All men and women created equal: Differences in brain size between sexes are ‘tiny and inconsistent’

NORTH CHICAGO, Ill. — Maybe men aren’t from Mars and women are from Venus after all. A recent study reveals that the only difference between the brains of the two sexes is size, with women’s brains being 11% smaller. However, this is about the same as physical size difference from men. Although, different mental illnesses and conditions are more common in men than women – and vice versa – researchers say the explanation isn’t a result of fundamentally different brains.

While previous research suggests differences in volume and thickness of different brain regions, researchers say they have found no consistency. Therefore, they say, there is no proof any difference exists. These findings are the result of a three decade study, including hundreds of the most cited scientific papers.

The research team, from Rosalind Franklin University, looked at 13 distinct measures of alleged sex difference. However, for nearly every one, they found almost no differences that were consistent across studies, even those involving thousands of participants. Male-female brain differences are also inconsistent across different races – meaning there is no universal marker that distinguishes men and women’s brains across the human species.

“Men and women’s brains do differ slightly, but the key finding is that these distinctions are due to brain size, not sex or gender. Sex differences in the brain are tiny and inconsistent, once individuals’ head size is accounted for,” explains Dr. Lise Eliot, professor of neuroscience and executive chair of foundational sciences and humanities at the university, in a statement.

“Since the dawn of MRI, studies finding statistically significant sex differences have received increased attention by scientists and the media. Researchers have been quietly accumulating massive amounts of data comparing male and female brains, but it’s only the differences that get hyped. Unlike other areas of health research, women have been equally included in brain imaging from the outset,”adds Dr. Eliot. “The handful of features that do differ the most are quite small in magnitude. For example, the volume of the amygdala, an olive-sized part of the temporal lobe important for social-emotional behaviors, is a mere 1% larger in men across studies.”

She also says her study proves wrong a longstanding view that men’s brains are more lateralized – meaning each hemisphere acts independently. Women’s two hemispheres, on the other hand, are said to be better connected and to operate more in sync with each other. This difference could make males more vulnerable to disability following brain injury such as stroke

However, those studies show that the difference is extremely small, accounting for less than 1% of the range of left-right connectivity across the population. Also, large datasets have found no gender difference in aphasia – the loss of language, following a stroke in the left hemisphere.

The last focus of the study was on functional MRI. This method has been widely used to observe male-female differences during language, spatial and emotional tasks. It allows neuroscientists to see areas of the brain that “light up” during particular mental tasks.

Team members say they have found extremely poor reliability in sex difference findings. Nearly all specific brain areas that differed in activity between men and women were not repeated across studies. Dr. Eliot adds that studies which show gender differences are common as researchers try to find evidence in their data in the hopes of getting a paper published on the “hype” around the topic.

”Sex comparisons are super easy for researchers to conduct after an experiment is already done. If they find something, it gets another publication, and, if not, it gets ignored. Publication bias is common in sex-difference research because the topic garners high interest,” said Dr. Eliot.

“Sex differences are sexy, but this false impression that there is such a thing as a ‘male brain’ and a ‘female brain’ has had wide impact on how we treat boys and girls, men and women. The truth is that there are no universal, species-wide brain features that differ between the sexes. Rather, the brain is like other organs, such as the heart and kidneys, which are similar enough to be transplanted between women and men quite successfully,” added Dr. Eliot.

The paper is published in the journal Neuroscience and Biobehavioral Reviews.

SWNS writer William Janes contributed to this report.