Infants are sensitive to eye expressions of fear and direction of focus. These responses operate without conscious awareness. ~ German psychologist Tobias Grossmann
Both the pupil and sclera – the blacks and whites of the eyes – act as social signals.
Pupils marginally dilate when viewing something of extraordinary interest. Women tend to be more aware of this social signal than men, though that may be said for most subtle social signals.
The sclera provides a powerful social signal. Humans are the only one of 221 primates that have whites of the eyes that are easily seen.
By ~7 months, babies know to follow the direction a person is looking by the sclera, not the head.
Apes and humans readily follow the gaze direction of others. But, owing to the high-contrast white sclera, only humans can discriminate between the eyes and the head. Apes follow the direction of the head, not the eyes.
Coordinating visual attention is important in cooperation. Infants acquire language through joint activities with others. Sharing a focus of attention lets a little one readily learn a word for an object or activity in question.