TheFacial Action Coding System (F.A.C.S.) is a system totaxonomize humanfacial movements by their appearance on the face, based on a system originally developed by a Swedishanatomist namedCarl-Herman Hjortsjö.^[1] It was later adopted byPaul Ekman andWallace V. Friesen, and published in 1978.^[2] Ekman, Friesen, and Joseph C. Hager published a significant update to F.A.C.S. in 2002.^[3] Movements of individualfacial muscles are encoded by the F.A.C.S. from slight different instant changes in facial appearance. It has proven useful topsychologists and toanimators.

In 2009, a study was conducted to study spontaneous facial expressions in sighted and blind judo athletes. They discovered that many facial expressions are innate and not visually learned.^[4]

Using the F.A.C.S.,^[5] human coders can manually code nearly any anatomically possible facial expression, deconstructing it into the specific "action units" (A.U.) and their temporal segments that produced the expression. As A.U.s are independent of any interpretation, they can be used for any higher-order decision-making process includingrecognition of basic emotions, or pre-programmed commands for an ambient intelligent environment. The F.A.C.S. manual is over five hundred pages in length and provides the A.U.s, as well as Ekman's interpretation of their meanings.

The F.A.C.S. defines A.U.s as contractions or relaxations of one or more muscles. It also defines a number of "action descriptors", which differ from A.U.s in that the authors of the F.A.C.S. have not specified the muscular basis for the action and have not distinguished specific behaviors as precisely as they have for the A.U.s.

For example, the F.A.C.S. can be used to distinguish two types ofsmiles as follows:^[6]

• the insincere and voluntaryPan-Am smile: contraction ofzygomatic major alone
• the sincere and involuntaryDuchenne smile: contraction ofzygomatic major and inferior part oforbicularis oculi.

The F.A.C.S. is designed to be self-instructional. People can learn the technique from a number of sources including manuals and workshops,^[7] and obtain certification through testing.^[8]

Although the labeling of expressions currently requires trained experts, researchers have had some success in using computers to automatically identify the F.A.C.S. codes.^[9] One obstacle to automatic FACS code recognition is a shortage of manually coded ground truth data.^[10]

Baby F.A.C.S. (Facial Action Coding System for Infants and Young Children)^[11] is a behavioral coding system that adapts the adult F.A.C.S. to code facial expressions in infants aged 0–2 years. It corresponds to specific underlying facial muscles, tailored to infant facial anatomy and expression patterns.

It was created by Dr. Harriet Oster and colleagues to address the limitations of applying adult F.A.C.S. directly to infants, whose facial musculature, proportions and developmental capabilities differ significantly.

The use of the F.A.C.S. has been proposed for use in the analysis ofdepression,^[12] and the measurement of pain in patients unable to express themselves verbally.^[13]

The original F.A.C.S. has been modified to analyze facial movements in several non-human primates, namelychimpanzees,^[14]rhesus macaques,^[15]gibbons, andsiamangs,^[16] and orangutans.^[17] More recently, it was developed also for domesticated species, including dogs,^[18] horses^[19] and cats.^[20] Similarly to the human F.A.C.S., the non-human F.A.C.S. has manuals available online for each species with the respective certification tests.^[21]

Thus the F.A.C.S. can be used to compare facial repertoires across species due to its anatomical basis. A study conducted by Vick and others (2006) suggests that the F.A.C.S. can be modified by taking differences in underlying morphology into account. Such considerations enable a comparison of the homologous facial movements present in humans and chimpanzees, to show that the facial expressions of both species result from extremely notable appearance changes. The development of F.A.C.S. tools for different species allows the objective and anatomical study of facial expressions in communicative and emotional contexts. Furthermore, an interspecial analysis of facial expressions can help to answer interesting questions, such as which emotions are uniquely human.^[22]

The Emotional Facial Action Coding System (E.M.F.A.C.S.)^[23] and the Facial Action Coding System Affect Interpretation Dictionary (F.A.C.S.A.I.D.)^[24] consider only emotion-related facial actions. Examples of these are:

F.A.C.S. coding is also used extensively incomputer animation, in particular forcomputer facial animation, with facial expressions being expressed asvector graphics of A.Us.^[25] F.A.C.S. vectors are used as weights forblend shapes corresponding to each A.U., with the resulting face mesh then being used to render the finished face.^[26][27]Deep-learning techniques can be used to determine the F.A.C.S. vectors from face images obtained duringmotion capture acting,facial motion capture or other performances.^[28]

For clarification, the F.A.C.S. is an index of facial expressions, but does not actually provide any biomechanical information about the degree of muscle activation. Though muscle activation is not part of the F.A.C.S., the main muscles involved in the facial expression have been added here.

Action units (A.U.s) are the fundamental actions of individual muscles or groups of muscles.

Action descriptors (A.D.s) are unitary movements that may involve the actions of several muscle groups (e.g., a forward‐thrusting movement of the jaw). The muscular basis for these actions has not been specified and specific behaviors have not been distinguished as precisely as for the A.U.s.

For the most accurate annotation, the F.A.C.S. suggests agreement from at least two independent certified F.A.C.S. encoders.

Intensities of the F.A.C.S. are annotated by appending letters A–E (for minimal-maximal intensity) to the action unit number (e.g. A.U. 1A is the weakest trace of A.U. 1 and A.U. 1E is the maximum intensity possible for the individual person).

• A Trace
• B Slight
• C Marked or pronounced
• D Severe or extreme
• E Maximum

There are other modifiers present in F.A.C.S. codes for emotional expressions, such as "R" which represents an action that occurs on the right side of the face and "L" for actions which occur on the left. An action which is unilateral (occurs on only one side of the face) but has no specific side is indicated with a "U" and an action which is bilateral but has a stronger side is indicated with an "A" for "asymmetric".

These codes are reserved for recording information about gross behaviors that may be relevant to the facial actions that are scored.

• Computer facial animation
• Computer processing of body language
• Emotion classification
• Facial electromyography
• Facial feedback hypothesis
• Facial muscles
• Microexpression

• Paul Ekman's articles relating to F.A.C.S.
• Paul Ekman's Facial Action Coding System (F.A.C.S.)
• More information on the different animal F.A.C.S. projects
• New Yorker article discussing F.A.C.S.
• Details from 1978 edition of F.A.C.S.
• Site at WPI
• download ofCarl-Herman Hjortsjö, Man's face and mimic language"Archived 2022-08-06 at theWayback Machine (the original Swedish title of the book is: "Människans ansikte och mimiska språket". The correct translation would be: "Man's face and facial language")

• Facial expressions
• Encodings
• Anatomical simulation
• Animal communication
• 1978 introductions

• Webarchive template wayback links
• Articles with short description
• Short description is different from Wikidata