You know those people. You might even envy them. They seem go out every night, have hundreds of friends and give every appearance of living a fabulous life.
You may have thought it was their charm or good looks, but new research suggests it might be all in the amygdala, an almond-shaped structure in the brain.
People with large, highly complex social networks tend to have larger amygdala regions than those with fewer friends, according to a study published in Nature Neuroscience.
It's the first study to demonstrate a link between amygdala volume and social network characteristics within a single species.
Using scans of the brain taken with magnetic resonance imaging machines, the researchers studied the amygdalas of 58 people aged 19 to 83.
They found that the gregarious types, those who reported having regular contact with comparatively large numbers of people from a variety of social groups, tend to have larger amygdala volumes.
"Humans are inherently social animals. We play, work, eat and fight with one another. A larger amygdala might enable us to more effectively identify, learn about and recognize socio-emotional cues in [other humans]" the authors write. That would also allow those with larger amygdalae to "develop complex strategies to co-operate and compete."
People in the study with smaller amygdala volumes, in the range of 2.5 to 3.0 cubic millimetres, reported having regular contact with between five and 15 people. Those with larger volumes, from 3.5 to 5.0 cubic millimetres, reporting having 20 to 50 contacts. The trend holds true for both young and old, despite the fact that the elderly tend to have smaller amygdalas. The size of a brain region is, according to the study, one indicator of its processing capacity.
The amygdala is already known to play an important role in how humans process visual cues to determine the identity and trustworthiness of others. Earlier this month scientists published research on a woman known as SM whose amydala had been damaged. She was described as being unable to feel fear, and as a result had led a very dangerous life that included being held at knifepoint and wanting to touch poisonous snakes and spiders.
But it's still not clear whether a bigger amygdala is better for social functioning, they write. That will have to be a question for further research.
It's also not clear whether having hundreds of friends on Facebook or other social networking sites is related to amygdala size.
The research builds on the social brain hypothesis, which suggests that there is an evolutionary selection process that favours those who are able to navigate the challenges of living in social groups.