Pupil dilation patterns spontaneously synchronize across individuals during shared attention.

Human social behavior relies on the coupling of minds. Here we show that patterns of pupil dilations reveal mental coupling between speakers and listeners. Speakers were videotaped and eye-tracked as they discussed positive and negative autobiographical memories. An independent group of listeners were then eye-tracked while they watched these videos. As pupillary dilations reflect the dynamics of conscious attention, we computed the morphological similarity of speaker-listener pupillary time-series data as a metric of shared attention. The emotional salience of each narrative was also assessed, dynamically, by independent raters. Collective pupillary synchrony between speakers and listeners was greatest during the emotional peaks of a narrative, and decreased as narratives became less engaging. Individual differences in speaker expressivity and listener empathy revealed greatest synchrony in high expressive-high empathic dyads. Together, these findings suggest that pupillary synchrony is an implicit corollary of shared attention that can be used to track mental coupling in real time.