What makes a successful smile?

A touch of asymmetry and the right amount of teeth.

To help people who can’t form facial expressions because of stroke or injury, researchers wanted to know what makes a smile successful. So they went to the Minnesota State Fair and showed people computer animated smiles, with each smile varying by mouth angle, extent of smile, and the number of teeth on show, as well as how symmetrically the smile developed. Fairgoers were then asked to rate smiles based on effectiveness, genuineness, pleasantness and perceived emotional intent. We spoke to author Nathaniel Helwig about the results of the PLOS ONE study.

ResearchGate: What did your study set out to determine?

Nathaniel Helwig: Sofia Lyford-Pike, a facial plastic and reconstructive surgeon, wanted to know how to create the perfect smile for her patients, in order to help them better reintegrate back into society after a stroke, disease, or injury. To discover the perfect smile, she sought out the help of Stephen Guy, a computer scientist, and myself, a psychologist and statistician. Together, we designed an experiment to determine which combinations of spatial and temporal features join to create successful smiles.

RG: Why study smiling in such detail?

Helwig: Decades of psychological research has revealed that different facial expressions can communicate different emotional intents. Smiles are arguably the most important facial expression, given their frequent use in day-to-day interpersonal interactions. Partial facial paralysis robs an individual of their ability to smile, which can have significant psychological and social consequences. To improve outcomes for these individuals, it is imperative to have a detailed understanding of what exactly constitutes a “successful smile.”

RG: What does it mean for a smile to be “successful?”

Helwig: In our study, we defined a successful smile as one that is rated to be effective, pleasant, and genuine in the colloquial sense of these words. We purposely avoided providing study participants with a concrete definition of a “successful smile,” so that our results could be interpreted in a colloquial—instead of a clinical—sense. Our study aims to fuse many colloquial opinions to improve the clinical definition of what makes a smile “successful.”

RG: How did you determine the characteristics of a successful smile?

Helwig: We created a collection of 27 computer-animated facial expressions by systematically manipulating three features that are commonly assessed in facial reconstructive surgery: the angle of the smile, the extent of the smile, and the amount of teeth displayed during the smile. We then asked over 800 individuals from the Minnesota State Fair to rate the smiles in terms of their effectiveness, genuineness, pleasantness, and perceived emotional intent.

Dental show effect at different levels of angle-extent. (a) Two smiles with smaller angle-extent combinations. (b) Two smiles with larger angle-extent combinations. Increasing dental show makes the smile worse (i.e., less successful) for (a) and better (i.e., more successful) for (b). Credit: Helwig et al (2017).

RG: So what are your results?

Helwig: The results reveal that no single smile is "perfect" compared to the others. Instead, there exists a window of parameters, or “smile sweet spot,” which creates successful smiles. We observed somewhat of a Goldilocks Phenomenon, such that successful smiles needed just the right amount of teeth for the given smile angle and extent. Also, we found that too much smile angle and extent produced fake and creepy smiles, which contradicts the idea that “more is always better.” Interestingly, we discovered that smiles with slight timing asymmetries are more successful than perfectly symmetric smiles, but asymmetries larger than 125 milliseconds were detrimental.

RG: Did you just look at the mouth? How important are the eyes in a smile?

Helwig: Past research has revealed that the eyes are important—particularly for distinguishing between “Duchenne” (genuine) versus “Pan Am” (fake) smiles. However, our study looked at just the mouth for two reasons. One, smiling impairment due to restricted mouth motion increases depressive symptoms for patients with partial facial paralysis. Two, existing surgical interventions have shown particular success in restoring mouth movement. Our results reinforce the idea that the lower half of the face, particularly mouth movement, is a prominent factor for determining the emotional intent of a facial expression.

RG: Should people with huge smiles train themselves to smile differently?

Helwig: A future goal is to provide personalized biofeedback information on how an individual can retrain their smile to be more successful. Our current study suggests that “successful smiles” need the correct amount of teeth for the given angle and extent. Thus, individuals with larger smiles should show larger amounts of teeth, and individuals with limited mouth movement should be encouraged to form closed-mouth smiles.

RG: What applications could this research have?

Helwig: The knowledge gained from this research will improve facial reanimation surgeries and therapies by providing precise spatial and temporal benchmarks for creating a successful smile. Beyond these applications, our research can help animators create more believable animation for virtual humans in movies and virtual reality. Digital animations of facial expressions can be used to develop personalized biofeedback applications (e.g., for a smart phone or tablet) to help people better display and recognize different facial expressions of emotions, which could be useful for treating disorders such as autism.

Featured image courtesy of Alvin Mahmudov.