Robert R. Provine (1989).
Contagious yawning and infant imitation.
Bulletin of the Psychonomic Society 27. 2 (Mar): 125-126.

Suggests that the neonate's presumed ability to imitate the facial expressions or gestures of adult models may be the result of ethological fixed-action patterns released by sign stimuli. Contagious yawning (CY) of adults is a precedent for such a facial fixed-action pattern (i.e., a yawn) triggered by a facial stimulus (i.e., an observed yawn). The study of CY can provide insights into both the problem of infant imitation and the more general issues concerning the detection and processing of information about faces. CY also provides a reliable classroom demonstration of released behavior.

Provine, Robert R. (1989).
Faces as releasers of contagious yawning:
An approach to face detection using normal human subjects.
Bulletin of the Psychonomic Society 27. 3 (May): 211-214.

360 psychology students were divided into 12 experimental groups and participated in a single experimental session. The yawn-evoking potency of variations in a 5-min series of 30 videotaped repetitions of a yawning face were compared with each other and with a series of 30 videotaped smiles to determine the ethological releasing stimulus for the fixed-action pattern of yawning and to understand the more general process of face detection. Animate video images of yawning faces in several axial orientations evoked yawns in more Subjects than did featureless or smiling faces, and no single feature, such as a gaping mouth, was necessary to evoke yawns. The yawn recognition mechanism is neither axially specific nor triggered by an isolated facial feature.