How Mouth Shape Affects Expression

Facial expressions have always been one of the most valuable indicators of others’ emotions. But have you ever seen someone smile and felt something was slightly off? Nathaniel Helwig, faculty, collaborated with other University of Minnesota researchers to examine what a smile is composed of, pinpointing exactly how millimeter-level changes in mouth shape determine whether a face looks genuinely happy, contemptuous, or angry. In the article, “Regularized Multilevel Multinomial Regression for Select-All-That-Apply Responses and High-Dimensional Predictors with Applications to Perception of Facial Expressions,” published in Psychometrika, the researchers outlined how they studied three aspects of mouth placement: mouth angles, the extent of the expression, and dental show (the amount of teeth revealed). The former two mouth placements were positively correlated in all of the 27 smiles that the researchers examined, so they were often paired together during analysis. Additionally, the researchers found a common pattern where an expression’s angle/extent matching the dental show (e.g., a wide smile that shows many teeth) indicated a positive emotion. The reverse was also true: when the angle/extent and the dental show contradicted each other, an expression was perceived as negative.

Using these results, Helwig and his collaborators created a model that can be manipulated to more accurately display different expressions. They hope that this model can help them further study and understand the subtle variations that are integral to what emotion people perceive from a facial expression. They also suggest that future studies could use the same methodology to examine other expressions or face types.