The Facial Action Coding System (FACS) is a system to taxonomize human facial movements by their appearance on the face, based on a system originally developed by a Swedish anatomist named Carl-Herman Hjortsjö.^[1] It was later adopted by Paul Ekman and Wallace V. Friesen, and published in 1978.^[2] Ekman, Friesen, and Joseph C. Hager published a significant update to FACS in 2002.^[3] Movements of individual facial muscles are encoded by the FACS from slight different instant changes in facial appearance. It has proven useful to psychologists and to animators.

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 FACS^[5] human coders can manually code nearly any anatomically possible facial expression, deconstructing it into the specific "action units" (AU) and their temporal segments that produced the expression. As AUs are independent of any interpretation, they can be used for any higher order decision making process including recognition of basic emotions, or pre-programmed commands for an ambient intelligent environment. The FACS manual is over 500 pages in length and provides the AUs, as well as Ekman's interpretation of their meanings.

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

For example, the FACS can be used to distinguish two types of smiles as follows:^[6]

• the insincere and voluntary Pan-Am smile: contraction of zygomatic major alone
• the sincere and involuntary Duchenne smile: contraction of zygomatic major and inferior part of orbicularis oculi.

The FACS 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 FACS codes.^[9] One obstacle to automatic FACS code recognition is a shortage of manually coded ground truth data.^[10]

For clarification, the FACS is an index of facial expressions, but does not actually provide any bio-mechanical information about the degree of muscle activation. Though muscle activation is not part of the FACS, the main muscles involved in the facial expression have been added here.

Action units (AUs) are the fundamental actions of individual muscles or groups of muscles.

Action descriptors (ADs) 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 AUs.

For the most accurate annotation, the FACS suggests agreement from at least two independent certified FACS encoders.

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