Everyone knows the difference between male and female brains. One is talkative and a little nervous, but never forgets and takes good care of others. The other is quieter, although more impulsive, but can ignore gossip to get the job done.

These are stereotypes, of course, but they have a surprising influence on the way brain science is conceived and interpreted. Since the dawn of MRI, neuroscientists have worked tirelessly to find differences between the brains of men and women. This research is attracting a lot of attention because it is so easy to try to relate a particular brain finding to a difference in behavior between the sexes.

But as a long-time neuroscientist in the field, I recently completed a careful analysis of 30 years of research into gender differences in the human brain. And what I have found, with the help of excellent contributors, is that hardly any of these claims have been proven to be reliable.

With the exception of the simple difference in size, there are no significant differences between the brain structure or activity of males and females that are maintained in various populations. Neither does any of the so-called brain differences explain the familiar 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 human brains are “sexually dimorphic”. It is a very scientific term that biologists use to describe a structure that occurs in two distinct forms in males and females, such as deer antlers or the genitals of males and females.

As regards the brain, some animals indeed exhibit sexual dimorphism, such as certain birds whose brain contains a song control nucleus which is six times larger in males and is responsible for the naked display song reserved for males. But as we demonstrate in our exhaustive investigation, nothing in the human brain comes remotely close to this.

Yes, men’s overall brain size is about 11% larger than that of women, but unlike some songbirds, no specific brain area is disproportionately larger in men or women. The size of the brain is proportional to the size of the body, and the brain difference between the sexes is actually smaller than other internal organs, such as the heart, lungs and kidneys, which range from 17% to 25% more great in men.

When overall size is properly controlled, no individual brain region varies by more than about 1% between males and females, and even these tiny differences do not consistently occur across geographically or ethnically diverse populations.

Other much touted brain sex differences are also a product of size, not gender. These include the ratio of gray matter to white matter and the ratio of connections between, relative to the inside, the two hemispheres of the brain. Both of these ratios are higher in people with smaller brains, whether they are male or female.

What’s more, recent research has totally dismissed the idea that the tiny difference in connectivity between the left and right hemispheres actually explains any difference in behavior between men and women.

A zombie concept

Yet “sexual dimorphism” will not die. It’s a zombie concept, with the latest revival using artificial intelligence to predict whether a given brain scan is from a man or a woman.

Computers can do this with 80% to 90% accuracy, except, again, that accuracy drops to 60% (or not much better than a toss) when you have proper head size control. Most annoying is that these algorithms do not translate between populations, such as Europeans and Chinese. Such inconsistency shows that there are no universal characteristics 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 “real” or species-wide sex differences in the brain. But the truth is, as studies have developed, the sexual effects have diminished.

This collapse is a telltale sign of an issue known as publication bias. Small, early studies that found a significant difference between the sexes were more likely to be published than research that found no brain differences between men and women.

Software vs. hardware

We have to do something right, because our challenge to the dogma of brain sex has been repelled by both ends of the academic spectrum. Some have called us science “deniers” and make fun of us for political correctness. At the other extreme, we’re shunned by women’s health advocates, who believe that research has overlooked women’s brains – and that neuroscientists should step up our research into gender differences to better treat female-dominated disorders, such as depression and Alzheimer’s disease.

But there’s no denying the decades of real data, which shows that the gender differences in the brain are miniscule and overwhelmed by the much greater variance in brain measurements of individuals in the population. And the same goes for most behavioral measures.

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About ten years ago, teachers were urged to separate boys and girls for math and English lessons based on alleged gender differences in learning. Fortunately, many refused, arguing that the range of abilities is always much greater among boys or girls than between each gender as a group.

In other words, gender is a very imprecise indicator of the type of brain a person will have. Another way to think about it is that each individual brain is a mosaic of circuitry that controls the many dimensions of masculinity and femininity, such as emotional expressiveness, interpersonal style, verbal and analytical reasoning, sexuality and gender identity itself.

Or, to use a computer analogy, gendered behavior arises from running different software on the same basic hardware.

The brain’s lack of binary sex characteristics also resonates with the growing number of people who identify as non-binary, queer, non-conforming, or transgender. Whatever influence biological sex has directly on the circuits of the human brain, it is clearly not sufficient to explain the multidimensional behaviors that we group together under the complex phenomenon of gender.

Rather than “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.