Infants Understand That Surprise Is Caused by Novel Information

Infants use others' emotional expressions to make inferences about the world, for example by inferring candidate causes of their happiness or sadness. However, one emotion-surprise-is less clearly linked to specific external triggers. Rather, surprise can be invoked by any entity or event, so long as it offers novel information. Surprise is thus unique among the emotions, because once an observer has encountered something new it ceases to be surprising. Do infants understand this dynamic relationship between surprise and information? Here we addressed this using a violation-of-expectation paradigm with 17- to 24-month-old infants. In Experiment 1 (N = 36), we found that infants who saw a person express surprise each time she peered into a box inferred that the person must have seen multiple things inside, whereas infants made no such inference after seeing the person repeatedly express happiness. In Experiment 2 (N = 36), we found that infants did not merely expect to see one object revealed for each instance of surprise expressed; infants recognized that surprise can also be triggered by the absence of objects. Finally, in Experiment 3 (N = 36) we found that infants recognize that surprise is a response to information that is new with respect to a particular observer. Infants were sensitive not only to the number of surprise expressions produced, but also to which person in a multi-person scene had expressed surprise. Together, these results suggest that by the end of their second year, infants represent surprise as the reflection of dynamic changes in the epistemic states of others.