A

Psychologists hoping to unlock the secrets of human emotion have focused on infants, as young babies can provide indicators of early emotional growth and development. Among these changes are modifications of facial expressions that provide cues for determining how an infant's reaction to a given experience or situation conveys meaning. Infants' facial expressions are particularly useful for understanding the nature of human emotion because they have not yet been significantly conditioned by social norms and conventions; the lack of social conditioning is perhaps best expressed when infants smile, which makes this facial expression an especially interesting object of study.

B

The most fundamental type of smile in infants is the endogenous smile, which can occur in the first week following birth and is usually associated with sleeping. Endogenous smiles happen automatically due to unconscious changes in the nervous system and are not caused by external stimuli – such as tickling or a pleasing sound – is known as an exogenous smile. Exogenous smiles occur when the infant is awake, and they begin to appear by the second or third week of life.

C

An important type of exogenous smile is the 'social smile', which, as its name suggests, arises through social interaction. The social smile is particularly associated with grinning in response to seeing the faces of mothers and other caregivers as well as hearing their voices. Yet social smiles also occur in interactions with strangers, and the term may even be extended to describe how an infant smiles at inanimate, though familiar, objects like teddy bears. According to psychologist Daniel G. Freedman, generalised social smiles develop by the end of the first month, and at around five weeks old, selective social smiling begins. It is in this selective stage that the child learns to smile in response to familiar faces.

D

Experts view the social smile as a strong indicator of infants' growing curiosity and a higher level of engagement with their surroundings. It follows, therefore, that the social smile, as one of explicit interaction, also indicates a child's growing awareness of himself or herself as a distinct individual and active agent in a social environment. Of crucial importance is what is known as social referencing, in which an infant looks to his or her caregiver before reacting to a situation. Many studies have revealed that around seven months of age, infants begin to become scared of strangers and often show fear or distress when encountering unfamiliar faces. However, a study by the psychologists Feiring, Lewis, and Starr revealed that by fifteen months, babies were cautiously attentive to their mother's response to strangers. In an experiment, the researchers found that if the mother smiled at a stranger, the baby was far more likely to smile.

E

Technically, some experts solely use the term social smile for smiles that involve only the zygomaticus major muscles, which are responsible for raising the corners of the lips. If smiles are accompanied by open mouths and contraction of the orbicularis oris muscles – movements that make the outer corners of the eyes wrinkle – they are said to be 'emotional' smiles. This view is partly a result of reductionistic attempts to directly connect human emotions with anatomical movements, which have been proved to be incorrect. One example of this was Paul Eckman's Facial Action Coding System (FACS) which attempted to give emotions an objective basis and allow investigators to identify emotional expressions without cultural bias.

F

According to this method of analysis, the social smile is primarily a false smile. This argument has some merit because the social smile can happen even in the absence of emotion. Infants sometimes merely imitate what they see, and the social smile at times may be nothing more than an attempt to get some form of support from the mother. And it is well known that the social smile continues into adulthood. It is common to use a social smile in numerous social contexts in a way that may be fake. For instance, imagine a grown person receiving a birthday present that is a huge disappointment. Rather than expressing a true emotion via a frown, the person receiving the gift is far more likely to smile.

G

The difficulty with distinguishing 'social' from 'emotional' smiles, however, is that there is a broad spectrum of facial expressions and emotional states between the two extremes. Certainly, a smile can be both social and emotional at the same time. Viewed this way, social smiles can vary tremendously in the extent of emotion they convey, from simple grins that may show a less intense emotion to fuller smiles involving the whole face that are displays of pure joy.