Development of category formation for faces differing by age in 9- to 12-month-olds: An effect of experience with infant faces.

We examined category formation for faces differing in age in 9- and 12-month-olds, and the influence of exposure to infant faces on such ability. Infants were familiarized with adult or infant faces, and then tested with a novel exemplar from the familiarized category paired with a novel exemplar from a novel category (Experiment 1). Both age groups formed discrete categories of adult and infant faces, but exposure to infant faces in everyday life did not modulate performance. The same task was conducted with child versus infant faces (Experiment 2). Whereas 9-month-olds preferred infant faces after familiarization with child faces, but not child faces after familiarization with infant faces, 12-month-olds formed discrete categories of child and infant faces. Moreover, more exposure to infant faces correlated with higher novel category preference scores when infants were familiarized with infant faces in 12-month-olds, but not 9-month-olds. The 9-month-old asymmetry did not reflect spontaneous preference for infant over child faces (Experiment 3). These findings indicate that 9- and 12-month-olds can form age-based categories of faces. The ability of 12-month-olds to form separate child and infant categories suggests that they have a more exclusive representation of face age, one that may be influenced by prior experience with infant faces.