Gleitman (1977), who report that correlations between input complexity and measures of the child's linguistic maturity, while positive and often significant, are not usually very large. An interpretation of this finding is that caretakers are not taking aim exactly at i + 1. The input they provide for children includes i + 1, but also includes many structures that have already been acquired, plus some that have not (i + 2, i + 3, etc.) and that the child may not be ready for yet. In other words, caretakers do not provide a grammatically based syllabus! (For a more complete review of rough-tuning, see Krashen 1980, 1981.)

A third characteristic of caretaker speech that concerns us is known as the "here and now" principle. It is well established that caretakers talk mostly about what the child can perceive, what is in the immediate environment. Discourse with children is far more likely to deal with what is in the room and happening now ("See the ball?") than what is not in the room and not current ("What will we do upstairs tomorrow?"). As Newport et al. (1977) points out, this is a topical constraint--the "here and now" principle reflects the common interests of the caretaker and child.

While there is no direct evidence showing that caretaker speech is indeed more effective than unmodified input, the input hypothesis predicts that caretaker speech will be very useful for the child. First, it is, or aims to be, comprehensible. The "here and now" feature provides extra-linguistic support (context) that helps the child understand the utterances containing i + 1. As MacNamara (1972) pointed out, the child does not acquire grammar first and then use it in understanding. The child understands first, and this helps him acquire language.

As discussed earlier, roughly-tuned caretaker speech covers the child's i + 1, but does not focus on i + 1 exclusively. Part (3) of the input hypothesis claims that this is optimal. Rough-tuning has the following advantages in child first language acquisition:

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