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We propose a novel theoretical and empirical approach to studying group-level social functions of emotions and use it to make new predictions about social consequences of gratitude. Here, we document the witnessing effect: In social groups, emotional expressions are often observed by third-party witnesses—family members, coworkers, friends, and neighbors. Emotional expressions coordinate group living by changing third-party witnesses’ behavior toward first-party emotion expressers and toward second-party people to whom emotion is expressed. In 8 experiments (N= 1,817), we test this for gratitude, hypothesizing that third-party witnesses will be more helpful and affiliative toward a first party who expressed gratitude to a second party, as well as toward the second party, and why. In Experiments 1–3, participants who witnessed a “thank you” in 1 line of text, expressed to someone who previously helped the …