Beyond the depths of current brain research lies a strongly debated topic that has resided over neuroscientists and brain research for decades. Surprisingly, it proves to be quite simple for society to disregard the imminent biological differences between males and females in this field.
As time passes, we continue to push this gap under the shadows without a second thought.
People have searched for sex differences in human brains since at least the 19th century. In the early 19th century, American physician Samuel George Morton collected skulls to compare brain size and “measure intelligence” among certain populations. Beyond the inaccuracy of his study, the results were biased, as they deemed intellectual superiority amongst Caucasian groups.
In the later 19th century, French scientist Gustave Le Bon found men’s brains to be larger than those of women, leading philosopher Alexander Bain and psychologist George Romanes to fabricate the argument that the size difference resulted in higher intelligence among males. Yet, philosopher John Stuart Mill pointed out in the 19th century that this seemingly simple criterion couldn’t be true as it would mean that elephants and whales should be smarter than humans.
Thus, researchers focused their efforts on the relative sizes of brain regions to provide explanations. Researchers suggested that the frontal lobe, the part of the cerebrum above the eyes, is the most crucial determinant of intelligence, and is proportionally larger in men. Meanwhile, the parietal lobe, located behind the frontal lobe, is proportionally larger in women. However, neuroanatomists soon argued that the parietal lobe is more important for intelligence and that the parietal lobes in men were, on average, larger.
Frontal Lobe: controls important cognitive skills in humans, such as emotional expression, problem solving, memory, language, judgment, communication; the “control panel” of our personality.
Parietal Lobe: processes sensory information it receives from the outside world, mainly relating to touch, taste, and temperature. Damage to the parietal lobe can lead to dysfunction of the senses.
In the 20th and 21st centuries, researchers began to look for distinct “female” or “male” characteristics in smaller brain subdivisions.
As established decades earlier, the female brain is smaller than the male brain by approximately 10%. Other than this difference, both brains are seemingly identical in structure, despite the continued influx of neuroanatomical data.
This might be the only difference that researchers agree on.
Today, scientists stand on uneven ground. Concepts such as neurosexism and scientists such as Lise Eliot, Neuroscience Professor at Chicago Medical School and author of Pink Brain, Blue Brain; and Gina Rippon, British neuroscientist and author of the book Gender and Our Brains, stand very strongly on their beliefs towards the subject.
For every study that claims that the male and female brains largely differ structurally, there is another claiming that both are “more alike than we might think.”
In her book, Gender and Our Brains: How New Neuroscience Explodes the Myths of the Male and Female Minds, Gina Rippon argues that there’s no such thing as the “male” or “female” brain.
As she sifts through centuries of research into supposed differences in areas including behavior, skills, and personalities, she shows how external factors including gender stereotypes and real-world experiences are quite affluent.
Furthermore, she demonstrates that the differences among women as a group or among men as a group are significantly larger than the differences between both men and women as genders.
In her book, Rippon cites a 2015 study looking at 1,400 brain scans comparing 160 brain structures — identifying areas that were, on average, larger in either males or females. The study concluded that there weren’t any scans proving certain traits to be either “male,” or “female,” whether that was weight or tissue thickness. As Rippon claims, “We’re trying to force a difference into data that doesn’t exist.”
Starting the Search
According to Rippon, this hunt started about 200 years ago as scientists began to observe the role of the brain in cognition and ability. Looking into society, researchers found women to be intellectually “inferior,” with significantly smaller educational achievements.
Due to this status quo, Rippon says that researchers sought to prove differences between both genders in this regard.
And that seems to be their goal from the beginning; to simply explain the status quo, rather than to find observable differences within the brain.
One of the early studies conducted in this field, published in February of 1995, was a linguistic study, attempting to demonstrate how men and women processed language differently. The study was based on a long-held hypothesis claiming that language functions are more likely to be highly lateralized in males and to be represented in both cerebral hemispheres in females.
The study looked at 38 right-handed subjects; 19 male and 19 female. Researchers used echo-planar fMRI imaging to observe the brains of the participants during orthographic (letter recognition), phonological (rhyme), and semantic (semantic category) tasks. Ultimately, the research found virtually no differences in data between sexes in two of the categories and a slight difference in one between a small group of participants.
Rippon claims that this is an example of a study that has been overscaled; many still believe that there is a very profound sex difference between men and women in language processing, proving that we might have actually been manipulating our data from the beginning….
An Unintentional Manipulation
Through brain research, we have access to immensely complicated data. But when carrying out studies, researchers say that we tend to highlight differences that are found — creating an inaccurate representation of what is actually going on within the brain.
