The differences in men and women even extend to the way our brains are built.
In the largest study yet on sex differences in the physical makeup of the human brain, researchers from the University of Edinburgh in Scotland have shown that men and women do, in fact, have different brain structures and sizes. Women tend to have thicker cortices, which are associated with intelligence, whereas men’s brains tend to be bigger overall. Although these differences can’t prove that men and women behave differently, they could shed light on why some medications work better in men better than women, and vice versa.
Researchers looked at the brains of over 5,200 participants older than 40, roughly half men and half women. This group was part of the larger UK Biobank study, which is in the midst of collecting health data on over 500,000 individuals. For this particular study, patients lay down in a structural magnetic resonance imaging. These MRIs are able to parse out different types of brain tissues, like the neurons and the connections between them, which can give scientists a picture of the various brain regions.
They found that on average, men’s brains were larger. But women’s brains had larger subregions of the cortex—the cortical subregions are discrete parts of this particular brain section associated with memory, sensory input, learning, and making choices. Additionally, there was a lot of variation in the sizes of different brain regions in men; women’s brains tended to be more similar to each other. The research, which hasn’t yet been peer-reviewed, was published in BioArXiv earlier this month.
These findings aren’t brand new. But most neuroscience studies to date only looked at a sample size of a few hundred participants. The thousands of brains here validate a lot of previous work. The fact that men’s brains had more differences among them “fits with a lot of other evidence that seems to point toward males being more variable physically and mentally,” Stuart Ritchie, a psychologist and lead author of the paper, told Science. Similarly, it wasn’t surprising to find that women tended to have thicker cortices over all based on previous findings (paywall).
The differences between men and women’s brains were small enough that it’d be impossible for scientists to determine a person’s sex by looking at his or her brain alone. Brain size and composition are characteristics kind of like nose shape: they depend on a lot of different genetic factors, and can take on countless different forms. And although a lot of men have larger noses (and brains) than women, that’s not always the case.
And it’s important to consider that different brain sizes and regions don’t necessarily translate to actual behavioral differences, like intelligence. “Our manuscript is just about describing the differences, and we can’t say anything about the causes of those differences,” Ritchie told New York Magazine. Different environmental and social factors play a huge role in determining the ways we think and interact with each other.
Ritchie is confident, though, that understanding the structural variability can help determine why certain diseases affect men and women differently. Understanding variations in brain structure can help develop better, sex-specific treatments for them.