Lead researcher Dr Richard Watts, an adjunct professor at the University of Canterbury’s Faculty of Health, says that challenges the accepted view there is little to distinguish between them, once individual variation in size is accounted for.
The structural contrasts were most apparent in regions of the brain strongly associated with mental health disorders, such as the thalamus, hippocampus and amygdala, which are involved in emotions, memory, learning and attention.
This suggests there may be a biological driver behind a marked gender gap in mental health.
Women are nearly twice as likely to develop anxiety, depression and other mood disorders, while autism, ADHD and antisocial behaviour are far more common in men. Women also tend to experience more side effects from drugs such as antidepressants and antipsychotics.
The research, which was conducted in association with the University of California San Diego, has attracted international interest, including a story in Newsweek magazine and an interview with Watts on Finland’s public radio station.
“People would say it’s all socialisation and the environment that’s driving things,” he says.
“We were really surprised to find huge differences in the structures of grey matter deep within the brain, so you can look at an individual scan and, with reasonable accuracy, say, ‘This has come from a man’ or ‘This has come from a woman.’ Before, you just couldn’t separate them.”
The study, which has been published in the prestigious Proceedings of the National Academy of Sciences (PNAS) journal, also found an association between the MRI measurements and self-reported psychiatric symptoms, although the numbers were small.
Further investigation will be required to confirm whether a biological mechanism is indeed at play, and both the cost and accessibility of MRI scans currently make their routine use prohibitive.
However, Watts says being able to identify pre-existing risk factors via brain imaging would be a breakthrough in the proactive treatment of mental health problems, potentially enabling early diagnosis, sex-specific interventions and a more personalised approach.
“Instead of just looking at your symptoms, we could look more closely and say, ‘Well, maybe this is why you have these symptoms and for you as an individual, this is the treatment that’s most likely to work.‘”
Registered child and adolescent clinical psychologist Sarah Watson, founder of the Totally Psyched private practice clinics, says the study aligns with her own professional experience.
“Clinically speaking, there’s always been a difference between genders. You’d have to be blind not to notice that,” she says. “So it doesn’t really surprise me, to be honest. I’m surprised it’s taken this long to figure out.
“But we’re not just one gender, as is signified in our transitioned youths, and we’re not just one culture. We’re made up of so many millions and millions of differences. Everything is a sliding scale. When we look at these things, I think we need to understand that.”
While the statistical divide in susceptibility to different mental health disorders has long been recognised, Watson says it can be difficult to differentiate whether those differences are based on gender or “gendered acculturation” – the socialising of females versus males.
“Especially around mental health and what socially we’re allowed to experience, men often feel they need to be more emotionally suppressed and females often feel their emotions are invalidated because they’re considered dramatic.
“So having this level of data based on physiological science is potentially very helpful to clarify a little more on the nature-nurture side.”
Neurology or neurosexism?
Research comparing male and female brains has been somewhat controversial, with feminist critics accusing scientists of “neurosexism” – an alleged bias in how cognitive data is presented, reinforcing harmful gender stereotypes that portray women as inferior or unsuitable for certain roles.
A 2015 study that was also published in the PNAS journal, but used less-advanced imaging technology, found minimal differences between male and female brains, describing a “unique mosaic” of features.
Daphna Joel, a psychology professor who led the research at Tel Aviv University, found the influence of biological sex on the brain was variable and could be highly masculine on one feature but highly feminine on another.
“We have to treat each person according to what he or she is and not according to the form of their genitals,” she said, at the time.
While much previous research has focused on fixed sex differences in the brain, the deep cellular structures involved in Watts’ study are not “hard-wired” but grey matter that can be modified.
Instead of the current trial and error process often involved in the treatment of mental health disorders, high-tech MRI scans could potentially be used to measure whether specific interventions – from medication to behavioural changes – are having a positive effect.
“You might have two patients who pretty much have the same symptoms, but the underlying cause could be different,” Watts says. “If there are multiple pathways to a disease and something biological is going on, maybe this is why some treatments work for some people and some don’t.”
The scan data used in the research was acquired from the Human Connectome Project, which is funded by the United States National Institutes of Health.
Participants were asked to self-report their gender as male or female. Watts says future work will try to uncover the extent to which brain differences relate to biological sex or gender, compared to other factors such as hormones, genetics and environmental influences, “such as how we are socialised to think and behave differently as a man or a woman in societies that have very different expectations of each gender”.
Watts is also involved in the Adolescent Brain and Cognitive Development (ABCD) Study, which is following more than 10,000 American adolescents from the age of 9 or 10, conducting MRI scans every two years.
A previous paper Watts worked on used those scans to look at obesity in children, finding strong correlations between body mass index (BMI) and structures in the brain that are known to be involved in the reward circuitry.
As a result of this latest study, he’s interested in tracking sex differences in the brain as males and females go through puberty and into their teens and early 20s, when there’s an increased risk of developing mental health issues such as eating and alcohol disorders.
Until the past decade or so, typically only male animals were used in drug testing, to avoid greater hormone fluctuations in female animals confounding the results. However, when some therapies that had proved reasonably effective in animals were tested in human clinical trials, women didn’t respond as well or had completely different side effects.
Watts says physical variations between the brains of male and female animals have been well-recorded. “If you look down a microscope, you do see differences, and these differences have fairly large effect sizes, as well.”
Manipulating sex hormones in animals has been shown to influence behaviour, but other aspects are harder to explain. In one study, rhesus monkeys were given plush toys and toys with wheels: the males preferred the toys with wheels while the females played with both equally.
“That’s not because their mothers and fathers have been buying them plush toys or toys with wheels, so it’s difficult to attribute it to some environmental difference,” Watts says.
“I’m not saying the social effect [on humans or animals] is zero, but there must be some things that are biologically based.”
Watson shares his hope that objective data obtained from brain scans could be used in the future to aid more accurate diagnoses. Females are often better at masking symptoms of ADHD and autism, for example, which could be distorting the statistics.
“We also see people who have led incredibly complex lives with a number of different challenges, and then it can be really challenging for a professional to be able to get inside their experience and truly understand what is what. Is this autism or is this trauma?” she says.
Being able to identify a biological trigger would also help counter the prejudice and stigma that surrounds mental health problems, which are often simply dismissed as bad behaviour or blamed on poor parenting.
“We have people who are on the autism spectrum or suffer from anxiety and clearly have a really over-responsive amygdala. Their fight-or-flight response is on a hairpin trigger,” says Watson.
“How wonderful and validating would it be if there was a scan that says, ‘Yep, you’ve got a really overreactive amygdala response’ or something like that. You’re not being overly dramatic – all these things we say to put people down. Yes, you need to do something about it, but actually, it’s just the way your brain’s made.”
Joanna Wane is an award-winning senior feature writer in the New Zealand Herald’s Lifestyle Premium team, with a special interest in social issues and the arts.
Leave a Reply