If your social life is a blur of friends and family, you might want to thank an almond-shaped clump of nerves at the base of your brain.
Researchers have found that part of the brain called the amygdala, a word derived from the Greek for almond, is larger in more sociable people than in those who lead less gregarious lives.
The finding, which held for men and women of all ages, is the first to show a link between the size of a specific brain region and the number and complexity of a person's relationships.
The amygdala is small in comparison with many other brain regions but is thought to play a central role in coordinating our ability to size people up, remember names and faces, and handle a range of social acquaintances.
Researchers at Massachusetts general hospital in Boston used magnetic resonance imaging (MRI) scans to measure the amygdalas of 58 people aged 19 to 83 and found the structure ranged in size from about 2.5 cubic millimetres to more than twice that.
As part of the study, each of the volunteers completed a questionnaire giving the number of people they met on a regular basis. They also commented on the complexity of each relationship. For example, one friend might also be a boss, meaning the person had to adapt their behaviour with the person depending on the nature of their encounter.
The team, led by psychologist Lisa Feldman Barrett, found that participants with larger amygdalas typically had more people in their social lives and maintained more complex relationships.
Those with the smallest amygdalas listed fewer than five to 15 people as regular contacts, while those with the largest amygdalas counted up to 50 acquaintances in their social lives. Older volunteers tended to have smaller amygdalas and fewer people in their social group.
Writing in the journal, Nature Neuroscience, Barrett's team cautions that the finding is only a correlation, meaning they cannot say whether there is a causal link between the size of the amygdala and the richness of a person's social life. However, previous studies with primates show that those that live in large social groups also have bigger amygdalas. "People who have large amygdalas may have the raw material needed to maintain larger and more complex social networks," said Barrett . "That said, the brain is a use it or lose it organ. It may be that when people interact more their amygdalas get larger. That would be my guess.
"It's not that someone with a larger amygdala can do things that someone with a smaller amygdala cannot do. People differ in how well they remember people's names and faces and the situation in which they met them. Someone with a larger amygdala might simply be better at remembering those details," Barrett added.
Previous studies have found that parts of the brain enlarge to cope with more demanding tasks. In 2000, a team of neuroscientists led by Eleanor Maguire at UCL showed that in London taxi drivers, part of the brain called the hippocampus grows to help them remember a mental map of the city.
Barrett's MRI scans revealed no other brain structures that varied in size according to the extent and complexity of a person's social life.
The work builds on previous research by Robin Dunbar, director of social and cultural anthropology at Oxford University, who found a theoretical limit to the number of meaningful relationships a person can maintain. The figure is rough but considered to be about 150.
Barrett did not look at whether amygdala size varied with the number of contacts a person had on social networking websites like Facebook or Twitter, in part because it is unclear whether these require the same cognitive effort to maintain as more traditional relationships.
Barrett's group is now looking at other brain regions to see which others are involved in social behaviour, and how abnormalities or injuries to the brain can impair a person's social life.
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