The amygdala is a small almond-shaped body buried deep within the brain's temporal lobe.
Previously it was known to be linked to empathy and fear responses but now a new study has shown that people with larger amygdalas have wider and more complex networks of friends and colleagues.
"We found that the amygdala volume positively correlated with the size and complexity of social networks in adult humans," said study leader Professor Lisa Feldman, a psychologist at Massachusetts General Hospital in Boston, US.
Prof Feldman's team asked 58 volunteers aged 19 to 83 to complete questionnaires which measured how many regular social contacts they had, and in how many different groups.
Magnetic resonance imaging scans were carried out to examine their brain structures.
Participants with the largest amygdalas also had the richest social lives, the scientists discover ed.
The same association, observed in all ages of men and women, was not seen with other parts of the brain.
Prof Barrett said the findings, reported in the journal Nature Neuroscience, were consistent with the "social brain hypothesis". This suggests the human amygdala evolved to deal with an increasingly complex social world. Other studies of primates have shown that those living in larger groups tend to have larger amygdalas.
"Further research is in progress to try to understand more about how the amygdala and other brain regions are involved in social behaviour in humans," said Prof Barrett.