Reliable facial muscle activation enhances recognizability and credibility of emotional expression.

We tested Ekman's (2003) suggestion that movements of a small number of reliable facial muscles are particularly trustworthy cues to experienced emotion because they tend to be difficult to produce voluntarily. On the basis of theoretical predictions, we identified two subsets of facial action units (AUs): reliable AUs and versatile AUs. A survey on the controllability of facial AUs confirmed that reliable AUs indeed seem more difficult to control than versatile AUs, although the distinction between the two sets of AUs should be understood as a difference in degree of controllability rather than a discrete categorization. Professional actors enacted a series of emotional states using method acting techniques, and their facial expressions were rated by independent judges. The effect of the two subsets of AUs (reliable AUs and versatile AUs) on identification of the emotion conveyed, its perceived authenticity, and perceived intensity was investigated. Activation of the reliable AUs had a stronger effect than that of versatile AUs on the identification, perceived authenticity, and perceived intensity of the emotion expressed. We found little evidence, however, for specific links between individual AUs and particular emotion categories. We conclude that reliable AUs may indeed convey trustworthy information about emotional processes but that most of these AUs are likely to be shared by several emotions rather than providing information about specific emotions. This study also suggests that the issue of reliable facial muscles may generalize beyond the Duchenne smile.