opportunity; they were found to exhibit more ‘masculine’ behaviours, higher levels of aggression, and a higher likelihood to be in top CEO positions than control females without CAH [13, 16]. As we know, the majority of leadership roles are held by men, and so seemingly this adds to the argument that sexual anatomy gives men and women certain roles in society.

Some have pointed out, though, that parents will tend to treat girls with CAH differently, whether subconsciously or not, since they tend to be born with masculinised genitalia and subsequently given surgery. This is an incredibly controversial process. A wide societal misunderstanding of intersex people (not a rare occurrence as many would believe, but instead affecting 1.7% of people worldwide [17]) has led to many children immediately given unnecessary surgeries which violate their personal autonomy in order to fit the child into the ‘normal’ expectations of children and gender. While some surgeons insist that these surgeries are paramount to maintaining the child’s physical and mental health, many are becoming aware of the fact that the treatment of intersex people is at best ignorant and at worst, violent. Why should intersex people be forced into boxes of one sex or another without their consent? Growing research, along with the voices of intersex people being heard more often nowadays, shows that there are little to no benefits to these surgeries, and hiding a child from a key part of themselves just leads to further confusion and distrust of medical professionals and the people around them [18]. As we discuss the nuances of gender, it is worth keeping in mind that its chromosomal counterpart is also a spectrum, as we mentioned earlier. The intersex community should not be made into an anomaly violating the ‘proper’ view of gender and sex, but rather a major part in the discussion of biological fluidity.

There also seems to be conflicting evidence surrounding CAH and sex differences - one study finds no difference between the cognitive abilities of females and males with CAH as opposed to those without [19]. Another study finds evidence of high prenatal testosterone, typically found in males, having effects on brain lateralisation. Earlier we were reminded that lateralisation is associated with the more masculine focussed cognitive behaviour [20]. 

There is indeed a relationship here between hormones, brain structure, and masculine or feminine typical behaviour. As we will come to see, this is by no means the be-all and end-all, but it does add some perspective to the picture we are building of where these differences begin. Hormones will have an effect on brain anatomy. The question now is, how directly does this link to the sex differences in behaviour?

Neuroplasticity

In all the years of neuroscientific research, we are still not quite sure how the brain works - but the one thing we do know is that it is the most flexible and adaptable of the organs. New neural pathways are formed and influenced every day, which is exactly why every person on this earth exhibits such different behaviour and personality traits. So why do we still use the term ‘wired’ when it comes to male vs female brains? No one brain is designed for any one purpose: if that were the case, humans would not be the complex, living, conscious organisms we know we are. Indeed, Patrice Voss sets out the importance of critical periods in neural development and how there are many factors (for example, ageing) that can affect not only changes in brain circuitry but the types and prevalence of these changes [21].

An interesting theory comes to light in the research of sex differences by Geert De Vries. His idea states that the sex differences in the anatomy of the brain are supposed to balance gaps between the sexes, such that sex differences do not grow to an undesirable (in evolutionary eyes) dimorphism between males and females because of hormones and genetics [22]. The hormones at play regarding the sex of a person may have a huge effect on their actions, behaviour, and decisions, but de Vries argues that these differences may not be ideal for the good of an organism - therefore, there must be more differences in the brain to make up for this. This may be a big part of the discussion at hand because what we are lacking information on is why exactly, especially on an evolutionary basis, sex differences are needed - if they are at all. 

”Girls Can’t Do Maths…”

Behaviour. Ever since we evolved the ability to look at ourselves and ask ‘why?’, behavioural psychology has been the name of the game. Women and men have different perspectives, make decisions differently, spend their free time differently, prioritise differently...or so we think. However, as previously mentioned, the core aspect of neuroscience we need to consider here is neuroplasticity. Behaviour is nothing if not learned, and if we are surrounded from a young age with subtle stereotypes and unwritten rules about what gender we are and what that is supposed to look like, how can we not subconsciously attempt to fulfill this standard? A 1977 study found that women who believed that they were suffering from premenstrual symptoms when they were not, reported many of the same emotional symptoms as they would if they were actually about to start their cycle [23]. Now, of course, hormones do have a great effect on our emotional and physical wellbeing. It is interesting, though, to see how easily our behaviour and perspectives can change based on a sudden change of information, almost as if our brains have to correct for new data just like a computer algorithm.

