Humans are very good at reading facial expressions of others, but the smile has proved to be more difficult to interpret.
Take, for example the enigmatic smile of the Mona Lisa, which has baffled viewers for centuries.
“There is a wide range of more ambiguous expressions, from which it is difficult to deduce the underlying emotional state. A typical example is the expression of happiness,” said David Beltrán Guerrero, researcher at the University of La Laguna in Spain, an expert who has analyzed the smile’s capacity to distort people’s innate deductive ability.
Beltrán added that while the smile is usually associated with happiness, it can also mask negative or ambiguous feelings.
To see how powerfully smiles can hide other emotions, Beltran and fellow researchers created several faces with differing expressions ranging from smiling mouths and smiling eyes to emotionless or neutral eyes and mouths. The faces were shown to volunteers who were asked to read the different expressions.
Four out of ten participants identified ambiguous expressions as happy. Furthermore, participants were not able to see the subtle differences between a smiling mouth and non-smiling eyes. When the participants had to say whether the faces were happy, sad or something else, the power of the smile was diminished somewhat.
“A smile can cause us to interpret a non-happy expression as happy, except when we are involved in the emotional assessment of said expression,” Beltran said.
The reason smiles can often mask other feelings, the research showed, was that it has a high visual salience. In other words, smiles are attention grabbers and since they’re associated with happiness, that tends to be how people view them.
Other studies have also shown that the smile dominates many of the initial stages of the brain’s processing of faces. Genuinely happy expressions prompt similar electrical activity in the brain as do ambiguous expressions with smiles and non-happy eyes.
By measuring eye movements, it was observed that an ambiguous expression is confused and categorized as happy if the first gaze falls on the area of the smiling mouth, rather than the area of the eyes.