Everybody everywhere smiles. Across all cultures, people smile. In fact, smiling may be an evolutionary throwback to the silent bared-teeth display of nonhuman primates.
Although babies start smiling in the womb, social smiles develop at around 6 weeks of age. Between 8 and 12 months, infants smile to communicate, such as to share interest and attention with their parents.
Not all smiles are equal. Genuine smiles (also called Duchenne smiles) involve the muscles affecting the eyes, mouth, cheeks and eyebrows; they are hard to feign. Genuine smiles are a response to positive emotion or can convey things like joy, pleasure, encouragement and appreciation.
Fake smiles, however, don’t involve all these facial features. A fake smile could be motivated by a desire to hide, justify or moderate something that’s negative like a criticism or lie.
In a 2016 study published in Evolution and Human Behavior, Song and co-authors examined at which age young children are first able to recognize real smiles as well as their preference for real smiles. The researchers also examined the kids’ understanding of how genuine smiles are tied to prosocial behavior. Of note, prosocial behavior is voluntary behavior intended to help or benefit others, such as honesty and cooperation.
In total, Song and colleagues assessed 168 children aged between 2 and 5 years. Here are some of the results:
People need to cooperate with others to survive; it’s imperative that people choose others with whom they can cooperate. In many contexts, we can identify who will be cooperative based on either reputation or experience with this person.
Without such insight, however, we need other authentic signals that indicate cooperation. Because genuine smiles are hard to fake, they make good signals. The presence of a genuine smile could indicate who is cooperative as well as who won’t be exploitative or dishonest.
This research suggests that even at a very young age, children can discriminate between real and fake smiles and thus decide with whom to cooperate, interact and trust.
Furthermore, according to the researchers: “These results demonstrate that the origins of this evolutionarily important form of partner choice appear early in development.”
On a related note, a young child’s ability to discriminate between real and fake smiles is reminiscent of developmental milestones. A milestone is a behavior or physical ability predictably demonstrated in an infant or child during growth and development. For example, babbling, walking, talking, skipping and drawing a circle are all milestones.
Mai, X., Ge, Y., Tao, L., Tang, H., Liu, C., & Luo, Y. (2011). Eyes are the windows to the Chinese soul: Evidence from the detection of real and fake smiles. http://dx.doi.org/10.1371/journal.pone.0019903
Song, R., Over H., & Carpenter M. (2016). Young children discriminate genuine from fake smiles and expect people displaying genuine smiles to be more prosocial. Evolution and Human Behavior, 37, 490-501.