Categorical responding in 15-month-olds: Influence of the noun-category bias and the covariation between visual fixation and auditory input

According to constraints/bias accounts of word learning, children learn words rapidly and accurately because they possess the uniquely linguistic knowledge that nouns refer to objects in a category. These accounts predict that (a) when input is provided, children will organize objects categorically in the presence of words or nouns but not in their absence, and (b) when nouns are present, manipulation of nonlinguistic variables should not disrupt categorical responding. Using a familiarization-discrimination paradigm, a preliminary experiment confirmed that, for the target category, 15-month-olds did not respond categorically in the absence of input. Experiments 1 and 2 (labeling input) and Experiments 3 and 4 (instrumental music input) revealed successful categorization when either input was perfectly correlated with an infant's fixation of an object. However, in all four experiments, when this perfect covariation was degraded, infants did not categorize, even when nouns were present (Experiments 1 and 2). These outcomes are not consistent with the predictions of bias accounts and they considerably weaken the case for a psychologically real noun-bias prior to the vocabulary explosion. The reported findings are more consistent with children's use of manifold sources of information as cues to responding categorically.