Movement of facial muscles, often in response to physiologically arousing or emotionally laden events. Particular configurations of movements can be reliably labeled (e.g., ‘fear’, ‘happy’), universally recognized, and serve as an important vehicle for communicating emotion to other people. Facial expressions (or rather movements) begins early in prenatal life given that the innervation of well-formed facial muscles begins at about eight weeks and is complete by 18 to 21 weeks gestational age. Furthermore, all but one of the facial actions seen in adults can be identified in preterm newborns born in the last trimester of pregnancy.
See Action unit, Communication, Display rules