From Scientific American:
Imitation is thought to be the sincerest form of flattery—even when the mimic and model are unaware of the mimicry. Now, new evidence from a study of capuchin monkeys shows a possible evolutionary benefit to being a clueless copycat (or copy-capuchin, in this case).
“We've known for awhile that we humans imitate each other all the time, unintentionally and unconsciously,” says Annika Paukner, a comparative behaviorist at the National Institute of Child Health and Human Development and lead author of the study published online today in the journal Science. “When people start using the same words or the same body language, it seems to help social interaction. People [who are imitated] say they like the other person better. It builds rapport and empathy.”
More here.