The findings are in line with past animal studies that have shown that species with larger social groups have relatively larger amygdalas, when brain and body size are taken into account, compared with less social animals. “Our question was, could we see this variation within a single species?” says lead author Lisa Barrett, director of the Interdisciplinary Affective Science Laboratory at Northeastern University.
Understanding the relationship between the size of an individual’s amygdala and his or her social relationships could help lead to treatments for a variety of conditions that involve difficulties with social connections, such as depression or autism.
So what does the amygdala actually do? “[It’s] strongly connected with almost every other structure in brain. In the past, people assumed it was really important for fear. Then they discovered it was actually important for all emotions. And it’s also important for social interaction and face recognition,” Barrett says. “The amygdala’s job in general is to signal to the rest of brain when something that you’re faced with is uncertain. For example, if you don’t know who someone is, and you are trying to identify them, whether it is a friend or a foe, the amygdala is probably playing a role in helping you to perform all of those tasks.”
Barrett says it is commonly assumed that the size of a structure reflects its computational capacity, noting that if your larger amygdala easily allows you to identify people you’ve met before at a cocktail party, you will have a much easier time connecting and socializing. “You can imagine that might be one thing someone with big amygdala might be better at and that might lay the foundation for easier formation of social bonds,” she says.
The research, which was published in Nature Neuroscience, found a moderate correlation between amygdala size and the number and complexity of social relationships in 58 healthy adults aged 19 to 83.
Interestingly, however, amygdala size was not related to the quality of those relationships or to whether or not people enjoyed socializing. “We looked at measures of ‘How much do you enjoy social interaction?’ and ‘Are you satisfied with your social support?’ and that was not related to amygdala volume,” says Barrett.
Prior research has shown that people with autistic spectrum disorders have smaller amygdalas, which could help explain their social problems. But these studies cannot determine cause or effect — whether having a small amygdala makes socializing difficult, or whether lack of social interaction shrinks the amygdala — or whether both factors interact and result in a smaller brain region. For example, it may be that the amygdala requires a certain amount of social experience in order to develop properly; not receiving that, it may remain small but capable of further growth given the right social exposure.
“This study represents an important initial study in human neurosociology — the study of the neurobiology of human living groups. The findings, while preliminary, suggest that the structure and functional capacity of our brain is influenced by the nature, quality and quantity of relational connections we — and our extended relational community — have,” says Dr. Bruce Perry, senior fellow at the ChildTrauma Academy who was not associated with the research. (Full disclosure: Perry and I have co-authored two books.)
While this study did not look at the size of people’s online social networks, the researchers plan to include those measures in future research to determine their influence.