The range of mirror-like processes under investigation should be broadened to include contagious behavior, a class of social behavior that is largely neglected by mirror neuron (MN) researchers. The study of contagious behavior offers the highly desirable properties of identifiable stimuli and motor responses. In contrast, mirror neurons (MNs) seem lost in thought, like disembodied computers not hooked up to printers – full of potential, but short on demonstrated function. Once the relation between the brain's inputs and outputs are defined, we will be well on the way to understanding the mechanism of mirror-like behavior. The study of contagious behavior also offers an economic incentive; it can be a low-budget affair that requires only behavioral observation, not the pricey technology of neurophysiology labs or fMRI machines.

Upon first hearing about MNs, students often ask about their involvement in contagious yawning, laughter, and the like. They are surprised to learn that what seems obvious to amateurs is often ignored by professionals, including the authors of the target article. My present comments focus on yawning and itching/scratching, representative contagious behaviors. These and other contagious acts – laughing, coughing, nausea/vomiting, and vocal crying – are reviewed and contrasted in my recent book, Curious Behavior: Yawning, Laughing, Hiccupping, and Beyond (Provine Reference Provine2012).

Yawns are propagated, being passed from one person to another, in a behavioral chain reaction. This mindless connectedness involves social behavior of the most primal sort. When you yawn contagiously, you do not consciously decide to imitate the observed yawn – it happens automatically. The rippling of yawns through a group is heritable, neurologically programmed social behavior that synchronizes the physiological and behavioral state of the group. Details about the mechanism of this contagion, its evolution, and its development are still being worked out. However, it is clear that the motor act of yawning is phylogenetically ancient, characteristic of most vertebrates, and develops early in prenatal life. Contagious yawning, in contrast, is phylogenetically more modern, confined in various degrees to great apes and, perhaps, dogs and other highly social mammals, and develops several years after birth (see reviews in Provine Reference Provine2005; Reference Provine2012; Walusinski Reference Walusinski2010).

One of the most striking features of adult human yawning is its extreme contagiousness. Almost anything associated with yawning can be a vector for the contagious response, including viewing yawning faces, hearing yawn-related sighs, thinking about yawning, or even reading about yawning, as you are now doing. Given the broad, multimodal spectrum of yawn stimuli, the involvement of a single, narrowly tuned detector or MN sensitive to a specific aspect of a yawning person seems unlikely. Instead, contagion is probably mediated by a variety of detectors of yawn-related stimuli, each capable of producing a yawn. In the pre-MN research era, I proposed that an ethological releasing stimulus was responsible for triggering the stereotyped motor act of contagious yawning (Provine Reference Provine1986; Reference Provine, Heyes and Galef1996). Although the stereotypy of yawning is unchallenged, the detector activated by the releasing stimulus is much more broadly tuned than I had anticipated.

Itch and associated scratching, like yawning, are highly infectious, and the stimulus vector for their contagion is broadly tuned and multimodal (Provine Reference Provine2012). Although eczema, contact dermatitis, and other skin irritation can trigger itch, so can such abstract stimuli as hearing a lecture about itch, viewing itch-causing parasites, or seeing someone else scratching, especially among individuals with pre-existing dermatological conditions (Holle et al. Reference Holle, Warne, Seth, Critchley and Ward2012). The itch/scratch complex provides intriguing research opportunities because it has more potential response variability than stereotyped behaviors such as yawning. For example, Ward et al. (Reference Ward, Burckhardt and Holle2013) investigated how the behavior of a model influences the specific site of itchiness and scratching of an observer. When participants in their study viewed a movie depicting scratching, they were more likely to scratch themselves, but the hand that they used to scratch (left or right) and the site of scratching did not necessarily match the model. Although the model scratched only the arms and chest, the majority of participants viewing the video directed their scratching upward toward their face and hair. The authors concluded, “contagious itchiness may be more driven by vicarious perception of the feeling state (itchiness/unpleasantness) than contagion of the motor act or bodily target” (Ward et al. Reference Ward, Burckhardt and Holle2013, p. 2).

Many questions about contagious behavior and MNs remain. Building on the above results for contagious yawning and itching/scratching, should we conclude that the sufficiency of a variety of multimodal stimulus triggers is evidence against the behavioral involvement of MNs, instances of broadly tuned or multiple MNs, or examples of a different class of mirror-like acts that do not involve MNs? Do environmental contingencies influence the tuning of stimulus triggers or MNs, possibly contributing to the acquisition of multimodality? And what about other contagious behaviors? To what extent does the contagiousness of nausea/vomiting, coughing (but not sneezing), vocal crying, laughing, and yawning involve shared feeling states or another, more specific trigger (Provine Reference Provine2012)? The answers to these questions may come from developmental, comparative, and perceptual studies that are now underway. Whatever the outcome, such research will broaden our understanding of the neurological basis of sociality.