Faces are a complex and rich source of social, emotional and linguistic signals. We rely on all of these signals to communicate with one another through a complex and dynamic dance that depends on each partner being able to read the other’s signals. Interestingly, even when we can see whole faces, we often have trouble telling what other people are feeling. For instance, as the psychologist Lisa Feldman Barrett has noted, we can interpret a smile as meaning “I’m happy,” “I like you” or “I’m embarrassed”. So, seeing partially visible faces robs us of a plethora of linguistic signals that are essential for communication.
Babies and young children see and hear communicative signals and learn to attach meanings to them through their everyday interactions with their caregivers and social partners. Take, for example, a baby at a birthday party or in a day care center where several masked people can be heard and seen talking. To figure out which face goes with which voice, that baby must learn that the mouth is the source of spoken language and that looking at the mouth is essential for figuring out whether a particular person’s face goes with a particular voice.Masks Can Be Detrimental to Babies’ Speech and Language Development – Scientific American