Facial Action Coding System provide methodology of coding facial expressions on the basis of facial muscle movement. The face as a whole communicates emotional states. The seven universally recognized emotions shown through facial expressions are happiness, sadness, surprise, anger, fear and contempt. But human faces are capable of more than 10,000 different expressions (sometimes very subtle) using muscles of facial expressions. 
For instance, autism is a developmental disorder involving qualitative impairments in social interaction. One source of those impairments are difficulties creating facial expressions of emotion. Difficulties with facial expressions may arise from deficits in a motivation to express positive emotions with others. The difficulties may also stem from physiological problems in physically creating expressions that are due to damage to areas of the brain that control the facial nerve (which produces those expressions). Source : https://www.autismspeaks.org/science/grants/automated-measurement-facial-expression-autism-deficits-facial-nerve-function