There have been studies that show men outperforming women in maths and systematically based tasks, while women tend to show more verbal skill and better emotional intelligence. Here lies a psychological trap, however, in the form of gender priming and stereotype threat. Gender priming occurs when a participant is in some way forced to acknowledge either their gender or a stereotype about their gender. A group of women reminded of their gender did worse on average in maths and science performance tests than women who were neutrally primed [24].

This effect can be combined with another, called stereotype threat, in which a group will suffer from anxiety about confirming a negative stereotype about themselves. This can affect performance and decrease space in working memory, as some studies show [25, 26]. When you have been told your whole life that the social group you belong to is wired to be bad at something, it is just accepted as a fact, the psychological effects are vast and dangerous. The stakes are much, much higher.

Since we know how mouldable the brain is, we can now understand how stereotype threat and gender priming can reform negative connection pathways and reinforce this perceived gender gap. We will come onto this when we discuss the influences of our environment on gender, but why is it these aspects are rarely taken into account when “proving” sex differences? Perhaps we must dig deeper - right into the core of this evidence itself.

…Or Can They?

Research into biological and behavioural differences between the sexes is full of conflicting and insufficient evidence. As with any theory, we must look at the evidence and be critical of all of it, whether it supports what we want to believe or not. The Systemising Quotient and Empathising Quotient were key methods of figuring out whether a person took a more focussed, systematic approach to life or a more varied, emotional approach [27]. The quotients were lists of statements along the lines of ‘I can feel what others are feeling,’ and participants were asked to rank each statement on a scale of Strongly Agree to Strongly Disagree. Women generally scored high on the EQ and lower on the SQ, while for men it was the other way around. One of the other hypotheses tested in this study was that autistic people showed a brain more like the typical biologically male brain [28]. However, this conclusion should be treated with caution - autism research has historically (and in many cases continues to be) focussed on the experiences of boys and men, resulting in under- and mis-diagnosis of women and girls who do not align with the gendered stereotype that is systemic in diagnostic tools.

Now, this evidence could be used in favour of the fact that girls and boys are drastically different. However, this study used a sample set of just 278 adults from the UK and Canadian public, half of which were students from University of Cambridge. While it is absurd to think a theory can be tested in every single person alive, this is an incredibly small data set, and trends within it are much more likely to be coincidental, and if not by chance, centred around just two countries. Perhaps if these male-female brain trends were observed in other countries and cultures, it could be an argument for the hardwired behaviour of the sexes; but we have seen on multiple occasions that the environment a participant is raised in is hugely relevant in their perspective of others and themselves.

Let us also think about just how difficult it is to control behavioural experiments like this, even with the much larger data set in the other EQ/SQ study. Experimental evidence, while important, is incredibly hard to control in the way that we know a reliable experiment must be. Self-report questionnaires are far from reliable, requiring the participant to analyse themselves, and human behaviour will very easily be changed to fit in with either the participant’s own or the societal expectations for their gender. They are, on the one hand, a good insight into how a person sees life through their own eyes, but they also open the doors to unconscious bias in the form of gender priming and stereotype threat. Scientists Naomi Weinsstein and Gina Rippon often point out in their writings this deep rooted urge to take a certain approach, and it can be further extended to the scientists who carry out the research - sampling bias could allow the data set to be tainted with people who are more likely to answer in a way that will confirm their hypothesis. 

