As was the case in monkeys, some brain areas were enlarged and better connectedin people with larger social networks. In humans, these areas were the temporal parietal junction, the anterior cingulate cortex and the rostral prefrontal cortex, which are part of a network involved in "mentalization"—the ability to attribute mental states, thoughts and beliefs to another.

"These different brain regions are all singing different songs," Noonan said. "Networked areas are all singing the same song, and when they're connected better, they're singing more harmoniously with each other."

The researchers also tested whether the size of a person's social networkwas linked with changes in white-matter pathways, the nerve fibers that connect different brain regions.

Again, they found that white-matter tracts were better connected in people with bigger social networks. "The nerves were more like a Los Angeles freeway than a country road," Noonan said.

The researchers couldn't say whether social interaction caused these changes in brain structure and connectivity, or whether the brain determined how innately social someone was.

In the case of the monkeys, the researchers dictated the size of the animals' social network, so they concluded that social-group size was causing the brain differences.

It can be inferred that a similar process takes place in human brains, but to prove this, long-term studies are needed, Noonan told LiveScience.

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