A genuine smile is a universally acknowledged sign of honesty and moral intention. Therefore, it is of evolutionary importance that humans have the ability to recognise a genuine smile during social interactions.
According to the Facial Action Coding System (FACS) there are two muscle groups involved in the formation of a genuine smile. Action Unit 6 (AU6) raises the cheeks and crinkles the eyes and Action Unit 12 (AU12) lifts the corners of the mouth. Previous research has shown that most people find it impossible to deliberately operate AU6, which makes it very difficult to fake a genuine smile. Therefore, a genuine smile is a reflection of honest emotions and the ability to distinguish a genuine smile helps us to maintain relationships with trustworthy people and avoid being exploited by deceitful people.
Human infants begin to smile in the womb, but do not use smiles as a form of communication until the age of 8-12 months. At preschool-age, children are selective about who they interact with, and they can tell the difference between real and bogus emotions. However, it is not clear what age children develop the ability to distinguish between subtler cues of emotion including genuine and fake smiles.
A series of studies recently published in Evolution and Human Behaviour conducted by Ruiting Song (Max Planck Institute for Evolutionary Anthropology), Harriet Oliver (University of York) and Malinda Carpenter (University of St Andrews) show that this evolutionarily important ability to discriminate between trustworthy and dishonest individuals appears early in development.
A total of 168 children aged between 2 and 5 took part in the study where they were shown pictures of different people and asked ‘who is really smiling?’, ‘who do you like better?’ and ‘who is the nicer person that would share more resources with you?’. The authors also used an eye tracking test to see whether children look longer at genuine or fake smiles when deliberating between them.
The results of the study showed that from the age of 4 children can identify genuine smiles and can discriminate between genuine and fake smiles by the age of 3. Additionally, 4-5 year-old children associate a genuine smile with prosocial meaning; they expect people with genuine smiles to be nicer. This expectation was most clearly observed in older children and girls in particular. The results build on our understanding of a child’s ability to make appropriate choices when deciding who to trust and interact with.
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