When we cast a watchful eye on the many issues with psychological experiments, sex differences between male and female brains seem to be a self-fulfilling prophecy. Empathy is associated with being inherently feminine, and of course, that stereotype will affect how a participant sees themselves and in turn how they answer the questions. Are there better ways of researching these differences?

Figure 2 - Patient having an MRI scan. Reprinted from Ref [29].

We have now come full circle back to the anatomy of the brain. Brain imaging has been the focus of the past few decades; once we can see what is really going on inside the billions of neural pathways in our heads, it will be much easier to link this activity to how it is presented in our behaviour. It is difficult, however, to directly link activity to function. Even functional MRIs (Fig. 2), which provide moving real time images of blood flow in the brain, do not directly prove how neurons interact, what purposes they work together for, or why certain areas are ignited during certain behaviours. Imagine trying to work out the exact steps in a game of chess just by looking at a picture of the board at the beginning and one at the end. Brain imaging is a fantastically advanced technology, but since we do not know a lot about the areas of the brain, it definitely has its limitations. This is especially true when it comes to behavioural links to neurological function [30].

As technology advances further, we can paint a clearer picture of how brain function and brain anatomy are related. The Human Brain Project, a ten year plan started in 2013, is, among other advancements, looking at computational methods of tracking neural pathways and investigating brain structures [31]. Perhaps by 2023 we can physically see what it is about the brain that causes people to behave so differently. The point still stands, though, that our social context does have a huge effect on the brain and its pathways, which we must take into account when researching the brain.

Congratulations, it’s a Human Being!

It is very easy to take for granted the influence of our environment. We are taught that biology begins with DNA, genetic units that are remarkably unchangeable throughout generations. It is often tempting to accept that different sexes simply have different functions. However, the brain is quite the opposite of unchangeable. The age-old debate of nature versus nurture is frankly outdated; nature and nurture are locked in balance with each other, each affecting the other in just the right way to create the perfectly mouldable human being. 

Given the name of selective pressure in evolutionary biology, our environment has everything to do with how we turn out, and the same applies to our behaviour. In the past, our environment would have been our conditions, predators, prey, and location. We changed our perspectives with the society we know today, meaning that ‘environment’ now also refers to the socioeconomic factors, relationships with family, friends, and partners, and stereotypical views that surround us.

Naomi Weinsstein, a psychologist and feminist, argues that the psychology of behavioural sex differences needs to take into account social context [32]. This links back to the discussion of foolproof research methods. We all know the importance of control variables that would otherwise have a hidden effect on our results, and drawing this idea up to a larger scale, children are very much affected by the socio-cultural stigmas around their sexes, even before they can walk and talk. Studies show children learn from an early age what is appropriate and acceptable behaviour for their sex by imitating the adults around them that they recognise as part of their sex [33]. We live in an inherently gendered society as clearly seen in everyday objects, like pink versus blue toys, to the things that are harder to acknowledge, like how female babies are described as cute and males are described as strong. As babies, they watch as 28% of mothers stay at home, while only 7% of fathers do (an increase from previous decades, but the difference is still astounding) [34]. This phenomenon is something babies will inevitably pick up on, especially as they are in the most vital and changeable part of their development. Even when they grow up, possibly more aware of the sexism infiltrating the world, their subconscious’ still internalise the stark difference in male and female CEOs.

Furthermore, some have discovered that there are little to no differences in early mathematical development between males and females [35, 36]. One of these studies was conducted using functional MRI among other methods, which has limitations but is still a good starting point to think more deeply into where this academic gender gap begins.

Picking up on the earlier point about our gendered society, it is true that everything we do, say, and think, can be somehow linked back to the importance of gender constructs in our world. On an extreme level, there are, regrettably, thousands of cases where men and women are disadvantaged dangerously, such as men being more likely to have problems with drugs and alcohol to women having higher rates of depression [37]. Not to mention, cases of extreme violence towards women have been an ongoing issue for generations, in the form of femicide and female genital mutilation. Even in the more subtle ways of the world, gender plays a huge role in the way our society works, which is exactly why the neuroscience of it must be discussed.

