TUESDAY, Oct. 12 (HealthDay News) -- Close friends trigger a stronger brain response than strangers, even if you have more in common with some of the strangers, a new study finds.

This may seem obvious, but some neuroscientists used to think it was the other way around.

"There are psychological and evolutionary arguments for the idea that the social factors of 'similarity' and 'closeness' could get privileged treatment in the brain -- for example, to identify insiders versus outsiders or kin versus non-kin," said study co-leader Fenna Krienen, a graduate student at Harvard University, in a Society for Neuroscience news release. "However, these results suggest that social closeness is the primary factor, rather than social similarity, as previously assumed."

The researchers studied the brain activity of volunteers as they answered questions about friends and strangers. When discussing friends, there was increased activity in the participants' medial prefrontal cortex, a part of the brain involved in processing social information.

This increased activity did not occur when the participants answered questions about strangers, even if the stranger had more in common with the participant.

The findings suggest that social alliances are stronger than shared interests, the researchers said.

The study appears in the Oct. 13 issue of the Journal of Neuroscience.

More information

For more on the importance of friendship, visit the American Psychological Association's Friendship Blog.