For decades, many popular books on the differences between the brains of men and women have been published and widely accepted by the general population. From such publications, ideas about the differences between males and females are explained as being fundamentally, biologically predetermined; e.g., men are problem solvers; women are caretakers; men don’t listen, and women can’t read maps. These arguments are so prevalent that they have become deeply rooted in our western psyches, and the ideas seem to have become bigger than the science behind them. But are there real differences in the brains of men and women, and if so, do these differences manifest significantly between genders? Do these variations account for intrinsic distinctions in personality development, self-awareness, and socio-cultural influences on male/female child development? Or, to what extent does the idea of these differences shape and reinforce stereotypes and gender norms? 

Men Have Bigger Brains

Many studies have shown anatomical differences between male and female brains. A 2010 paper offering a review of “Gender differences in the Human Brain,” claimed that while size and weight of the brain are roughly comparable when accounting for body weight, sexual dimorphism, or differences in the structures between males and females, occur in terms of brain volume (male brains have larger amygdalas and hypothalamuses and women have larger hippocampuses); grey vs. white matter (men have more grey and women have more white matter); as well as differences in Wernicke’s and Broca’s areas (this variation attends to the assumption that men process verbal language unilaterally, or in one hemisphere, while women’s brains activate both) (Zaidi, 2010). 

While the data for sexual dimorphism is growing, there could be other factors contributing to these differences. The age of the participants, as well as environmental and experiential circumstances could be contributing to dissimilarity in brain structure. Additionally, many studies show that the variations between men and women, while statistically significant, are still very very small, (Ritchie, et al., 2018, p. 7-8).

Gender Matters

Gender can be defined in many ways. It can be viewed as the difference between having two x chromosomes versus one x and one y. Gender can be assigned at birth, and gender identity can align or conflict with this appraisal. There is also sexual orientation, sexual preference, and a myriad of other divisional factors. Much of the argument for sex differences in the brain hinges on the role of biological factors, hormones and prenatal hormones, as well as genetic effects. It relies on gender distinctions on a chromosomal level, while taking into consideration individual factors that compromise these distinctions. It entails that structural, chemical, and processing differences in the gendered brain result in observable variance in the thinking, emotion, and behavior of humans, (Cohen, 2002, 2003). For example, differences in amygdala  volume “may relate to behavioral evidence for sex differences in emotion processing,” (Zaidi, 2010, p. 38).  

Simon Baron-Cohen is one of the most prominent contemporary figures supporting the role of biological sex differences in men and women today. He distinguishes between two brain systems: empathizing (predicting and interpreting the emotions and actions of others) and systemizing (analyzing and constructing variables within a system,) and claims that, “on average, males spontaneously systemize” more than females, (Cohen, 2002, p. 248). Building from his Theory of Mind studies with autism, Cohen argues that the female brain is designed towards empathy (‘Type E,’ or female brain), while males are more likely to “predict” and “control” their world by interpreting it as a “system,” (‘Type S,’ or male brain). Cohen claims that while some men will have Type E brains, and some women may be Type S, the majority of people will stay true to their gender, and that individuals of both genders with autism are more often categorized as extreme male brains/Type S. It is also interesting to note that Cohen’s model also includes a ‘Type B,’ or balanced type. While Cohen does not argue strict binary distinctions, he does emphasize the significance of average differences in males and females.

Building on this E-S Theory of Gender, Cohen co-authored a study in 2006 that looked at prenatal hormone differences and their effect on an individual’s ability to empathize (Chapman, et al., 2006). In the study, children of mothers who had undergone amniocentesis during pregnancy had fetal levels of testosterone recorded. On average, male fetuses produce more testosterone than female fetuses. The children were given empathy/systemizing tests at the age of 8 years old. The results showed that children with higher levels of prenatal testosterone had more difficulty in reading faces. In the systemizing test, which consisted of an image with a shape hidden within the overall design, children with higher levels of testosterone were quicker and more accurate at finding the part in the whole (Chapman, et al., 2006). These results support Cohen’s E-S Theory about biological differences in gender correlating to male/female brain functionality.

In 2000, Cohen also co-authored a study that looked at sex differences in social perception in newborns. In the study, 44 male and 58 female infants were shown an image of a female face and a geometric image. These images were shown separately and in a randomized order, and the amount of time an infant spent looking at each image was recorded. The results of this study showed that infant girls looked longer at the female face, while infant boys looked longer at the geometric image. These infants were only 24 hours old, and the researchers concluded that prenatal biology is at the root of the differences in preference of social (female) vs. non-social (male) stimulus, (Connellan, et al., 2000), building further support for the E-S theory.

