This study examines the role of exposure to speech in children's early vocabulary growth. It is generally assumed that individual differences in vocabulary depend, in large part, on variations in learning capacity. However, variations in exposure have not been systematically explored. In this study we characterize vocabulary growth rates for each of 22 children by using data obtained at several time points from 14 to 26 months. We find a substantial relation between individual differences in vocabulary acquisition and variations in the amount that particular mothers speak to their children. The relation between amount of parent speech and vocabulary growth, we argue, reflects parent effects on the child, rather than child-ability effects on the parent or hereditary factors. We also find that gender is an important factor in rate of vocabulary growth.The work reported here was supported by grants from the Spencer Foundation and the Smart Family Foundation to Janellen Huttenlocher. We thank Peter R. Huttenlocher and James W Stigler for their helpful comments on the manuscript.

A model of category effects on reports from memory is presented. The model holds that stimuli are represented at 2 levels of detail: a fine-grain value and a category. When memory is inexact but people must report an exact value, they use estimation processes that combine the remembered stimulus value with category information. The proposed estimation processes include truncation at category boundaries and weighting with a central (prototypic) category value. These processes introduce bias in reporting even when memory is unbiased, but nevertheless may improve overall accuracy (by decreasing the variability of reports). Four experiments are presented in which people report the location of a dot in a circle. Subjects spontaneously impose horizontal and vertical boundaries that divide the circle into quadrants. They misplace dots toward a central (prototypic) location in each quadrant, as predicted by the model. The proposed model has broad implications; notably, it has the potential to explain biases of the sort described in psychophysics (contraction bias and the bias captured by Weber's law) as well as symmetries in similarity judgments, without positing distorted representations of physical scales.

The present longitudinal study examines the role of caregiver speech in language development, especially syntactic development, using 47 parent-child pairs of diverse SES background from 14 to 46 months. We assess the diversity (variety) of words and syntactic structures produced by caregivers and children. We use lagged correlations to examine language growth and its relation to caregiver speech. Results show substantial individual differences among children, and indicate that diversity of earlier caregiver speech significantly predicts corresponding diversity in later child speech. For vocabulary, earlier child speech also predicts later caregiver speech, suggesting mutual influence. However, for syntax, earlier child speech does not significantly predict later caregiver speech, suggesting a causal flow from caregiver to child. Finally, demographic factors, notably SES, are related to language growth, and are, at least partially, mediated by differences in caregiver speech, showing the pervasive influence of caregiver speech on language growth.The present paper concerns the relation of caregiver speech to child language development. A major issue for acquisition theories concerns the sources of language growth, especially for syntax. While all investigators recognize that exposure to incoming speech is essential for acquiring a language, different theories make alternative claims about the influence of input on child language. Constructivists argue that variations in input are critical to language outcomes in children (e.g

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The authors tested a model of category effects on stimulus judgment. The model holds that the goal of stimulus judgment is to achieve high accuracy. For this reason, people place inexactly represented stimuli in the context of prior information, captured in categories, combining inexact fine-grain stimulus values with prior (category) information. This process can be likened to a Bayesian statistical procedure designed to maximize the average accuracy of estimation. If people follow the proposed procedure to maximize accuracy, their estimates should be affected by the distribution of instances in a category. In the present experiments, participants reproduced one-dimensional stimuli. Different prior distributions were presented. The experiments verified that people's stimulus estimates are affected by variations in a prior distribution in such a manner as to increase the accuracy of their stimulus reproductions.