Heart-rate deceleration and smiling in 3-month-old infants

The relation between heart-rate deceleration and the expression of positive affect in infancy is examined by testing the hypothesis that intensity of orienting (as indexed by heart-rate deceleration) is predictive of intensity of positive affect (smile size and duration). A social-stimulation procedure was utilized to elicit smiles in 41 3-month-old infants. The first smile elicited was coded for smile size and duration, and the preceding heart-rate deceleration was coded for magnitude, duration, and slope of deceleration. Slope and duration of heart-rate deceleration proved to be reliable predictors of smile size, but none of the heart-rate measures correlated with smile duration. Results suggest that the steeper and more rapid the deceleration, the larger the ensuing smile. Possible mechanisms for this finding are discussed.