Sex-based discrimination is frequently the byproduct of differences between males and females and the stereotypes surrounding those distinctions. Parents excuse unnecessary violence from male children with the commonly uttered phrase, “boys will be boys'', and employers reject qualified female applicants from being hired using the logic that “girls are not tough enough for my industry”. Surprisingly, the reason why stereotypes such as these are hard to eradicate is that they originate from a semblance of truth. It is correct that males and females are psychologically dimorphic. In fact, just as a computer algorithm can easily identify whether a face is male or female based on a composite of notable facial features, a computer algorithm can also distinguish male individuals from female individuals when given the personality metrics of an anonymous individual (Giudice et al., 2012). From these findings it is evident that there is a strong correlation between the sex of an individual and the likelihood that they possess certain personality traits. However, these studies do not justify the prevalent stereotypes. Giudice’s study, and other similar studies, emphasize that variation in personality traits are only “small to moderate” (Weisberg et al., 2011). Additionally, the complex web of social, cultural, biological, and evolutionary factors that dictates the average traits of a sex cannot determine the characteristics of any individual. Thus, sex differences should never be maladapted for judgemental uses. To reduce sexism, we must educate ourselves on the scientifically proven differences between males and females, understand how and why these differences arose, and create policies to better accommodate our diversity. In this essay I will argue that we cannot just ignore the psychological differences between sexes and the impact they have on sexism and modern society.

To understand the underlying causes of our differences, it is essential to look at the origins of psychological sexual dimorphism in early humans. In order for ancient humans to succeed as a species, humans had to develop intricate brains, and when our species faced strong selective pressures, cognitive function continuously improved. This focus on mental development to overcome existential obstacles is also observed in the ways our species became sexually dimorphic. Professor Archer at the University of Central Lancashire has found that in prehistory, constant fighting for resources between neighboring groups of homo sapiens, specifically “inter-male competition” caused human populations to become more aggressive over time (Archer, 2019). Aggression was disproportionately selected for in members of the male sex because males throughout history exhibited greater muscle mass on average, lending them a competitive edge for hunting, physical labor and fighting. Additionally, just like how two male bucks lock antlers when fighting over a doe, primitive human males often fought for mates, and the more assertive male gained better mate choices and a reproductive advantage (Gat, 2000). In this way, enhanced aggression directly increased fitness for males. The disparity in aggression and physicality between males and females meant that males left the home to hunt and fight, so females were left to undertake domestic jobs, such as caring for children and cooking food. Over many generations, both sexes evolved physical and mental traits that suited their respective jobs. In more recent history, further division of labor has increased sex differences. For example, before the invention of the plow, both men and women participated equally in agricultural jobs, but after the plow was introduced to human populations, females saw a wholesale decrease in agricultural labor participation. This can be explained by the upper body strength required to direct plows that women often lacked (Alesina et al., 2011). As a result, males in those societies were valued for their strength that helped them use plows, while females were valued for emotional intelligence and other qualities that were beneficial for raising children and caring for the household.
However, the evolutionary history of humans does not tell the full story of psychological differences between sexes. Researchers have long theorized that social norms have an even bigger impact on sexual dimorphism than natural evolution does, since social change occurs at a much faster rate than evolutionary change. Whereas evolution only acts on populations of humans over several generations, social expectations can change within an individual’s lifetime and are key influences on how humans express their identity and personality. Using the same example of male aggression, a large factor in the disparity of violence between males and females is the widespread acceptance of male aggression in society. Fuentes, an anthropologist at Princeton, asserts that “a kind of inevitability in our communal sense of violence and society… makes violence a natural default state of masculinity” because for countless generations boys have been encouraged to express aggression while growing up (Fuentes, 2021). Even in the present, boys are often told that in order to grow up they need to “be a man” for their family, which often includes hiding emotion and dealing with confrontation in a physical manner. On the other hand, girls are often told to “be seen and not heard” from their early childhood, preventing the expression of assertiveness in females. This phenomenon of inheriting societal ideals from previous generations is called cultural transmission, and is present in all human societies. Cultural transmission allows stereotypes about sexes and gender roles to be directly passed from parent to child, changing how the brain thinks (Hiller and Baudin, 2016). Furthermore, the ideas we receive from our parents are deep-rooted in our psychology, even as adults. When studying college students, researchers determined that the difference in success and lifestyle choices between male and female students was not affected by the college experience, but in fact influenced by factors from the students’ childhoods (Sax, 2005). Although we can choose to consciously reject our parents’ values, Sax demonstrates that cultural transmission forms the framework of our internal biases and can be difficult to unlearn.

