Two experiments are described which explore the relationship between parental reports of infants' receptive vocabularies at 1 ; 6 (Experiment 1a) or 1 ; 3, 1 ; 6 and 1 ; 9 (Experiment 1b) and the comprehension infants demonstrated in a preferential looking task. The instrument used was the Oxford CDI, a British English adaptation of the MacArthur-Bates CDI (Words & Gestures). Infants were shown pairs of images of familiar objects, either both name-known or both name-unknown according to their parent's responses on the CDI. At all ages, and on both name-known and name-unknown trials, preference for the target image increased significantly from baseline when infants heard the target's label. This discrepancy suggests that parental report underestimates infants' word knowledge.

During the second year of life, infants develop a preference to attach novel labels to novel objects. This behavior is commonly known as “mutual exclusivity” (Markman, 1989). In an intermodal preferential looking experiment with 19.5‐ and 22.5‐month‐olds, stimulus repetition was critical for observing mutual exclusivity. On the first occasion that a novel label was presented with 1 familiar object and 1 novel object, looking behavior was unsystematic. However, on reexposure to the same stimuli, 22.5‐month‐olds looked preferentially at the novel object prior to the re‐presentation of the novel label. These findings suggest a powerful memory mechanism for novel labels and objects, enabling mutual exclusivity to emerge across repeated exposures to potential referents.

Studies report that infants as young as 1 ; 3 to 1 ; 5 will seek out a novel object in response to hearing a novel label (e.g. Halberda, 2003; Markman, Wasow & Hansen, 2003). This behaviour is commonly known as the ‘mutual exclusivity’ response (Markman, 1989; 1990). However, evidence for mutual exclusivity does not imply that the infant has associated a novel label with a novel object. We used an intermodal preferential looking task to investigate whether infants aged 1 ; 4 could use mutual exclusivity to guide their association of novel labels with novel objects. The results show that infants can successfully map a novel label onto a novel object, provided that the novel label has no familiar phonological neighbours. Therefore, as early as 1 ; 4, infants can use mutual exclusivity to form novel word–object associations, although this process is constrained by the phonological novelty of a label.