Children vary greatly in the number of words they know when they enter school, a major factor influencing subsequent school and workplace success. This variability is partially explained by the differential quantity of parental speech to preschoolers. However, the contexts in which young learners hear new words are also likely to vary in referential transparency; that is, in how clearly word meaning can be inferred from the immediate extralinguistic context, an aspect of input quality. To examine this aspect, we asked 218 adult participants to guess 50 parents' words from (muted) videos of their interactions with their 14-to 18-mo-old children. We found systematic differences in how easily individual parents' words could be identified purely from this socio-visual context. Differences in this kind of input quality correlated with the size of the children's vocabulary 3 y later, even after controlling for differences in input quantity. Although input quantity differed as a function of socioeconomic status, input quality (as here measured) did not, suggesting that the quality of nonverbal cues to word meaning that parents offer to their children is an individual matter, widely distributed across the population of parents.language acquisition | word learning | SES C hildren's vocabularies vary greatly in size by the time they enter school (1, 2). Because preschool vocabulary is a major predictor of subsequent school success (3), this variability must be taken seriously and its sources understood. Some of this variability resides in the individual capacities and temperament that infants bring to the word learning task (4, 5). However, environmental influences are also bound to play instrumental roles. Accordingly, we examined the contextualized speech input parents provide to infants during the second year of life as a potential source of the massive vocabulary differences found at school entry.It is already known that the sheer quantity of linguistic input is an important determinant of vocabulary size; overall, the more words children hear early in development, the larger their subsequent vocabularies. This relationship holds true both for types (different words) and tokens (number of words heard, including repetitions) (6, 7). These quantity differences are correlated with socioeconomic status (SES). Children from low SES homes are typically exposed to fewer words early in development (8, 9) and have smaller vocabularies at school entry than children from high SES homes (10).Taken alone, the correlation of vocabulary size with amount of input is puzzling because as a general rule language learners do not seem to require a large number of exposures to a word to acquire its meaning (11). In experimental settings, for example, children have been shown to acquire and retain a new word heard only once or a very few times (12-14). The likelihood, then, is that certain exposures to a new word are especially informative, supporting secure and rapid inferences to meaning. For example, common sense insists that it will be ...