Genuine smiles convey enjoyment or positive affect, whereas fake smiles conceal or leak negative feelings or motives (e.g. arrogance, contempt, embarrassment), or merely show affiliation or politeness. We investigated the minimum display time (i.e. threshold; ranging from 50 to 1,000 ms) that is necessary to distinguish a fake from a genuine smile. Variants of fake smiles were created by varying the type of non-happy (e.g. neutral, angry, sad, etc.) eyes in blended expressions with a smiling mouth. Participants judged whether faces conveyed happiness or not. Results showed that thresholds vary as a function of type of eyes: blended expressions with angry eyes are discriminated early (100 ms), followed by those with disgusted eyes, fearful, and sad (from 250 to 500 ms), surprised (750 ms), and neutral (from 750 to 1,000 ms) eyes. An important issue for further research is the extent to which such discrimination threshold differences depend on physical or affective factors.