Understanding category members as "the same sort of thing": explicit categorization in ten-month infants.

Although it is often acknowledged that classification exists in the first year of life, it has been suggested that infants are capable only of implicit categorization, recognizing that something is or is not familiar. In contrast, older children are thought to compare the stimuli or objects they categorize and to explicitly equate different category members. 2 habituation experiments were conducted in an attempt to determine whether 10-month-old infants are capable of explicit categorization. The approach taken was to insert nonmembers into the familiarization sequence in a categorization task. The nonmembers were designed so as to have a predictable effect on infants' performance whether or not these stimuli were included as infants processed the categorical information. The results suggest that infants do explicitly equate category members. Infants appeared to disregard the nonmembers in the familiarization sequence, basing their categorization response instead on the set of instances that were "the same sort of thing."