The continued evolution of the human brain has created multiple differences in our brain anatomy. Using a three-dimensional convolutional neural network, researchers were able to find distinguishable variation in major brain structures between men and women. Among these observations, the genu of the corpus callosum, which connects our two cerebral hemispheres, was found to be larger in females than males, indicating that the male brain localizes activity more than the female brain (Xin et al., 2019). This conclusion about the varying methods of cognition between sexes is further substantiated by a Yale study where participants underwent FMRI scans while studying lines and letters. In all the female participants, significant neural activity was detected in both hemispheres of the frontal lobe in areas that are dedicated to language recognition. However in males, all neural activity was confined to the right frontal lobe, providing evidence that the difference in corpus callosum size across sexes has a tangible outcome on the way we think (Shaywitz et al., 1995). Another difference in brain anatomy that we see between sexes occurs within the limbic system, our brain’s emotional center. Evidence suggests that women often have a larger hippocampus, while men have a larger amygdala and middle cerebellar peduncle (Xin et al., 2019).
Beyond studying how the differences in brain anatomy affect brain function, scientists have also established a link between the structure of our brain and the differing ways males and females act. When neurons that respond to progesterone are destroyed in the hypothalamus, females exhibit lower receptivity to sexual advances, and in contrast, when the same neurons are destroyed in males, it lowers their aggression instead (Yang et al., 2013). Progesterone is one of the major sex hormones responsible for mating behavior in humans, and while it is widely known that it is influential on human behavior, we now know that progesterone has fundamentally different effects in males versus females, explaining why male and female sexual behavior is often vastly distinct. In a different area of the brain, research on the workings of the rat hippocampus has demonstrated that beyond differing in size, hippocampi have “biochemical differences in ectonucleotidase activities” between sexes (Kruger et al., 2004). Essentially, male rats released less adenosine diphosphate from their hippocampus when exposed to electric shocks than female rats did. As the hippocampus is responsible for the formation of memories, the results of the study suggest that males and females process painful memories differently. An unrelated study conducted on humans provides observations that support this hypothesis. When males and females were exposed to a psychological stressor, and then placed into a fear conditioning program, males were more likely than females to exhibit a conditioned response (a change in behavior evoked by a stimulus associated with stress) (Jackson et al., 2005). Using skin conductance, males were measured to be consistently more anxious than females during the program. From these studies, it appears possible that the different way painful memories are created in the male hippocampus causes males to remember their fear more vividly.

Considering that males and females are distinctly psychologically different, and that these differences do in fact tangibly impact the lives of people, we must take action to ensure that an individual’s sex does not impact their quality of life. Parents, teachers, and other community figures are responsible for allowing children of any sex and gender identification to be comfortable expressing themselves in their own ways, and not confine them to gender stereotypes. For example, our children’s education has a profound effect on the individuals they become. Two professors of education, Duke and McCarthy, found that in the status quo, “elementary schools often reinforce the homophobia, heterosexism, and sexism that characterize contemporary U.S. society” (Duke and McCarthy, 2009). To address this, teachers need to make small teaching adjustments, such as encouraging their female students to pursue learning in fields that have been dominated by men, so that more women enter those fields. This way, we can break the chain of cultural transmission and start tearing down the walls of gender norms. The benefits of taking action for equality between sexes are massive. In a study focused on the European Union, scientists discovered that enacting gender equality policies would improve overall GDP per capita by 3.6% more than current projections over 30 years. These policies would also create 10.5 million additional jobs (Maceira, 2017). However, this is not to say that the myriad of psychological variation between males and females should be minimized or ignored. Surprisingly, in a recent study, improving equality between sexes has been shown to increase sexual dimorphism on a personality index. Scientists theorize that a population which accepts different traits allows every individual to express themselves in their own ways. Computational analysis revealed that “there was a strong correlation (r = .69) between a country's sex differences in personality and their Gender Equality Index” (Mac Giolla and Kajonius, 2018). Perhaps, instead of forcing people into either traditional male or female roles, gender-equal societies create sexual dimorphism by allowing the organic expression of one’s own gender. Additionally, this focus on understanding the differences in psychology between sexes has practical uses. Mental illnesses such as depression can be better treated with personally tailored treatment plans based on the sex of the patient (Garnefski et al., 2004). No matter the approach, we have all the tools at our disposal to understand psychological dimorphism and combat sexism.

Using empirical research from evolutionary biology, neuroscience, and sociology, I have demonstrated that males and females are psychologically different in ways that distinctly impact their lives. Among these differences, males and females have differently sized brain systems, process fear differently, and express aggression in different ways. We can trace these differences to the way the homosapiens species evolved, and our institutionalized cultures. Understanding our true sex differences and discrediting outdated gender roles will allow us to address the issue of modern day sexism. From workplace discrimination to medical diagnoses, it is paramount that we realize how much psychological dimorphism impacts each of us, and we must work together to continue our societal evolution in a way that equally benefits males and females.

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