The richer and more varied a person’s social network, the bigger their amygdala, a structure deep in the brain that has been linked to size and complexity of social groups in other primate species, said researchers from Massachusetts General Hospital (MGH) in the US.

You can read about their study in the 26 December advance online issue of Nature Neuroscience.

The amygdala comprises a pair of symmetrically placed small almond shaped structures deep within the temporal lobe. It has many connections with other brain regions, and is thought to be involved in a variety of behavioral functions.

Study co-leader Dr Lisa Feldman Barrett, of MGH’s Psychiatric Neuroimaging Research Program and a Distinguished Professor of Psychology at Northeastern University told the press that:

“We considered a single primate species, humans, and found that the amygdala volume positively correlated with the size and complexity of social networks in adult humans.”

They found the link was just as strong when they adjusted for age (older people have on average smaller amygdala volumes than younger people) and when they analyzed left and right amygdalas separately, ” indicating no lateralization of the effect”.

Co-lead author Dr Bradford C Dickerson, of MGH’s Department of Neurology and the Martinos Center for Biomedical Research, said:

“This link between amygdala size and social network size and complexity was observed for both older and younger individuals and for both men and women.”

Dickerson is also an associate professor of Neurology at Harvard Medical School.

For the study, Barrett, Dickerson and colleagues invited 36 men and 22 women aged between 19 and 83 years and of average age 52.6, to fill in a survey about their social lives, and give information about the size and complexity of their social networks by answering questions based on two scales of the Social Network Index.

From the responses the researchers were able to measure the total number of regular contacts each participant maintained plus the number of different groups the contacts belonged to. The first is a measure of overall social network size and the second is a measure of network complexity.

The researchers then took magnetic resonance imaging (MRI) scans of the participants’ brains to examine the structure of various regions, and the volume of the amygdala.

They found that the link between amygdala volume and social network size and complexity was just as strong when they corrected for overall brain volume, and seems to be specific to the amygdala and not other subcortical structures.

“An exploratory analysis of subcortical structures did not find strong evidence for similar relationships with any other structure, but there were associations between social network variables and cortical thickness in three cortical areas, two of them with amygdala connectivity,” they wrote.

They also found that the volume of the amygdala was not linked to other social variables in the life of humans such as life support or social satisfaction.

They concluded that their findings “indicate that the amygdala is important in social behavior”.

Barrett said their findings were similar to results of other studies that have compared the size and complexity of social groups in other primate species:

“We know that primates who live in larger social groups have a larger amygdala, even when controlling for overall brain size and body size,” she said.

But she and her colleagues wrote that to their knowledge, this study is the first to show a link “between amygdala volume and social network characteristics within a single species”. They also wrote that their findings were consistent with the “social brain hypothesis” which purports that the human amygdala evolved partially to deal with the increasing complexities of social life.

Barrett said further research was under way to find other aspects of human social behavior that might involve the amygdala and other regions of the brain.

They also hope to discover how “abnormalities in these brain regions may impair social behavior in neurologic and psychiatric disorders,” she added.

“Amygdala volume and social network size in humans.”
Kevin C Bickart, Christopher I Wright, Rebecca J Dautoff, Bradford C Dickerson & Lisa Feldman Barrett.
Nature Neuroscience, Published online: 26 December 2010.
DOI:10.1038/nn.2724

Additional source: Massachusetts General Hospital press release.

Written by: Catharine Paddock, PhD