Humans generate a number of different movements of their facial muscles to express emotions. Some basic facial expressions originally served an adaptive, biological function, such as the expression of fear signalling a threat to others. The ability to perceive and understand facial emotion also evolved to act as ‘social glue’, allowing humans to work together and communicate successfully in complex groups.
Scientists code facial movements using the Facial Action Coding System. Each separate movement — such as jaw dropping, nose wrinkling or brow lowering — is called an action unit (AU) and given a number (e.g. AU26: jaw drop). Different facial expressions can then be categorised as a combination of AUs. For example, two types of smile are distinguished — insincere and voluntary (AU12: contraction of the muscles at the lip corners) and sincere and involuntary (AU12+AU6: additional contraction of muscles to raise the cheeks).
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