Grace Browne

The social brain

Brain scans reveal how men and women respond differently to relationships

The "human social brain" looks different across sexes.

We all deal with social interactions differently – or these days, as communities prioritize and practice social distancing, the lack thereof. But how you’re coping with this newfound isolation may be impacting your brain differently depending on your sex, a new study suggests.

Findings published Wednesday in the journal Science Advances demonstrate that “all studied parts of the human social brain” show “some degree of divergence between male and female brain anatomy,” co-author Danilo Bzdok, a neuroscientist at McGill University, tells Inverse.

There was no social behavior or social dimension that they examined that did not show some degree of difference between males and females, Bzdok says.

To discover this, Bzdok and his colleagues used data collected by the UK Biobank database and studied the brains of 10,129 participants, both male and female. The researchers accounted for how the participants differed on a number of factors, both on a sociodemographic level and in lifestyle. This included variables such as whether they have siblings, how social their job is, how lonely they feel, and how many people they live with.

Then, the scientists took a dive into MRI data that detailed participant’s brains. The team specifically examined the volume of 36 different areas of the brain closely related to social behavior — what they refer to as the “social brain atlas.” This “atlas” is a group of brain regions that are known to show consistent neural activity changes during social task experiments.

Ultimately, the scientists discovered that the quality, frequency, and type of social interactions affect the “social brain atlas” differently, depending on the sex of a person. These findings establish a link to how rich one’s social life is and variation in brain volume.

The largest study on sex differences in the human brain

This new research builds on a concept called the “social brain hypothesis”, which proposes that the reason primates have developed such large brains over time is due to their social environments growing in complexity. The findings of this new study suggest that this relationship seems to change depending on sex.

"I think that sex-gender research has maybe been asking the wrong question."

This work — the largest study on sex differences in the human brain to date — investigates to what degree male and female brains differ, as opposed to whether they are just different, point-blank.

“I think that sex-gender research has maybe been asking the wrong question,” says Bdzok. “Much previous research tried to impose binary, black or white, yes or no type of conclusions."

Older studies that tried to get to the bottom of this question also relied on small sample sizes — often only a few dozen participants or so — which pales in comparison to this study’s sample size of 10,000.

Large quantities of data, like what was used here, allows for more fine-grained analysis, Bzdok says. With these factors, “it becomes always more apparent that there’s really a spectrum between males and females,” Bzdok explains.

Brains reflect how socially rich a life is

This study revealed that some of the hotspot areas of the social brain regularly studied by neuroscientists (like the amygdala and the nucleus accumbens) showed some almost opposite effects between males and females, depending on the social environments of their everyday lives.

For example: The team discovered that for the more socially stimulated women, there were some neuroanatomical associations in the amygdala, the region of the brain that processes emotions. Meanwhile, these effects were barely visible in men.

On the other hand, in the brain region called the ventromedial prefrontal cortex, which is found deep in the frontal lobe and thought to play a role in emotional cognition, they observed opposite effects in males that live alone, largely alone, or are more socially stimulated at home, a finding that was not seen in the female participants. Men who lived in less socially rich environments at home had, on average, a ventromedial prefrontal cortex with a volume that’s different than men who lived in more socially stimulating homes.

They also observed that men who had less vibrant social lives showed similar volume effects in components of the social brain that are tied to rewards, such as the nucleus accumbens.

Social dynamics affect the volume of certain parts of the brain.Danilo Bzdok

Women who had a lower number of social ties — fewer friends, or less social support — were found to show deviations that were similar across their limbic systems, a collective term for the parts of our brains that plays a role in our behavioral and emotional responses to things.

Sex specific differences in parts of the brain were linked to frequency and intensity of social contact.Danilo Bzdok

Why would men and women’s brains react differently to social environments?

“The ways in which men and women navigate in and cope with the social environment – friends, family, job – they may have, evolutionarily speaking, led to partly distinct adaptations in the brain, and some specific behavioral tendencies,” Bzdok tells Inverse.

Depending on whether you're male or female, your social habits may have effects that go right up to the top (the top being your brain, that is).

Abstract: In human and nonhuman primates, sex differences typically explain much interindividual variability. Male and female behaviors may have played unique roles in the likely coevolution of increasing brain volume and more complex social dynamics. To explore possible divergence in social brain morphology between men and women living in different social environments, we applied probabilistic generative modeling to ~10,000 UK Biobank participants. We observed strong volume effects especially in the limbic system but also in regions of the sensory, intermediate, and higher association networks. Sex-specific brain volume effects in the limbic system were linked to the frequency and intensity of social contact, such as indexed by loneliness, household size, and social support. Across the processing hierarchy of neural networks, different conditions for social interplay may resonate in and be influenced by brain anatomy in sex-dependent ways.
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