People can sense a genuine smile before it even appears on a face.
But a forced or polite smile does not transmit the same signals, meaning we only detect it when it is visible, reports journal Psychological Science.
Researchers say the study reflects the unique social value of a heartfelt smile, which involves specific movements of muscles around the eyes.
A team from Bangor University had noted that pairs of strangers getting to know one another not only exchanged smiles, they almost always matched the particular smile type, whether genuine or polite.
But they responded much more quickly to their partners’ genuine smiles than their polite smiles, suggesting that they were anticipating the genuine smiles.
In the lab, the results were repeated and data from electrical sensors on participants’ faces revealed that they engaged smile-related muscles when they expected a genuine smile to appear but showed no such activity when expecting polite smiles.
The different responses suggest that genuine smiles are more valuable social rewards, said Dr Erin Heerey.
She said: “These findings give us the first clear suggestion that the basic processes that guide responses to reward also play a role in guiding social behaviour on a moment-to-moment basis during interactions.
“No two interactions are alike, yet people still manage to smoothly coordinate their speech and nonverbal behaviors with those of another person.”
She said that polite smiles typically occur when sociocultural norms dictate that smiling is appropriate.
Genuine smiles, on the other hand, signify pleasure, occur spontaneously, and are indicated by engagement of specific muscles around the eye.
She said the study could help those who find social interactions tricky.
She explained: “As we progress in our understanding of how social interactions unfold, these findings may help to guide the development of interventions for people who find social interactions difficult, such as those with social anxiety, autism, or schizophrenia.”