Social neuroscience is a rapidly growing discipline that examines the relationship between the brain and social behavior. The “social brain hypothesis” posits that, over evolutionary time, living in large, social groups favored the physical growth of brain regions important for social behavior. In non-human primates, some evidence indicates that the size of the amygdala is related to social behavior. Little is known, however, about this relationship in humans. A provocative new study finds that the volume of a key component of the social brain, the amygdala, is directly related to the size and complexity of social networks in adult humans.

A research team headed by Lisa F. Barrett, Ph.D., Distinguished Professor of Psychology at Northeastern University, used brain imaging to study the relationship between the volume of the amygdala and social network size and complexity in both males and females.

To investigate, the researchers recruited 58 healthy adults (22 women and 36 men, aged 19-83 years, with average age of 53) with normal intelligence and no major mental disorders to participate in the study. Each participant underwent magnetic resonance imaging (MRI) in order to assess the volume of multiple brain regions, including the amygdala (important for emotions such as fear) and the hippocampus (important for learning and memory). Importantly, head size was corrected for in the measurement of each participant’s brain volume. In addition, the researchers used two versions of a questionnaire called the Social Network Index to evaluate the size (reflected by the total number of contacts), as well as the complexity (reflected by the number of different groups that contacts belong to), of each participant’s social network. Subsequently, the research team used advanced statistical methods to evaluate the relationship between the volume of different brain regions and social network size and complexity.

In a study published in a recent issue of Nature Neuroscience, the research team reports that people having larger and more complex social networks have larger amygdala volumes. These results persisted even when biological (such as age and amygdalae on different sides of the brain) and social (such as life satisfaction and perceived social support) factors were taken into account. Importantly, the volume of the hippocampus (measured for purposes of comparison) was not related to either the size or complexity of social networks. Further analyses also demonstrated a similar lack of relationship between the size of additional brain regions and social network variables. Amygdala volume was not related to other social variables, such as life satisfaction or perceived social support. These results demonstrate the specificity of the relationship between amygdala volume and social network parameters. Interestingly, when the participants were considered separately by age and sex, the relationship between amygdala volume and social networks was less pronounced in older participants, as well as in males (regardless of age).

This study is the first to report that the volume of the amygdala is related to social network variables within a single species. The researchers suggest that this phenomenon may have served, in evolutionary time, to facilitate greater social intelligence in order to accommodate the demands of an increasingly complex social life. It should be noted, however, that the results merely indicate a correlation; that is, people who have greater amygdala volumes have larger social networks, but it is unknown whether having greater amygdala volume leads to having a larger and more complex social network, or vice versa. Dr. Barrett said “it is probably a little of both.”

Imaging studies of amygdala volume as related to social behavior have mostly been conducted in individuals with autism or psychopathy/antisocial personality and have yielded variable results. Even in normal humans lacking any such disorders, the exact relationship between amygdala volume and social functioning has remained somewhat elusive. So, is a bigger amygdala better? The answer remains to be definitely determined. However, a bigger amygdala is likely to be better equipped to accommodate more social information from more people in more contexts. In this sense, a bigger amygdala may facilitate greater social intelligence.

The research findings have potential significance for a growing body of literature suggesting that emotional memories in the amygdala can be rewired during a process known as “reconsolidation” (during which time old memories can be updated with new information) in order to help people suffering from post-traumatic stress disorder. “We hope to be able to discover how abnormalities in the amygdala and related brain regions may impair social behavior in psychiatric and neurological conditions,” said Dr. Barrett.

References

Bickart KC, Wright CI, Dautoff RJ, Dickerson BC, & Barrett LF (2011). Amygdala volume and social network size in humans. Nature neuroscience, 14 (2), 163-4 PMID: 21186358

Hartley CA, & Phelps EA (2010). Changing fear: the neurocircuitry of emotion regulation. Neuropsychopharmacology : official publication of the American College of Neuropsychopharmacology, 35 (1), 136-46 PMID: 19710632

Lewis KP, & Barton RA (2006). Amygdala size and hypothalamus size predict social play frequency in nonhuman primates: a comparative analysis using independent contrasts. Journal of comparative psychology (Washington, D.C. : 1983), 120 (1), 31-7 PMID: 16551162

Monfils MH, Cowansage KK, Klann E, & LeDoux JE (2009). Extinction-reconsolidation boundaries: key to persistent attenuation of fear memories. Science (New York, N.Y.), 324 (5929), 951-5 PMID: 19342552

Schiller D, Monfils MH, Raio CM, Johnson DC, Ledoux JE, & Phelps EA (2010). Preventing the return of fear in humans using reconsolidation update mechanisms. Nature, 463 (7277), 49-53 PMID: 20010606