Say you are carrying out a brain-imaging study in relation to memory. Let’s say you hook up each of your participants to a scanner of some sort and give them a simple task, such as reading words. Once you have collectively gathered data from each participant you would go about analyzing the data by trying to find some difference in brain activity between genders associated with the given task.
Even if the differences you find are insignificant in comparison to the other neuronal activity, they have a larger prevalence because it’s specifically what you were looking for.
Perhaps you choose to set a particular threshold — only observing data upon meeting a certain statistical significance. Even as you hone in on those differences and set your standard, you have an unproportional view towards the subsection of data you are studying, rather than analyzing it in the context of the complete set of neuronal activity.
In the end, some might say that you are simply looking at the product of a very complicated series of data manipulation and carefully evaluated statistical analysis.
However, it is important to note that researchers do not consider these findings fraudulent, rather, they don’t see their relevance as they believe that this research has simply been used to prove a preexisting hypothesis, based on cultural bias, or mirror a stereotype. In reality, the results may or may not be fully accurate.
Explaining Accepted Differences
There are certain mental tasks in information processing, such as spatial skills, map reading, or constructing and manipulating 3D objects, where men consistently outperform women.
Spatial skills display a large sex difference — and are claimed to be evidence of an innate and biologically programmed ability that works in favor of men.
But taking into account spatial experience adds a different perspective to the argument. Individuals who have played with certain toys or associated themselves with certain activities, such as high-activity video games, building cars, or using construction toys could be the true predictor of an individual’s spatial skills.
What looks like a sex difference might have actually risen from gendered stereotypes and opportunities that society may offer to individuals; or simply different biologically programmed interests.
Crossing The Bridge
In 2017, the January/February edition of the Journal of Neuroscience Research was the first issue of any neuroscience journal to be devoted entirely to the influence of sex differences on nervous system function.
Dr. Larry Cahoun, Neurobiology and Behavior Profesor at UC Irvine had a large role in this 70-article feature. He notes that based on research published in this edition and other research since then indicates that beyond the varying brain size between genders, a woman’s hippocampus, critical to learning and memorization, is larger than that of males and functions differently. On the contrary, the male amygdala, a structure associated with the experiencing of emotions and the recollection of such experiences, is bigger than the female amygdala. As Cahill’s research has shown, both structures differ not only structurally, but functionally as well.
In a previous study, Cahill studied the brains of men and women viewing either emotionally unsettling films or emotionally neutral ones. Those viewing the agitative films were expected to initiate strongly negative emotions and thus, imprinting, in the amygdala. As predicted, the activity in the amygdala during the viewing predicted the ability of the subjects to recall the viewed clips after the film. However, this pattern was observed only in the left amygdala of women, while in men, it was only in the right. Since then, several other researchers have come upon the same results.
Many researchers have also agreed that the two hemispheres of a woman’s brain communicate much more than in males. In a 2014 UPenn study, researchers imaged brains of 428 and 521 female youths (considered a large sample in relation to the study) and found that the female brains consistently showed stronger and more coordinated activity between hemispheres. Meanwhile, the brain activity in males was more tightly coordinated within local brain regions.
This finding correlates with the results of a smaller series of studies published earlier, which observed that the corpus callosum, the white-matter cable that crosses and connects the hemispheres, is bigger in women and that female brains tend to be more bilaterally symmetrical than males.
Beyond structural and functional differences, Halpern and other researchers have noted a multitude of observable and unobservable human behavioral differences, all of which have been replicated.
Among these researchers, it is widely understood that women excel in several measures of verbal ability, except for verbal analogies. Women’s reading comprehension and writing ability exceed that of men, and women outperform men in tests of fine-motor coordination and perceptual speed. Women are also more skilled at retrieving information from long-term memory.
On the other hand, men can, on average, more easily juggle items in their working memory. They have significantly higher visuospatial skills and can more accurately visualize 2D and 3D shapes being rotated in space, as well as more accurately determine angles from the horizontal, track moving objects, and aim at projectiles.
New technologies have generated growing evidence of inherent differences in neuronal wiring between genders.
Contrary to the argument of researchers who believe in neurosexism, these researchers have observed that many of these cognitive differences appear quite early in life. In her text, Halpern notes that sex differences in spatial-visualization ability in two-month-old and three-month-old infants are notable.
Infant girls often respond more readily to faces and begin talking earlier, while boys react earlier in their childhood to perceptual differences in their visual field. This continues into adulthood, as women remain more oriented towards faces and men to things.
