In a world of equal rights, pay gaps, and gender-specific toys, one question remains central to our understanding of the two biological sexes: are men's and women's brains wired differently? If so, how, and how is that relevant?
There are many studies that aim to explore the question of underlying differences between the brains of men and women. But the results seem to vary wildly, or the interpretations given to the main findings are in disagreement.
In existing studies, researchers have looked at any physiological differences between the brains of men and women. They then studied patterns of activation in the brains of participants of both sexes to see if men and women relate to the same external stimuli and cognitive or motor tasks in the same way.
Finally, the question that emerges is: do any of these differences affect the way in which men and women perform the same tasks? And do such differences affect men versus women's susceptibility to different brain disorders?
Often, there are no clear-cut answers, and scientists tend to disagree on some of the most basic aspects - such as whether there are any notable physiological differences between the brains of men and women.
In this article, we look at some of the more recent studies dealing with these questions and give you an overview of where current research stands.
Are there 'hardwired differences?'
Increasingly, online articles and popular science books appeal to new scientific studies to deliver quick and easy explanations of "why men are from Mars and women come from Venus," to paraphrase a well-known bestseller about heterosexual relationship management.
One such example is a book from the Gurian Institute, which emphasizes that baby girls and boys should be treated differently because of their underlying neurological differences. Non-differentiated child-rearing, the authors suggest, may ultimately be unhealthy.
Cars for boys, teddies for girls?
Dr. Nirao Shah, who is a professor of psychiatry and behavioral sciences at Stanford University in California, also suggests that there are some basic "behaviors [that] are essential for survival and propagation," related to reproduction and self-preservation, that are different in men and women.
These, he adds, are "innate rather than learned [...] [in animals] so the circuitry involved ought to be developmentally hardwired into the brain. These circuits should differ depending on which sex you're looking at."
Some examples brought to bear on these "innate differences" often come from studies on different primates, such as rhesus monkeys. One experiment offered male and female monkeys traditionally "girly" ("plush") or "boyish" ("wheeled") toys and observed which kinds of toys each would prefer.
This team of researchers found that male rhesus monkeys appeared to naturally favor "wheeled" toys, whereas the females played predominantly with "plush" toys.
This, they argued, was a sign that "boys and girls [may] prefer different physical activities with different types of behaviors and different levels of energy expenditure."
Similar findings have been reported by researchers from the United Kingdom about boys and girls between 9 and 32 months old - a period when, some researchers suggest, the children are too young to form gender stereotypes.
Apparent differences in preferences have been explained through a differential hardwiring in the female versus male brain. Yet, criticisms of this perspective also abound.
Refuting studies in monkeys, some specialists argue that, no matter how similar to human beings from a biological point of view, monkeys and other animals are still not human, and guiding our understanding of men and women by the instincts of male and female animals is erroneous.
As for studies on infants and young children, researchers often identify pitfalls. Boys and girls, some argue, can already develop gender stereotypes by age 2, and their taste for "girly" or "boyish" toys may be influenced by how their parents socialize them, even if the parents themselves are not always aware of perpetuating stereotypes.
The perspective that "gendered" preferences can be explained through hormonal activity and differences in the brains of men and women remains, therefore, controversial.
Different brain activation patterns
Still, there are a number of studies that pinpoint different patterns of activation in the brains of men versus women given the same task, or exposed to the same stimuli.
One such study evaluated sex-specific brain activity in the context of visuospatial navigation. The researchers used functional MRI (fMRI) to monitor how men's and women's brains responded to a maze task.
In their given activity, participants of both sexes had to find their way out of a complex virtual labyrinth.
It was noted that in men, the left hippocampus - which has been associated with context-dependent memory - lit up preferentially.
In women, however, the areas activated during this task were the right posterior parietal cortex, which is associated with spatial perception, motor control, and attention, and the right prefrontal cortex, which has been linked to episodic memory.
Another study discovered "rather robust differences" between resting brain activity in men and in women. When the brain is in a resting state, it means that it is not responding to any direct tasks - but that doesn't mean it isn't active.
Scanning a brain "at rest" is meant to reveal any activity that is "intrinsic" to that brain, and which happens spontaneously.
