When people guess at how genuine a person's smile is, or what emotion it's portraying, we don't know exactly what cues we're responding to.
But, the new study found that timing has a lot to do with how people interpret expressions, said study researcher Ehsan Hoque, of the Massachusetts Institute of Technology.
How quickly someone smiles can tell us what they actually mean by it, Hoque said. "Getting the timing right is very crucial if you want to be perceived as sincere and genuine with your smiles," Hoque was quoted as saying by LiveScience.
For their study, the researchers asked participants to act out, or fake, expressions of delight and frustration which were recorded by webcams.
Then the clips were compared with each other and with spontaneous expressions of real joy and frustration, looking for ways to distinguish between the varied emotional states.
When asked to fake frustration, 90 per cent of subjects didn't smile. But when given with a task that caused genuine frustration -- filling out a detailed online form, only to then find the information deleted after pressing the "submit" button -- 90 per cent of them smiled, the researchers found.
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Still images showed little difference between delighted and frustrated smiles, but video analysis showed that the progression of the two smiles was quite different -- Often the happy smiles built up slowly, while frustrated ones appeared quickly but faded fast, the researchers detailed in journal IEEE Transactions on Affective Computing.
When a computer was programmed with information on the timing of the two types of smiles, it was able to tell the difference with 90 per cent accuracy.
The analysis could also be useful in creating computers that respond in ways appropriate to the moods of their users. One goal of the research is to "make a computer that's more intelligent and respectful," Hoque said. (More)