How and why could smiling influence physical health? A conceptual review.

Smiling has been a topic of interest to psychologists for decades, with a myriad of studies tying this behavior to well-being. Despite this, we know surprisingly little about the nature of the connections between smiling and physical health. We review the literature connecting both naturally occurring smiles and experimentally manipulated smiles to physical health and health-relevant outcomes. This work is discussed in the context of existing affect and health-relevant theoretical models that help explain the connection between smiling and physical health including the facial feedback hypothesis, the undoing hypothesis, the generalized unsafety theory of stress, and polyvagal theory. We also describe a number of plausible pathways, some new and relatively untested, through which smiling may influence physical health such as trait or state positive affect, social relationships, stress buffering, and the oculocardiac reflex. Finally, we provide a discussion of possible future directions, including the importance of cultural variation and replication. Although this field is still in its infancy, the findings from both naturally occurring smile studies and experimentally manipulated smile studies consistently suggest that smiling may have a number of health-relevant benefits including beneficially impacting our physiology during acute stress, improved stress recovery, and reduced illness over time.