Reddy and Trevarthen explore what we can learn from emotionally engaging with babies. Theirs is a different approach from 20th-century psychology, in which doubt and detachment play a role in discerning other people's feelings and thoughts. Instead, the authors suggest that emotions are the key to psychological engagement. When interacting with an infant, they argue, anyone--including a researcher--must be emotionally and sympathetically involved to fully understand why an emotion has emerged, and what purpose or effect it may have in the child's experience of life. The authors take three examples of infant behavior--proto-conversation, coyness and shyness, and teasing--to make two points: 1) researchers never would have studied these phenomena had it not been for psychologists' openness to engaging with their own infants' emotions; and 2) engagement allows a richer, more useful interpretation of infant behavior than does detached observation. Furthermore, engagement is crucial not only for obtaining a fuller empirical picture of infant development, but also for the infant's development itself--for well-being, learning, and teaching. For instance, if an infant does not receive an emotion reaction to his emotions, such as his mother responding in kind to his excitement at seeing a flock of birds, he might stop expressing emotions or he might not experience them in quite the same way.