RESUMEN
Infants' nonverbal expressions-a broad smile or a sharp cry-are powerful at eliciting reactions. Although parents' reactions to their own infants' expressions are relatively well understood, here we studied whether adults more generally exhibit behavioral and physiological reactions to unfamiliar infants producing various expressions. We recruited U.S. emerging adults (N = 84) prior to parenthood, 18-25 years old, 68% women, ethnically (20% Hispanic/Latino) and racially (7% Asian, 13% Black, 1% Middle Eastern, 70% White, 8% multiracial) diverse. They observed four 80-s audio-video clips of unfamiliar 2- to 6-month-olds crying, smiling, yawning, and sitting calmly (emotionally neutral control). Each compilation video depicted 9 different infants (36 clips total). We found adults mirrored behaviorally and physiologically: more positive facial expressions to infants smiling, and more negative facial expressions and pupil dilation-indicating increases in arousal-to infants crying. Adults also yawned more and had more pupil dilation when observing infants yawning. Together, these findings suggest that even nonparent emerging adults are highly sensitive to unfamiliar infants' expressions, which they naturally "catch" (i.e., behaviorally and physiologically mirror), even without instructions. Such sensitivity may have-over the course of humans' evolutionary history-been selected for, to facilitate adults' processing of preverbal infants' expressions to meet their needs.