Ekman and Friesen's model

Ekman and Friesen’s model of facial expressions categorizes emotions based on specific facial movements. They identified the "facial action units"—distinct muscle movements—associated with different feelings like happiness, sadness, anger, fear, surprise, and disgust. Their work shows that some expressions are universal across cultures, caused by innate muscle movements, while others are learned. This model helps in understanding how emotions are revealed through the face and is widely used in psychology, law enforcement, and technology to interpret genuine emotional states through facial cues.