Many LGBTQ+ issues go hand in hand with the issues of modern-day sexism, particularly discussions around sex and gender. More and more people every day are realising that their gender is not the one they were assigned at birth. Sex in itself is a complex spectrum, as we have seen with the CAH cases, and an interesting finding in neuronal studies has shown that transgender women have neuron numbers similar to cisgender women, regardless of their biological sex at birth [4]. Sex is not simple, and gender even less-so.

It is important to note here that while the way we see gender is a construct, it is one that can still have a great effect on people’s psychology and feelings. Gender is chiefly a concept orchestrated by centuries of not simply biological, but environmental and socioeconomic influences. Therefore, it makes sense that a person’s gender will not necessarily ‘match’ their biology. As society evolves and changes, constructs such as gender will evolve with us and our psyches will too. It is not as simple as ‘genitalia = gender,’ and to suggest so is a misunderstanding of the culture we have cultivated based on a thousand stimuli. Gender fluidity is an important aspect of many non-Western cultures, and even within places such as the UK, the LGBTQ+ community is more often being heard. Fundamental to this journey to acceptance is a clear understanding of the intricacies of gender, and as we have seen, the biology behind it is far from black and white.

The Gender War Reignited

The majority of our population does not truly want inequality between the sexes. More women are breaking the ‘glass ceiling’ and taking up positions of power, just not quite as many as men named John in 2015 [38]. Perhaps it would be easiest to just assign people roles in society based on the ownership (or lack thereof) of a precious Y chromosome. This, however, is simply not good enough. We have greatly evolved since the origin of our species, and with us, a civilisation and society built on billions of rules and systems, but also inherent flaws, has formed.

It is ever so important that we look at the evidence and strong arguments, instead of outdated myths, to build these rules on. When we finally collect all the knowledge we can, and come to a conclusion about the male brain versus the female brain, the neuroscience and biology behind it may turn the scales of this debate definitively one way or another.

However, maybe this is not a one side versus another debate. There may very well be observed differences in the genders and observed differences within their brains. But our brains must rely on something more than just their original structure to change as wildly as they do and this something is most likely our social environment. Perhaps there are sex differences between male and female brains, but if our brains change, we must be responsible in some way. To put it another way, if there are sex differences in our brains, it might be because we made it that way. The problem is, there has been evidence against ‘male’ and ‘female’ brains. Yet, this dichotomy remains in our world because the alternative would be to actively fight against the social context that harshly holds so many people back. This is simply not right. Science was always supposed to provide clarity, but only if people are willing to accept it and look for it.

“[Physics was] invented and built by men, it’s not by invitation” [39]. It would be great to believe that this was said in 1800s Britain rather than by a prominent professor at CERN in 2018. Most women in STEM will have heard something along these lines at some point in their long or short careers. I certainly have. What should strike you here, and what has been the main point of the previous paragraph, is not the flippancy of this dismissal, but the claims that it is built on science. It is simply dangerous to not take into account all the evidence, especially if one claims to be a talented physicist.

Of the evidence we have discussed today, a lot of it contains subtle things to consider and a lot of it is conflicting, but we can see that gender is far more complex than just the male brain and female brain. It may be that anatomy and hormones do have an effect on behaviour for many different reasons. However, it is not the only factor at play, and our social environment has at least an equal, if not larger, stake in what kind of person we turn out to be. If you take anything from this article, it should be that biology is not and never has been a simple chain of events. When someone says women are just naturally worse at science, or men are just naturally less emotional, always question the use of the word ‘nature’ because, as we have seen, nature and nurture will always be intertwined. We owe it to ourselves as human beings to look deeper and harder to prove what we think we know - if only so that the next generations can build their civilisations on the correct ideas, not just the most convenient ones.

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