Psychologists studying the function and meaning of facial expression are mainly working in the domain of emotion psychology and/or nonverbal communication. Facial expressions play an important role in the communication of emotions and for the intrapsychic regulation of emotions. Additionally, facial expressions are important for signalling relationships, e.g. installing, maintaining or aborting a relationship. I will first try to show in a more systematic manner what kind of information the face can provide and why it is so difficult to draw correct inferences. Then I will give an introduction into a method for coding facial behaviour objectively: The Facial Action Coding System (FACS) developed by Ekman and Friesen (1978). Following the pioneering work of Duchenne (1872/1990) and Hjörtsjö (1970), FACS is based on the anatomy of the human face and allows the coding of any facial action in terms of the smallest visible unit of muscular activity. These smallest visible units are called Action Units, and each Action Unit is referred to by a numerical code. The FACS coding manual gives a detailed description of all the appearance changes occurring with a given Action Unit. This description lists the parts of the face that have moved and the direction of their movements, the wrinkles that have appeared or have deepened, and the alterations in the shape of the facial parts. With FACS, data collection is independent of data interpretation. There is no relation between the code and the meaning of the facial action.
In the final part, I will discuss the advantages and limits of FACS with respect to its possible application to classifying "mouthing" and "mouth gestures" in sign language. An important advantage of a comprehensive technique like FACS is that it provides a common nomenclature for descriptions of facial behaviour. If many investigators were to use the same comprehensive technique, comparisons of findings would be facilitated because investigators, even those who use it selectively, would choose their units from a single list of facial actions.