Psychologists specializing in facial expressions are still unsure as to whether a human smile is a tool used for communication or an involuntary expression that conveys our emotional state. Researchers from the Max Planck Institute for Evolutionary Biology in Plön and the Toulouse School of Economics have confirmed that putting on an honest smile that is genuine can influence people to cooperate with you by perceiving you as trustworthy.

Lead researcher Manfred Milinski, from the Max Planck Institute for Evolutionary Biology, and his colleagues recruited 84 participants who acted as trustees and 198 who acted as senders. Trustees were asked to make a 15-second video that would be viewed by senders and influence their decision to either “send” or “keep” a lower stake of four euros or higher stake of eight euros. When the money was sent, the stakes were tripled and the trustee had to decide whether to keep one-half, two-thirds, or all which would leave the sender empty handed.

Senders were much more likely to send money if they trusted the trustee with a smile they considered genuine. Trustees were more likely to produce a smile that was considered genuine when the stakes were higher at eight euros. Senders were also asked to rate the trustee based on the clip in terms of attractiveness, intelligence, trustworthiness, and whether that person’s smile was genuine. Smiles rated as genuine were strongly predictive of trustworthiness.

"The people whose smiles were rated as genuine were also judged to be trustworthy. Senders could actually use the smile as a basis for predicting whether the trustee would share the stakes,” Milinski said in a statement. "This means a smile perceived as genuine is an honest signal that displays a readiness to cooperate. It aims to encourage others to cooperate in situations that require justified trust.”

Smiling is considered a fundamental component of communication in humans as well as animals. For example, among apes, the socially inferior animals will draw back their lips and bare their teeth as a sign of submission to more dominant members of the group. Scientists speculate this may have been where the human smile evolved from. Humans learned to use a smile as a way to mimic a submissive gesture that would convince another human they are trustworthy.

Source: Centorrino S, Djemai E, Hopfensitz A, Seabright P, Milinski M. Honest signaling in trust interactions: smiles rated as genuine induce trust and signal higher earning opportunities. Evolution & Human Behavior. 2014.