A team of folks from the MIT Media Lab and tech company Affectiva  is studying the nuances of the human face. The goal: develop education products for people with autism spectrum disorders. Here, Affectiva shares some truths about smiles.
1. Researchers classify expressions based on muscle groups. An insincere smile uses the lip-corner pullers, which are voluntary muscles. A sincere smile also involves the cheek raisers and eye crinklers, which are involuntary.
2. Most people--90%, in fact--smile when they're frustrated, and these smiles of frustration have the same physical characteristics as happy smiles.
3. But happy smiles linger longer: After filling out a buggy online form, frustrated study participants smiled for just 7.5 seconds on average. When they watched a video of a baby laughing, smile time jumped to an average of 13.8 seconds.