For many years, the difference between male and female brains seemed rather obvious. One was more talkative but sometimes slightly anxious, yet always remembered names and knew how to care for people. Its counterpart is more relaxed, if not more calculating, though sometimes more impulsive, but knows the right moment to tune out the gossip when there’s a task to be completed.
This all sounds fairly dated, and even decades ago would be considered rather extreme stereotypes, but it’s an archetype that has also exerted considerable influence in the way that we process and interpret data when it comes to the human brain. From the beginning of MRI technology, neuroscientists have continuously worked to pinpoint any discernible differences between the brains of men and women. It’s possible to link just about any finding with the brain to differences in the way men and women behave, so the research has drawn the attention of multiple people in different fields — sometimes credible and some not so credible.
Dr. Lise Eliot, a professor of neuroscience from the University of Chicago, has been analyzing the field for a long time — looking at three decades of research on gender differences in the human brain, only to come to a conclusion that at once seems groundbreaking and yet not really all that surprising: There are no significant differences between men’s and women’s brain structure or activity that hold up across diverse populations. The purported brain differences also fail to explain the personality types and abilities that we so often tend to ascribe to either men or women.
Eliot’s study, titled “Dump the Dimorphism,” goes through an array of evidence to debunk the idea that human brains are “sexually dimorphic.” If you haven’t already heard the term before, it’s a description for structural differences in males and females of the same species — why deer bucks grow antlers, for example.
There are species who do show characteristics of sexual dimorphism when it comes to the brain. In the case of sparrows, a region of the brain interacting with the bird’s vocal cords is six times larger in males and is solely responsible for producing songs used for courtship. Even if human courtship rituals aren’t necessarily all that different from sparrows when it comes to music, the human brain doesn’t have any evolutionary equivalent to this.
While men’s brains are on average 11% larger than women’s, no given brain area is significantly larger in either men or women. The discrepancy in brain size is simply proportionate to body size. In fact, looking from this standpoint, the difference in brain size between the sexes is actually much less than when you look at size differences of other organs. Hearts, for example, can range to 17 to 25% greater in men than women.
Once you adjust properly for the overall size, then you find that no single brain region varies in size by more than 1% among men and women. Even that variance differs across populations when you factor in ethnicity or geographic diversity. It was once argued that differences in the gray matter to white matter ratio, or the degree of connectivity between the brain’s two hemispheres demonstrated a difference between the sexes, but these phenomena is found more often in people with smaller brains — both men and women. If anything, the latest research has thoroughly debunked the possibility that just a few connections between the left and right hemispheres demonstrate any behavioral differences between men and women, that these are more likely culturally ingrained.
A Concept That Has Outstayed Its Welcome
Even still, the idea of “sexual dimorphism” refuses to die out. It’s become something of a zombie concept, in its latest incarnation, with artificial intelligence experts developing models that boast the ability to predict whether a particular set of given brain scans were produced by a man or by a woman.
Computers are able to accomplish this with an 80% to 90% degree of accuracy. However, this level of accuracy drops to 60% (which is slightly better odds than correctly calling a coin flip) once you properly factor in differences for head size. Even more problematic is that you cannot use these algorithms to compute across populations, looking at populations in North America versus populations in Asia. These inconsistent variables indicate that there are no particular universal features that discern male and female brains in humans — that it’s an organ as interchangeable as a heart or kidneys that can be passed easily from a donor.
For some time, neuroscientists have been clinging to the hope that one day a bigger study and an improved set of methods would be the big one that could answer the question on the crucial or species-wide sex differences observed in the human brain. In fact, the opposite has happened. The larger the scale of the studies, the smaller distinctions in terms of sex. This body of literature suggests a problem that can occur with scientific research as a whole — a concern known as “publication bias.” In fact, the earliest literature on this subject with data that suggested sizable sex-linked brain differences was more likely to be printed in scientific journals — than research that could not find differences between male and female brains.
More From Brain World
- Basic Chemistry: Unraveling the Mysteries of Love, Sex, and Commitment
- How Our Minds, Neurosexism, and Society Create the “Male” and “Female” Brain
- Know Your Brain: Oxytocin — The “Love Hormone”
- Reaching For Gender Equality With Brain-Based Holistic Education
- What Neuroscience Tells Us About Transgender People
- Why We Experience Romantic Love: An Interview with Neuroscientist Lucy Brown