The Other Side

But what about the arguments for a more nuanced look at the role of sex differences between male and female brains? Factors such as environment and experience are at the heart of the argument against sexual dimorphism in the brain. In a 2014 paper by Rippon, et al., researchers outlined four concepts that should be taken into consideration “to increase the understanding of the interaction between the neurobiology of the individual and the environment in which humans develop and function,” (p.1). Those concepts are overlap, mosaicism, contingency, and entanglement. Overlap implies that men and women have more commonalities than differences in terms of “social, cognitive, and personality variables,” (Rippon, et. al, 2014. p. 2). Mosaicism refers to the ways in which female and male aspects are interwoven in the human brain, and argues that no individual can be strictly either/or. Contingency is the influence of complex and diverse cultural and socio-economic factors that influence gender behavior. Rippon, et al., claim that training can equalize perceived gender differences in relation to specific skills, such as video game training for improving proficiency in mental rotation tasks in women versus men. Entanglement is the process by which the brain continues to develop throughout an individual’s life, discrediting or building upon the conditions dictated at birth, and emphasizing the brain’s plasticity. Rippon, et al, conclude that if researchers integrate these concepts into their theoretical designs, it will increase the understanding between “the neurobiology of the individual” in context with environment circumstances when investigating and accounting for sex differences in the brain.

Another compelling argument against sexual dimorphism in the brain centers around scrutiny of the evidence presented by previous studies. In 2021, a meta-synthesis of human brain studies was published by Eliot, et al., and claimed that the variance in white and grey matter, as well as differences between “intra versus interhemispheric connectivity, and regional cortical and subcortical volumes in males,” become insignificant when the size of the brain is taken into account, (Eliot, et al., 2021, p. 1). In a much cited 2005 study by Haier, et al., MRIs showed that women had more white matter and less grey matter than men. The aim of the study was to investigate the differences between intelligence and gender, and while the researchers found that “different types of brain designs may manifest equivalent intellectual performance,” (Haier, et al., 2005, p. 320), the researchers concluded with the question as to whether “neuroanatomical differences” could be “associated with specific mental abilities,” (p. 326). In a review of the study by Haier et al., Science Daily reported that “gray matter represents information processing centers in the brain, and white matter represents the networking of – or connections between – these processing centers,” (University of California, 2005, p. 3). These interpretations have a far-reaching influence in the non-scientific world, and widely reinforce stereotypes about cognitive differences between males and females. However, it is important to note that the study by Haier et al. had 48 participants, and the researchers did not seem to take into account the differences in overall brain size when calculating their results. Still, the notion that men have significantly more grey matter than women is said to be one of the most “replicated and robust” sex differences revealed in similar studies, (Luders, et al., 2009, p.14,265).

Eliot and Rippon believe that the information behind NeuroImaging differences in sex and gender in the brains of males and females has not been “synthesized in a systematic manner,” (Eliot, et al., 2021, p. 3). In support of this claim, one can refer back to the previously mentioned Review of Gender Differences, and find that the author has summarized the findings of certain studies in cause and effect terms. For example, Zaidi writes that the distinctions in lateralization of brain hemispheres “helps explain why the male brain has a hard time expressing its feelings,” as opposed to women, who maintain “emotions in both hemispheres,” (2010, p. 39). These kinds of leaps between NeuroImaging differences and behavioral stereotypes are often conflated by the relatively small statistical variations reported by brain sex difference studies. As well, publication favors research where differences are shown to be significant and positive, while many of the studies showing no differences are never published at all. Eliot et al., claim that this type of bias “is coming under increasing scrutiny through systematic analysis and statistical proof of excess significance,” (2021, p.3). Despite Zaidi’s review, a 2008 study by Sommer, et al., found no differences in “language lateralization,” and “no sex difference in asymmetries of the Planum Temporale, dichotic listening or functional imaging findings during language tasks,” (p.76) between men and women.

In light of these arguments, how should one interpret brain sex differences shown by studies such as the one by Ritchie, et al.? Psychology often looks at small, statistically significant variations as the basis for theory and argument. Is brain gender therefore a biological variable? Are averages in brain sex differences valid, as Cohen believes? Or are these differences also the result of living in a gendered world; tempered by experience and culture? Eliot and Rippon do not deny the evidence put forth by other researchers, as much as they question the relevance of how the data is interpreted, with the focus primarily on the differences. The question remains: how do these differences actually manifest in individuals? Everyone is different, unique, and if the brain is gendered, the body does not always align with that input, and vice versa (see conditions such as congenital adrenal hyperplasia and androgen insensitivity syndrome). A 2015 paper by Joel, et al.supports the view of the brain as a human mosaic. These researchers looked at data from 1,400 brain scans, and corroborated that with 5,500 individuals, looking for relationships between internal workings and external behaviours and expressions. What they found was an “extensive overlap between females and males in personality traits, attitudes, interests, and behaviors,” as well as “substantial variability of gender characteristics,” and “extremely rare…internal consistency…even for highly gender-stereotyped activities,” (Joel, et al., 2015, p. 5). 

What next?

