The authors investigated the differences between 8-year-olds (n = 80) and adults (n = 80) in recognition of felt versus faked enjoyment smiles by using a newly developed picture set that is based on the Facial Action Coding System. The authors tested the effect of different facial action units (AUs) on judgments of smile authenticity. Multiple regression showed that children base their judgment on AU intensity of both mouth and eyes, with relatively little distinction between the Duchenne marker (AU6 or "cheek raiser") and a different voluntary muscle that has a similar effect on eye aperture (AU7 or "lid tightener"). Adults discriminate well between AU6 and AU7 and seem to use eye-mouth discrepancy as a major cue of authenticity. Bared-teeth smiles (involving AU25) are particularly salient to both groups. The authors propose and discuss an initial developmental model of the smile recognition process.