Originally published August 23, 2013
Here is an excerpt:
"The correlation between self and friend was remarkably similar," Coan said. "The finding shows the brain's remarkable capacity to model self to others; that people close to us become a part of ourselves, and that is not just metaphor or poetry, it's very real. Literally we are under threat when a friend is under threat. But not so when a stranger is under threat."
Coan said this likely is because humans need to have friends and allies who they can side with and see as being the same as themselves. And as people spend more time together, they become more similar.
The entire story is here.