Capacity to react to the cost of a request in relation to interlocutor's behavior is acquired early; ability to maintain good interactions by increasing the politeness of a request occurs later. Only from 9 years on do children use linguistic politeness as a criterion in judging a request's appropriateness according to addressee's status. (RH)

From earliest infancy, a typically developing child imitates or mirrors the facial expressions, postures and gestures, and emotional behavior of others. Where does this capacity come from, and what function does it serve? What happens when imitation is impaired? Synthesizing cutting-edge research emerging from a range of disciplines, this…