Faces and facial expressions have a special power over us as human beings. While friendly faces make us feel warm and fuzzy, those of our opponents evoke fear or even anger.
So, when do we as children learn to recognise faces and facial expressions? And what lessons can be learned by parents whose facial signals carry a huge amount of information for infants?
The importance of faces for children
Decades of research from many different labs suggest that faces are special for infants right from birth.
To demonstrate this, researchers showed newborns who were only 9 minutes old the image of a face or the image of a scrambled face. Researchers then moved the images along their line of sight. Newborns followed the images of faces for longer than the scrambled faces.
Only a few hours later, newborns will also become adept at differentiating between their mother's face and the faces of strangers, looking longer at images of their own mothers than at images of another woman.
And within a matter of days, they will learn to discriminate between different emotional facial expressions, like happy, sad and surprised faces.
Over the next few months, faces will become a newborn's favourite stimulus as he or she acquires more and more expertise at identifying familiar faces.
By the time infants are 5 months old, they will learn to match the image of an emotional expression (eg a sad face) with its corresponding vocal expression (ie a sad voice). By 5 years, children's ability to recognise and label facial expressions approaches the competence of most adults.
Researchers still aren't sure how infants learn about faces so quickly. Some argue that infants have a biological predisposition to prefer faces right from birth. Others suggest that the huge amount of experience newborns get with faces right away is enough to promote rapid learning.
Others take a middle-of-the-road approach, demonstrating that newborns aren't attracted to faces specifically, but instead prefer looking at any pattern that is top-heavy, having more "stuff" on top. This preference does indeed attract them to faces early in life, but isn't specific to faces until later on, after the infants gain more experience looking at faces.
Learning in the face of uncertainty
Nevertheless, infants' expertise with facial expressions becomes an extremely valuable tool for learning in the second half of the first year.
Around 8 to 12 months of age, infants learn that they can use information from other people's faces - especially their mother's - to help them figure out what to do in new situations.
For example, when infants who are first learning to crawl and walk are presented with a possibly dangerous slope, they look to their mothers' facial expressions for cues. They attempt to descend the slope only when their mothers offer an encouraging smile; they refuse when their mothers discourage them.
Narrowing their choices
This rapidly developing ability to identify different faces and facial expressions is of huge value for infants. However, this also leads to preferences for certain faces or an inability to identify some others.
For example, shortly after birth, infants show a preference for looking at faces judged by adults to be "attractive" over "unattractive" faces.
And 1-year-olds even behave differently around people with more attractive faces, smiling and playing more with attractive adults than with unattractive adults.
Perhaps even more surprising is that infants prefer the faces of their own race by 3 months of age, and have trouble distinguishing between faces of other races by 9 months.
Researchers call this phenomenon "perceptual narrowing": as infants become experts at identifying the faces they see most often, they lose the ability to differentiate between faces that look different from the ones that are most familiar to them.
The face you wear matters
The good news is that exposure to people from other races on a daily basis can erase this effect.
For example, if children live in neighbourhoods where they are exposed to people of other races, they will maintain the ability to differentiate between their faces. Similarly, brief daily exposure to photographs of individuals of other races helps infants maintain the ability to distinguish between them.
Given that a child's world is filled with uncertainty, the faces of those who are most familiar to them can provide an important source of information about what might bring joy, and what might bring fear.
And infants are experts at recognising any facial expressions. This might be worth a thought when reacting to a spider, a curse word or an annoying relative.
Vanessa LoBue is Assistant Professor of Psychology, Rutgers University Newark.