How does emotion regulation impact teammates? We present data from a dyadic experiment(N=266) that assessed in vivo stress responses in teammates during collaborative (a face-to-face product design task) and then individual work (a product pitch to evaluators). Throughout the experiment, one manipulated teammate reappraised their stress arousal (reappraisal), suppressed their emotional displays (suppression), or received no instructions (control). Their non- manipulated teammate received no instructions. Stress reappraisal benefited both teammates, eliciting challenge-like physiological responses (higher cardiac output, lower total peripheral resistance) relative to the suppression and control conditions. These effects were observed during both face-to-face collaborative work and later individual work. A mediation model suggests that the face-to-face social contagion effects of stress reappraisal fed forward to promote non- manipulated teammates’ improved stress responses during the individual performance task. These findings indicate that non-manipulated teammates exhibited improved stress responses simply by interacting with a person who reappraised their stress as functional.

Gratitude expressions play a key role in strengthening relationships, suggesting gratitude might promote adaptive responses during teamwork. However, little research has examined gratitude’s impact on loose tie relationships (like coworkers), and similarly little research has examined how gratitude impacts physiological stress responding or biological responses more generally. The present research uses an ecologically valid, dyadic teamwork paradigm to test how gratitude expressions impact in vivo physiological challenge and threat stress responding, assessed via a challenge–threat index composed of cardiac output and total peripheral resistance. Compared with a control condition, teammates (n = 190) who were randomly assigned to a gratitude expression manipulation showed improved biological challenge–threat responses while jointly completing an acutely stressful collaborative work task (developing a product pitch), and later while completing an individual performance task (pitching the product). During the collaborative task, gratitude expressions buffered against threat responses; during the individual task, gratitude expressions amplified challenge responses. Analyses of cardiac output (CO) and total peripheral resistance (TPR) aided in determining how cardiac outflow versus vascular constriction/dilation contributed to these effects. The finding that gratitude expressions promote adaptive biological responding at the dyadic level contributes to a growing literature on the social functions of positive emotions and gratitude, specifically. The present results also have wider implications for physiological stress in performance tasks and suggest that workplace gratitude interventions can promote adaptive stress responding in teams.