Quantifying Facial Expression Synchrony in Face-To-Face Dyadic Interactions: Temporal Dynamics of Simultaneously Recorded Facial EMG Signals

Human social interaction is enriched with synchronous movement which is said to be essential to establish interactional flow. One commonly investigated phenomenon in this regard is facial mimicry, the tendency of humans to mirror facial expressions. Because studies investigating facial mimicry in face-to-face interactions are lacking, the temporal dynamics of facial mimicry remain unclear. We therefore developed and tested the suitability of a novel approach to quantifying facial expression synchrony in face-to-face interactions: windowed cross-lagged correlation analysis (WCLC) for electromyography signals. We recorded muscle activations related to smiling (Zygomaticus Major) and frowning (Corrugator Supercilii) of two interaction partners simultaneously in 30 dyadic affiliative interactions. We expected WCLC to reliably detect facial expression synchrony above chance level and, based on previous research, expected the occurrence of rapid synchronization of smiles within 200 ms. WCLC significantly detected synchrony of smiling but not frowning compared to a control condition of chance level synchrony in six different interactional phases (smiling: d _z s = .85–1.11; frowning: d _z s = .01–.30). Synchronizations of smiles between interaction partners predominantly occurred within 1000 ms, with a significant amount occurring within 200 ms. This rapid synchronization of smiles supports the notion of the existence of an anticipated mimicry response for smiles. We conclude that WCLC is suited to quantify the temporal dynamics of facial expression synchrony in dyadic interactions and discuss implications for different psychological research areas.