Many of the ideas about the dimorphic brain that have resulted from thirty years of NeuroImaging technology. The question about the role that these differences play in the development of men and women is at the heart of this debate, as well as whether these differences are the result of biological processes that translate into observable differences in behavior, or whether they emerge as a result of living in a gendered world. If sex differences exist in the brain, then it could lead to critical changes in the way certain diseases, disorders, and psychological conditions are diagnozed and treated. If, however, these differences are exaggerated, and brains are independently changing throughout one’s life, with greater impacts coming from the environment, then our interpretation of these findings could be influencing and even limiting performance and evaluation along gender lines. 

Moving forward, there is still the need for replication. Neuroscientists will continue to study the brain, and look for relationships between activity and function. Differences in brains exist, just as differences in genders exist, as a complex result of biological and environmental factors. It seems fair to say that a scientist would be hard pressed to distinguish between a  male and female brain by looking at MRIs alone. Perhaps the field would progress more rapidly by subtracting, rather than adding differences to the concept of the gendered brain.

References

Baron-Cohen, S. (2002). The extreme male brain theory of autism. Trends in Cognitive Sciences, 6(6), 0–254. doi:10.1016/s1364-6613(02)01904-6 

Baron-Cohen, S. (2003). The Essential Difference: The Truth About The Male And Female Brain. Phi Kappa Forum.

Bian, L., Leslie, S.-J., & Cimpian, A. (2018). Evidence of bias against girls and women in contexts that emphasize intellectual ability. American Psychologist, 73(9), 1139–1153. https://doi.org/10.1037/amp0000427

Burman, D.D., et al. (2008). Sex differences in neural processing of language among children, Neuropsychologia, 46(5), 1349–1362. doi:10.1016/j.neuropsychologia.2007.12.021

Chapman, E., et al. (2006). Fetal testosterone and empathy: Evidence from the Empathy Quotient (EQ) and the “Reading the Mind in the Eyes” Test. Social Neuroscience, 1(2), 135–148. doi:10.1080/17470910600992239  

Connellan, J., et al. (2000). Sex differences in human neonatal social perception. Infant and Behaviour Development , 23(1), 113–118. doi:10.1016/s0163-6383(00)00032-1

Gilmore, J. H., Knickmeyer, R. C., & Gao, W. (2018). Imaging structural and functional brain development in early childhood. Nature reviews. Neuroscience, 19(3), 123–137. https://doi.org/10.1038/nrn.2018.1

Eliot, L. et al.(2021) Dump the “dimorphism”: Comprehensive synthesis of human brain studies reveals few male-female differences beyond size, Neuroscience & Biobehavioral Reviews, 125, 667-697. https://doi.org/10.1016/j.neubiorev.2021.02.026.

How to Academy Mindset. (2019). Is the brain gendered? The debate, [Video]. YouTube. https://www.youtube.com/watch?v=kxfaE-gWZ9I

Haier, R.J., et al. (2005). The neuroanatomy of general intelligence: sex matters. Neuroimage, 25(1), 320–327. doi:10.1016/j.neuroimage.2004.11.019. 

Ingalhalikar, M., et al. (2014). Sex differences in the structural connectome of the human brain. Proceedings of the National Academy of Sciences, 111(2), 823–828. doi:10.1073/pnas.1316909110 Joel, D., et al. (2015). Sex beyond the genitalia: The human brain mosaic. Proceedings of the National Academy of Sciences, (), 201509654–. doi:10.1073/pnas.1509654112 

Luders, E., et al. (2009). Why Sex Matters: Brain Size Independent Differences in Gray Matter Distributions between Men and Women. Journal of Neuroscience, 29(45), 14265–14270. doi:10.1523/jneurosci.2261-09.2009 

Ritchie, S., et al. (2018). Sex Differences in the Adult Human Brain: Evidence from 5216 UK Biobank Participants. Cerebral Cortex, 1–17. doi:10.1093/cercor/bhy109 

Rippon, G., et al. (2014). Recommendations for sex/gender neuroimaging research: key principles and implications for research design, analysis, and interpretation. Frontiers in human neuroscience, 8, 650. https://doi.org/10.3389/fnhum.2014.00650

Rogers, L.J. (2010). Sexing the Brain: The Science and Pseudoscience of Sex Differences. Kaohsiung J Med Sci, 26(6-supp-S), –. doi:10.1016/s1607-551x(10)70051-6 

Sommer, I.E., et al. (2008). Sex differences in handedness, asymmetry of the Planum Temporale and functional language lateralization. Brain Research, 1206, 76–88. doi:10.1016/j.brainres.2008.01.003

Town Hall Seattle. (2019). Gina Rippon: The Myth of the Gendered Brain, Town Hall Seattle, [Video]. YouTube. https://www.youtube.com/watch?v=wpPTpPvbcVo

University Of California, Irvine. (2005). Intelligence In Men And Women Is A Gray And White Matter. ScienceDaily. www.sciencedaily.com/releases/2005/01/050121100142.htm

Zaidi, Z. (2010). Gender Differences in the Human Brain: A Review. The Open Anatomy Journal. 2. 10.2174/1877609401002010037.