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Everyone knows the difference between male and female brains. You are talkative and a little nervous, but you never forget and take good care of others. The other is calmer, if more impulsive, but can turn off gossip to get the job done.
These are stereotypes, of course, but they have a surprising impact on the way that actual brain research is designed and interpreted. Since the beginning of MRI, neuroscientists have worked tirelessly to find differences between the brains of men and women. This research attracts a lot of attention because it is so easy to relate a particular brain finding to a gender difference in behavior.
As a longtime neuroscientist, I recently carried out a careful analysis of 30 years of research into gender differences in the human brain. And I have found, with the help of excellent staff, that virtually none of these claims have proven reliable.
Aside from the simple difference in size, there are no significant differences between the brain structure or activity of men and women that persist in different populations. None of the alleged differences in the brain actually explain the known, but modest, differences in personality and ability between men and women.
More the same than not
My colleagues and I titled our study “Dump the Dimorphism” to debunk the idea that the human brain is “sexually dimorphic”. This is a very scientific term that biologists use to describe a structure that occurs in two different forms in men and women, such as the antlers of deer or the genitals of men and women.
When it comes to the brain, some animals actually exhibit sexual dimorphism, such as: B. Certain birds whose brains contain a song control nucleus that is six times larger in males and is responsible for singing only in males. But as we show in our comprehensive survey, nothing in the human brain comes remotely close to that.
Yes, the total brain size of men is approximately 11% larger than that of women, but unlike some songbirds, no specific areas of the brain are disproportionately large in men or women. Brain size is proportional to body size, and the sex difference in the brain is actually smaller than that of other internal organs such as the heart, lungs, and kidneys, which are between 17% and 25% larger in men.
When overall size is properly controlled, no single brain region varies between men and women by more than about 1%, and even these tiny differences are inconsistent across geographically or ethnically diverse populations.
Other heavily touted gender differences in the brain are also a product of size, not gender. These include the gray matter to white matter ratio and the ratio of connections between and within the two halves of the brain. Both ratios are greater in people with a smaller brain, regardless of whether they are male or female.
In addition, recent research has completely dismissed the idea that the tiny difference in connectivity between the left and right hemispheres actually explains a behavioral difference between men and women.
A zombie concept
Still, “sexual dimorphism” will not die. It’s a zombie concept with the latest revival using artificial intelligence to predict whether a given brain scan was from a man or a woman.
Computers can do this with an accuracy of 80% to 90%, except that if you control the head size correctly, that accuracy drops again to 60% (or not much better than a coin toss). What is more problematic is that these algorithms cannot be translated into different population groups, e.g. B. in Europe or China. Such an inconsistency shows that there are no universal features that distinguish male and female brains in humans – unlike these deer antlers.
Neuroscientists have long hoped that larger studies and better methods would finally uncover the “real” or species-wide sex differences in the brain. But the truth is that the sexual effects have gotten smaller as the studies have gotten bigger.
This breakdown is a tell-tale sign of an issue known as publication bias. Small, early studies that found a significant gender difference were more likely to be published than research that found no difference between men and women.
Software versus hardware
We have to do something right because our challenge to brain sex dogma has been pushed back on both sides of the academic spectrum. Some have called us “deniers” of science and ridiculed us for political correctness. On the flip side, we’re being fired by women’s health advocates who believe that research has missed women’s brains – and that neuroscientists should step up our search for gender differences in order to better treat women-dominated conditions like depression and Alzheimer’s.
But the decades of actual data showing that the sex differences in the brain are tiny and inundated by the much greater variance in individual brain dimensions in the population are undeniable. The same applies to most behavioral measures.
About a decade ago, teachers were asked to separate boys and girls for math and English classes based on alleged gender learning differences. Fortunately, many refused, arguing that the range of skills in boys or girls is always much wider than that between the sexes as a group.
In other words, sex is a very inaccurate indicator of what type of brain a person will have. Another way of thinking about it is that each individual brain is a mosaic of circuits that control the many dimensions of masculinity and femininity, such as emotional expressiveness, interpersonal style, verbal and analytical thinking, sexuality, and gender identity itself.
To use a computer analogy, gender behavior relies on different software running on the same basic hardware.
The lack of binary brain gender traits also resonates with the increasing number of people who identify as non-binary, queer, non-conforming, or transgender. The direct influence of biological sex on the circuitry of the human brain is clearly insufficient to explain the multidimensional behavior that we group under the complex phenomenon of sex.
Rather than being “dimorphic”, the human brain is a sexually monomorphic organ – much more like the heart, kidneys, and lungs. As you may have noticed, these can be transplanted between women and men with great success.
Massive studies show few differences between men’s and women’s brains provided by The Conversation
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