Effects of normal and perturbed social play on the duration and amplitude of different types of infant smiles

Different types of smiling varying in amplitude of lip corner retraction were investigated during 2 mother-infant games--peekaboo and tickle--at 6 and 12 months and during normally occurring and perturbed games. Using Facial Action Coding System (FACS), infant smiles were coded as simple (lip corner retraction only), Duchenne (simple plus cheek raising), play (simple plus jaw drop), and duplay (Duchenne plus jaw drop). In addition, again using FACS, the amplitude of lip corner retraction was coded on a 5-point scale. Rather than a single smile expression that differs only in amplitude, the authors found a complex family of different smile expressions differing in their duration and amplitude as a function of game, setup versus climax of the game, and perturbation. Both type of smiling and amplitude of smiling appear to be controlled independently by the infant in relation to the context. These findings reveal systematic and context-specific nuances in infant smiles in the 2nd half of the first year.