All smiles are positive, but some smiles are more positive than others.

Disagreement as to whether all smiling or specific types of smiling index positive emotion early in life was addressed by examining when infants produced different types of smiling and other facial expressions. Thirteen infants were observed weekly from 1 to 6 months of age. Smiling alone—without cheek raising or mouth opening—was relatively more likely than periods without smiling both when mothers were smiling and when infants were gazing at their mothers' faces. Cheek-raise (Duchenne) smiling was relatively more likely than smiling alone only when mothers were smiling. Open-mouth (play) smiling was relatively more likely than smiling alone only when infants were gazing directly at mothers' faces. Smiling involving both cheek raising and mouth opening was relatively likely both when mothers were smiling and when infants were gazing at mothers' faces and became increasingly likely with age when both conditions co-occurred. The cheek-raise and open-mouth dimensions of smiling appear to be associated with, respectively, the amplification of processes of sharing positive affect and of visual engagement that are present to a lesser degree in smiling alone. (PsycINFO Database Record (c) 2010 APA, all rights reserved)