A 14-month-old infant plays on the floor with her mother when an unfamiliar person enters. The mother stands up, smiles, extends her hand, and walks over to the stranger. Observing this, the infant loses interest and goes back to her toys.

The infant has shown a skill–social referencing–that illustrates her ability to use expressions and actions to understand events. 1 Had the mother responded with fear or anger in her face, voice, or gestures the child would have acted quite differently. Infants learn to read and understand expressions, and use that information to guide their actions. This skill seems to develop rapidly, starting with a sensitivity to emotions, leading later to an understanding of those emotions. The as-yet-unresolved questions about this process are: What information do younger infants detect in expressions? Can they discriminate various expressions? When can they understand the meanings of those expressions?