When looking at the differences between male and female brains "at rest," the scientists saw a "complex pattern, suggesting that several differences between males and females in behavior might have their sources in the activity of the resting brain."
What those differences in behaviour might amount to, however, is a matter of debate.
An experiment targeting men's and women's response to perceived threat, for instance, highlighted a better evaluation of threat on the part of women.
The study, which used fMRI to scan the brain activity of teenagers and adults of both sexes, found that adult women had a strong neural response to unambiguous visual threat signals, whereas adult men - and adolescents of both sexes - exhibited a much weaker response.
Last year, Medical News Today also reported on a study that pointed to different patterns of cooperation in men and women, with possible underlying neural explanations.
Groups of male-male, female-female, and female-male couples were observed as they performed the same simple task involving cooperation and synchronization.
Overall, same-sex pairs did better than opposite sex pairs. But interbrain coherence - that is, the relative synchronization of neural activity in the brains of a pair performing a cooperative task - was observed in different locations in the brains of male-male versus female-female subjects.
Another study using fMRI also emphasized significant differences between how the brains of men and women organize their activity. There are different activation patterns in the brain networks of males and females, the researchers explain, which correlate with substantial differences in the behavior of men and of women.
Different activation patterns, but what does that mean?
A more recent study, however, disagrees that there are any fundamental functional differences, though the methodology of this research has been questioned. The authors of this work analyzed the MRI scans of more than 1,400 human brains, sourced from four different datasets.
Their findings suggest that, whatever physiological differences may exist between the brain of men and of women, they do not indicate underlying, sex-specific patterns of behaviour and socialization.
The volumes of white and gray matter in brains of people pertaining to both sexes do not differ significantly, the study found.
Also, the scientists pointed out that "most humans possess a mosaic of personality traits, attitudes, interests, and behaviors," consistent with individual physiological traits, and inconsistent with a dualistic view of "maleness" and "femaleness."
"The lack of internal consistency in human brain and gender characteristics undermines the dimorphic [dualistic] view of human brain and behavior [...] Specifically, we should shift from thinking of brains as falling into two classes, one typical of males and the other typical of females, to appreciating the variability of the human brain mosaic."
Susceptibility to brain disorders
That being said, many scientists continue to point toward evidence that the distinct physiological patterns of male and female brains lead to a differentiated susceptibility to neurocognitive diseases, as well as other health-related problems.
One recent study covered by MNT, for instance, suggests that microglia - which are specialized cells that belong to the brain's immune system - are more active in women, meaning that women are more exposed to chronic pain than men.
Yet another analysis of brain scans for both sexes suggested that women show higher brain activity in more regions of the brain than men.
According to the researchers, this heightened activation - especially of the prefrontal cortex and of the limbic regions, tied with impulse control and mood regulation - means that women are more susceptible to mood disorders such as depression and anxiety.
'Male-biased' and 'female-biased' conditions
A meta-analysis of studies related to sex-based differences in the brain confirms that men and women are susceptible to largely different brain disorders.
"Examples of male-biased conditions include autism, attention deficit/hyperactivity disorder, conduct disorder, specific language impairment, Tourette syndrome, and dyslexia, and examples of female-biased conditions include depression, anxiety disorder, and anorexia nervosa."
The authors suggest that it is important to take into account physiological differences in order to enhance preventive approaches and treatments.
An earlier study had also noted differentiated patterns of susceptibility to brain disorders between sexes, yet it also acknowledged some significant limitations.
First, the authors said, many previous studies did not manage to recruit similar numbers of participants of each sex, which may have led to gender bias. Additionally, they explained, "because women may seek treatment more than men, it may be easier for a researcher to recruit females."
"Both of these factors may lead to a patient sample predisposed to an uneven gender distribution," the authors admit, but their conclusion remains firm.
"[G]ender matching is essential in clinical functional imaging studies, and supports the idea of exploring male and female populations as distinct groups," the scientists urge, citing the wealth of studies that point to the same interpretation.
So, are brain differences fundamental to how men and women function? The answer is maybe. While so many studies noted different activation patterns in the brain, these did not necessarily amount to differences in the performance of given tasks.
At the same time, from a healthcare perspective, it may be important to take sex-based differences into account, so as to devise the best possible treatment plans for different individuals.