Facial Expressions of Emotion

All 8 Terms & Definitions

Facial Action Coding System (FACS)

Developed by Ekman and Friesen (1978), FACS is an anatomically based system for coding all visible facial movements. Each movement is described as one or more of 44 Action Units (AUs) corresponding to specific muscles. FACS-coded data allows objective comparison across studies and is used in clinical, legal, and AI research.

Action Units (AUs)

The basic units in FACS — each AU corresponds to the contraction of one or more facial muscles. For example, AU6 (orbicularis oculi) raises the cheeks; AU12 (zygomaticus major) pulls the lip corners up. Combinations of AUs define emotional expressions: a genuine smile requires AU6 + AU12.

Duchenne smile

A genuine smile involving both the zygomaticus major (AU12, lip corners up) and the orbicularis oculi pars orbitalis (AU6, crow's feet and raised cheeks). Named after the French neurologist Guillaume-Benjamin Duchenne, who first identified that the orbicularis oculi is involuntary and thus distinguishes genuine from posed smiles.

Micro-expressions

Extremely brief (40–200ms), involuntary facial expressions that may leak genuine emotional states when a person is attempting to conceal their feelings. Ekman proposed that trained observers can detect them, though evidence for reliable detection — even with training — is mixed.

Display rules

Culturally and socially learned norms that prescribe how, when, and to whom emotions should be expressed. Strategies include amplification (exaggerating an expression), suppression (reducing or hiding it), masking (replacing one expression with another), and deamplification. Display rules explain why Japanese participants showed more neutral expressions in the presence of an authority figure in Ekman's studies, despite identical physiological responses.

Social referencing

The process by which infants look to a caregiver's facial expression to determine how to respond to an ambiguous situation. By 12 months, infants reliably use a caregiver's fearful or happy expression to decide whether to approach or avoid a novel object — a finding that demonstrates the communicative function of facial expression early in development.

Facial feedback hypothesis

The proposal that the physical act of making a facial expression can influence the subjective emotional experience associated with it. Fritz Strack's 1988 pencil-in-mouth study (inhibiting vs facilitating smiling) was highly influential but failed to replicate in a large pre-registered study, highlighting the difficulty of testing this hypothesis.

Emotional contagion

The automatic, largely unconscious tendency to mimic and synchronise facial expressions, vocalisations, postures, and movements with those of another person — resulting in convergence of emotional states. Elaine Hatfield and colleagues documented this as a primitive form of empathy operating below the level of conscious simulation.