In order to get around the human ability to mask emotions and more clearly define facial expressions for particular emotions, Ekman and his associates:

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Ekman and his associates developed a special coding system called the Facial Action Coding System (FACS) to identify and analyze the numerous facial muscles involved in expressing emotions. This system allowed for more objective and reliable identification of facial expressions regardless of a person's ability to mask their emotions or display them in a subtle manner.

In Order to Get Around the Human Ability to Mask