Friday February 14 2014
Brain size doesn’t necessarily equate to intelligence
“Men really do have bigger brains,” reports the Daily Mail, going on to report that new research reveals “male and female brains are wired differently” with particularly big differences in the areas that control language and emotion.
It is well established that males and females have different predispositions towards developing different mental health conditions. For example, conditions such as autism spectrum disorder and dyslexia are more common in men, while depression and anxiety are more common in women. New research has pooled the results of 126 studies examining the differences in brain size between men and women to see if structural differences are part of the explanation.
It found that on average men had larger overall brain volumes than women. They also found differences between men and women in the volume of many different regions. These included regions previously associated with different mental health conditions. For example, men tended to have larger volumes in brain regions associated with survival instincts, memory and learning, while women tended to have larger volumes in areas of the brain dealing with language and emotions.
You could make a case that this differing mix of abilities means that it is mutually advantageous for the sexes to co-operate; a nice thought for Valentine’s Day.
However the media’s preoccupation with brain size is probably something of a distraction. The link between brain function and brain structure or size is still not clearly understood; so we can’t reliably conclude from this study how the differences in brain size influence physiology or behaviour.
Gender is influenced by both biological and social factors and it’s not yet clear how these interact to influence behaviour, personality or disease risk.
Where did the story come from?
The study was carried out by researchers from University of Cambridge and University of Oxford and was funded by the Wellcome Trust and Medical Research Council Behavioural and Clinical Neuroscience Institute.
The study was published in the peer-reviewed scientific journal Neuroscience and Biobehavioral Reviews and has been published on an open access basis so it is free to read online or download.
The UK media’s reporting is arguably over-speculative. The research was looking at structural differences only – it didn’t explore how these differences impacted disease, behaviour or intelligence, although it put forward plausible theories. And the Daily Star’s claim that “It has been revealed that male and female brains are completely different” is simply incorrect.
It is also probably simplistic to assume that there is a direct link between brain size and intelligence. It is thought that it is the complexity of the connections between individual brain cells that underpin cognitive ability and not the total amount of brain tissue.
For example, elephants have huge brains, weighing around five kilos. And while elephants are certainly bright creatures, renowned for their memory, it would be a bit of stretch to describe them as geniuses.
What kind of research was this?
This was a systematic review that searched the global literature aiming to identify published studies that used imaging (such as MRI scans) to examine the brain structure of men and women. The researchers then aimed to combine the findings and summarise any gender differences found.
The rates of many different mental health and neurological conditions are known to differ between men and women, as are their symptoms and age of onset. For example “male-biased” conditions include attention deficit hyperactivity disorder and autism, while “female-biased” conditions include depression and anxiety.
As the researchers say, understanding the different effects gender has on brain development may increase understanding of how and why male and female brains differ in their predisposition for, or resilience against, certain mental conditions.
Though several previous studies have examined gender differences in brain structures, the researchers say that theirs is one of the first to have compared the results of these various studies in meta-analysis. They first aimed to look at overall brain volume (size), then to look at differences in specific regions of the brain.
What did the research involve?
The researchers looked for studies published between 1990 and 2013. They included those that provided information on the overall brain volumes for male and females, and the volumes of specific regions of the brain, for example:
- grey matter (nerve cell bodies)
- white matter (nerve fibres)
- cerebrum (the main two large halves of the brain)
- cerebellum (area at the base of the brain controlling balance and movement)
- cerebrospinal fluid surrounding the brain and spinal cord
The researchers used statistical methods to combine the results of the studies, taking into account differences between the studies and their results, and potential biases (for example the possibility that only studies with positive results will be published).
The researchers identified 126 studies which provided data on the volume of the brain and how it differed by gender. Fifteen of these studies provided results for total brain volume that could be combined in meta-analysis, and nine studies provided information on brain tissue density that could be combined.
What were the basic results?
Males were found to have a larger overall brain volume than women. Studies found that brain volume was between 8% and 13% larger in a man than a woman.
When grouping the studies by age category they noticed that the size of the difference in brain volume between the genders varied according to the stage of life. However, this analysis was limited by the fact that the majority of studies had looked at “mature brains”. That is people in the 18 to 59 age category. There were few studies examining other age categories such as infancy or early childhood.
There were differences between males and females in the volume of various specific regions of the brain. These included regions that previous research has implicated to be involved in sex-biased mental health conditions. For example, the volume of the amygdalae (believed to be involved with survival-based emotions such as fear, anger and pleasure) and hippocampi (involved with memory and learning) were large in males. Meanwhile the volume of the insular cortex (involved with emotions, perceptions and self-awareness) was larger in females.
How did the researchers interpret the results?
The researchers conclude that their results “suggest candidate brain regions for investigating the asymmetric effect that sex has on the developing brain, and for understanding sex-biased neurological and psychiatric conditions”.
This study benefits from reportedly being one of the first to systematically search the global literature to identify published studies that have examined the differences in brain structure between men and women, and then combined these results in a meta-analysis.
It finds evidence that men have a slightly larger overall brain volume than women, with a brain size around 8% to 13% greater. This may be a result of their larger general size. They also find differences between men and women in the volume of many different regions. These include differences in regions that have previously been associated with different mental health conditions, tentatively suggesting an unproven link between brain structure and gender related differences in risk of disease.
Among the differences, men tended to have larger volumes in brain regions understood to be associated with survival instincts, memory and learning, while women tended to have larger volumes in areas of the brain dealing with emotions. This reinforces some commonly held gender stereotypes about the historical roles of men and women.
However it would be simplistic to think other factors, such as social pressures and environment, do not also play a role in how each gender may tend to think and behave.
The review also suggested there may be differences during different stages of our lives, for example as the brain develops during infancy and childhood. However, this is not possible to say for certain, given that very few studies have been conducted outside of the adult age bracket.
Overall this review contributes to the body of studies examining how the different brain structure of men and women may contribute to their propensity to different neurological and mental health conditions.
Findings may suggest that both genders work best when they co-operate on a common goal, rather than engaging in a “battle of the sexes”.
Analysis by Bazian. Edited by NHS Choices.
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