However, these observations originate from a pool of independent research, measuring largely different traits at varying scales. While statistically significant, the differences aren’t immense, which is the cause of the debate in the first place. Due to this reason, researchers like Rippon believe that we can safely ignore them.
Yet Halpern and others insist that the long list of behavioral tendencies in which male and female ratios are unbalanced may extend to both cognitive and psychiatric conditions.
Women are twice as likely as men to experience clinical depression and PTSD in their lifetimes, while men are twice as likely to develop drug addictions and 40% more likely to develop schizophrenia. The list continues for conditions such as autism, dyslexia, Parkison’s Disease, and more.
Seeking A Deeper Perspective
Do more recent studies prove Rippon’s argument otherwise? In fact, are both sides even seeking to explain the same thing?
Until now, the neurosexism argument has only examined studies that evaluate brains at a structural level.
But the brain is immensely more complicated than that.
Even Rippon acknowledges the role of hormones in cognition and behavior.
In her book, Rippon taps into the long-held concept of female inferiority due to “biological vulnerability” associated with the reproductive cycle. The norm in the 19th century was to shield women from education, as it was believed to affect the reproductive system.
We know that hormones affect female behavior; this is a fact that has been proven time and time again in a range of large-scale studies.
Researchers acknowledge the lack of consideration towards hormones in proving the difference between male and female brains.
They say that theoretically, neither should differ due to the structural uniformity.
However, our biochemistry proves to be more powerful than one might expect.
As Diane Halpern, an esteemed psychologist wrote in the first edition of one of her most acclaimed academic texts, Sex Differences in Cognitive Abilities,
“…it seemed clear to me that any between-sex differences in thinking abilities were due to socialization practices, artifacts and mistakes in the research, and bias and prejudice. … After reviewing a pile of journal articles that stood several feet high and numerous books and book chapters that dwarfed the stack of journal articles … I changed my mind.”
Could It Be Our Biochemistry?
Throughout their lifetimes, males and females have different chemicals driving their engines — namely, sex hormones and hormone levels.
In females, the major contenders are estrogen and progesterone. In males, it is testosterone and a few collective androgens, which resemble testosterone.
More importantly, during the embryonic development of males, the growing fetus is hit with a mid-gestational surge of testosterone, critical in not only shaping body proportions but also the brain.
Many studies suggest that the variations in estradiol, a form of estrogen, between genders has a key influence on synaptic plasticity, neurotransmission, neurodegeneration, and cognition, which we wrongly accredit to our environments and neurostructural organization.
Usually, brain regions that differ in size between men and women (the amygdala and hippocampus), contain especially high concentrations of receptors for sex hormones.
Neurotransmitters, other hormones, and chemical pathways also play a critical role in our observations. Hormones like catecholamines secreted by the adrenals (norepinephrine, and epinephrine), neurotransmitters like dopamine and serotonin, enzymes like aromatase, and pathways including the cholinergic system, which releases acetylcholine during the propagation of nerve impulses, and the opioid system, also known as the reward system, are just a glimpse of the notable differences between genders.
Another key variable in the brains of males versus females is in the sex chromosomes, which form one of the 23 pairs of human chromosomes in each cell. Females generally have two X chromosomes in this pair, while males have one X and one Y chromosome.
A gene on the Y chromosome is responsible for the initiation of developmental events that result in the male body and brain taking on “male” characteristics. Other genes on the Y chromosome are suspected to be involved in brain physiology and cognition.
All scientists acknowledge that the presence or absence of a single DNA base pair can change a life.
If a single base pair can significantly alter the life of an individual, it’s hard to imagine what a completely different chromosome could do, beyond external differences. The 1,500 genes on the X chromosome and the 27 genes on the Y only have some matching counterparts to the other respective chromosome. Essentially, this means that every cell in a female or male body each has a slightly different set of functioning genes from the sex-chromosomes that operate on a routinely basis, differing from the opposing gender.
The inexplicable symphony of biological processes, deeper than our structural observations, must affect at least some of our circuitry.
Essentially, gender-specific behavior boils down to a composite of these factors. We cannot be sure of anything at this point. What we know about the brain is so minuscule in comparison to the vast sea of research we are yet to accumulate in the following decades. We cannot be so quick to make assumptions or plant our feet firmly on any theory or belief.
As our technology is developing, neural imaging studies and animal research are growing in scale and magnitude to reveal more than we ever knew about humanity’s differences.
Despite the research, it’s nearly impossible to attribute really any of our findings regarding gender differences in the brain to “culture,” “structure,” or “chemistry,” in our society of interactions and experiences that we cannot fully account for.
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