third grade scores in the academic skill areas of reading, writing, spelling, or arithmetic.

It is important to recognize that this research on language input focuses largely on white, middle class children in the United States and on mothers' speech directed to their children. It does not explore the role that talk around and about the child might play in language acquisition. This may be particularly important in other cultures, in which children are more likely to be involved in relationships in which skilled conversation takes place around them, but is not directed at them (Rogoff et al., 1993). For example, in a Mayan Indian community studied by Rogoff and her colleagues, adults communicated to their children primarily through shared activity and group conversations, rather than in the context of one-on-one lessons or explanations directed to the child. Although, as we noted earlier, virtually all children learn language, the issue is whether there are qualitative differences across individuals that are correlated with differing types of input.

It is also important to note that this area of research is open to the criticism that it has not considered the sizeable role that genetic influences undoubtedly play in the development of verbal abilities. Mothers who talk more to their children may also share genetic endowments that facilitate language learning. One study, which took advantage of the fact that twins tend to lag behind singletons in language development, ruled out a variety of competing hypotheses to conclude that the quality and complexity of mother-child communicative interaction was responsible for the twin-singleton differences in language development (Rutter et al., 2000). Measures of mother-child verbal interaction at 20 months predicted language level at 36 months in both twins and singletons, and they accounted for the twin-singleton differences in language level. Nevertheless, the relative inattention to genetic factors in this area of research is a shortcoming that needs to be addressed.

Evidence of the importance of verbal input during the years when verbal development is proceeding rapidly has also emerged from research on child care. Children whose teachers talk with them a lot (and many don't!) have higher scores on tests of both verbal and general ability. This is especially the case when the talking consists of the teacher encouraging, questioning, and guiding the children's exploration and learning. Positive inputs are positive inputs, whether they happen at home or in child care. Vocabulary size, in turn, is highly correlated with IQ. Thus, environmental input can play a large role in determining the rate at which children acquire and use a particular aspect of language, and rate of acquisition and use may be an important factor in cognitive growth and cognitive functioning.

Furthermore, taking vocabulary as an example, the individual differences that characterize children at school entry are enormous. In one large, longitudinal study, children tested at kindergarten when they were 5 years