Getting in a Tangle Over Men’s and Women’s Brain Wiring

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  Filed under: Neuroscience
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Image: Brewbooks / Flickr


You may have heard there’s a new study that claims to have found that men’s and women’s brain’s are wired differently. Published in the respected journal PNAS, the researchers based at the University of Pennsylvania used a technique known as diffusion tensor imaging to plot the brain wiring maps of 949 people aged 8 to 22. Ragini Verma and her colleagues said their results showed “fundamentally different connectivity patterns in males and females.”
Specifically, they reported that men’s brains had more connectivity within each brain hemisphere, whereas women’s brains had more connectivity across the two hemispheres. Moreover, they stated or implied, in their paper and in statements to the press, that these findings help explain behavioral differences between the sexes, such as that women are intuitive thinkers and good at multi-tasking whereas men are good at sports and map-reading.
Not surprisingly the world’s media lapped this up. “The connections that mean girls are made for multi-tasking,” said the Daily Mail. “hardwired difference between male and female brains could explain why men are ‘better at map reading’,” announced The Independent. Your own newspaper probably said something similar.

Make no mistake, the technical wizardry involved in creating a brain wiring diagram – researchers call it a “connectome” – is awesome. I’m sure Leonardo Da Vinci, who used hot wax to create a cast of the brain’s ventricles (the fluid-filled hollows), would have been mightily impressed. But unfortunately, this wiring study and the subsequent press coverage has got a lot of things in a tangle. 

First of all, the differences in brain wiring between the sexes were not as noteworthy as the researchers imply. They say they are “fundamental,” but other experts have crunched the numbers and they state that although the differences are statistically significant, they are actually not substantive. And remember, these are average differences with a lot of overlap. It’s possible that my male brain is wired more like an average female brain than yours, even if you’re a woman.

A second key thing to bear in mind is that the new paper did not in fact look at behavioural differences between the sexes – things like intuitive thinking and multi-tasking. The researchers are only guessing about how any wiring differences might be related to behavioral differences between the sexes. They have published past research that tested the same sample on various tasks, but as Cordelia Fine points out, the sex differences they found were “trivially small” and they didn’t look at the kind of activities being cited in the media, such as map-reading.

The way Verma and her colleagues have arrived at the idea that their results support gender stereotypes about map reading, and so on, is via a logical mistake known as “reverse inference”. They looked at where in the brain they found wiring differences and then they’ve made assumptions about the functional meaning of those differences based on what other studies have suggested those brain regions are for. Unfortunately, Verma and co did this in an unsophisticated way. They dredged up old ideas about the left brain hemisphere being for analytical thought and the right hemisphere being intuitive. And the one brain region where men supposedly had more cross-hemisphere interconnectivity than women – the cerebellum – the researchers linked purely with motor function, which they said supports the idea that men are wired for action. Maybe they don’t realise, but modern research has shown that the cerebellum is involved in lots of other functions too. Writing an authoritative review in 2009, Peter Strick and his colleagues explained: “The range of tasks associated with cerebellar activation is remarkable and includes tasks designed to 

  • assess attention, 
  • executive control, 
  • language, 
  • working memory, 
  • learning, 
  • pain, 
  • emotion, and 
  • addiction.
So, the wiring differences between the sexes aren’t that large. And we don’t really know their functional significance, if any. Another issue is that the media coverage has implied that the wiring differences between the sexes are set in stone (“hard-wired”) and that they play a causal role in behavioural differences. In fact, it’s possible that the wiring differences, in part or whole, are a consequence of the way that society tends to shape people according to their gender. We know that cultural and societal factors affect how men and women perform on behavioral tasks. Remind people of gender stereotypes and they tend to perform in a way that reinforces them. A relevant meta-analysis published this year also that found spatial ability is related to how much men and women identify with masculinity. One possible interpretation of this result is that the more you identify with being male, the more likely you are to take an interest in activities with masculine connotations, and the better you become at related skills, such as spatial ability (possibly via changes to your brain wiring).
Other research shows that if you take away the pressure of societal expectations, gender differences in cognitive skills are reduced or disappear. Indeed, given that women’s brains and men’s brains are different, on average, in terms of size and other factors, it’s remarkable just how similar the two sexes behave and perform on psychological tests. Seen this way, any wiring or other differences could be solutions that allow for physically dissimilar brains to achieve the same ends, an idea known as the “compensation hypothesis”.
Finally, let’s set this new brain wiring study in the context of previous research. Verma and her team admit that a previous paper looking at the brain wiring of 439 participants failed to find significant differences between the sexes. What about studies on the corpus callosum – the thick bundle of fibres that connects the two brain hemispheres? If women really have more cross-talk across the brain, this is one place where you’d definitely expect them to have more connectivity. And yet a 2012 diffusion tensor paper found “a stronger inter-hemispheric connectivity between the frontal lobes in males than females”. Hmm. Another paper from 2006 found little difference in thickness of the callosum according to sex. Finally a meta-analysis from 2009: “The alleged sex-related corpus callosum size difference is a myth,” it says.
OK, one last thing. I don’t know if you saw it, but earlier this year another study involving hundreds of participants used a different technique (resting state fMRI) to examine connectivity in the brain, this time for the purpose of seeing if some people have more left-brain functional hubs and others have more right-brained hubs (they don’t). This obviously isn’t the same focus as the new PNAS paper, but if men and women’s brains really are wired up differently to optimise them for map reading or multitasking etc, you’d think there’s be some important sex differences in the way functional hubs are lateralised (distributed to one side of the brain or the other). In fact, “no differences in gender were observed,” the authors said.
In conclusion – Wow, those are some pretty wiring diagrams! Oh … shame about the way they interpreted them.
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