Face-to-face interactions of 56 sixth-grade friend and acquaintance pairs were videotaped, heart rate was recorded, and saliva cortisol was sampled. During interactions the friend dyads were more attentive, affectively positive, vocal, active, involved, relaxed, and playful, and their cortisol values suggested lower stress levels. They also spent more time together in mutually interested and animated states, and they assigned higher ratings to liking their interactions and interaction partners. Greater coherence in the friend pairs' behavior states and in the acquaintance pairs' vocal activity suggested that the friend pairs more often shared the same behavior state (e.g., playful), and the acquaintance pairs more often paid attention to each others' turn-taking signals, so that when one person talked, the other was silent.

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