Language growth is very rapid during the preschool years. From a vocabulary of no words at about one year of age, the two-year-old may have a vocabulary of two or three hundred words and can even form two- or three-word sentences. By the age of five, children can construct simple sentences and may use past and future as well as present tenses correctly. By this age, children also demonstrate considerable mastery of possessives and of definite and indefinite articles and may have an active vocabulary of about 2,500 words. Their passive vocabulary—the words they can hear and understand but do not usually use—is much higher and may be as large as 14,000 words. Averaged over the child's lifetime, she has acquired nine words a day since birth!

Although language growth is relatively easy to describe, it is more difficult to explain. Indeed, there are at least three different types of explanation of language growth, and there is some evidence to support each one.

One explanation of language development starts from the assumption that the language system is present from birth and follows its own path of development quite independent of the development of thought. This is the position of famed linguist Noam Chomsky. Those who look at language development from this perspective try to map the progress of the child's linguistic structures in relation to the child's experience and point to the fact that experience cannot account for the observed achievements in the evolution of language.

Young children's earliest words are often holophrases, or single-word utterances that are meant to convey as much meaning as a full sentence. A child who says "car" may mean, "I see a car," or "I want to be in the car," or "Where is the car?" As children become more proficient they may use telegraphic speech in which they evidence their understanding of word order by the ways in which they combine two or more words, such as "All gone," "Baby up," and "Daddy go bye-bye." Since children do not learn these combinations directly from adults, it is argued that they are evidence of an independently emerging language system.

Other language phenomena during early childhood also support this position. One of these is overregularization. A child may understand a grammatical rule but not its exceptions. For example, a child may say, "The boy runned home." Here the child is generalizing from the past progressive rule—add ed—to all verbs, but she does not appreciate that the verb to run is irregular. The understanding of grammatical rules, but not their exceptions, suggests that language acquisition is not simply a matter of copying adult speech. Adults, children's presumed language role models, do not make such errors.

More evidence for the independence of language development is the creativity of children's language. Children are always coming up with original speech utterance that are not copies of adult verbalizations. Parents do not say, "All gone milk" when they have finished their drink or "Mommy, sleep time" when they are ready for bed. Moreover, parents tend to reinforce the truth value of children's statements, not their grammatical correctness. A child who says, "Mommy dress red," is likely to be told "Yes" if she is correct factually though not grammatically. On the other hand, if the child says, "Mommy's dress is blue" she is likely to be told "no" even though she is grammatically correct. That children learn grammar even though it is not rewarded by parents is another evidence of its independence from adult instruction.

A different position with respect to language development is that of Piaget and his colleagues. According to this view, language development is regulated by intellectual development. The child's language limitations always reflect her cognitive limitations. In one study, preoperational and concrete operational children were asked to describe three blocks of different sizes. Preoperational children often described them in nonquantitative ways, such as lithe baby block, the mommy block, and the daddy block." Concrete operational children, in contrast, described the blocks in quantitative terms: "This one is taller and wider. That one is shorter and thinner."

Sometimes children's failures on the conservation tasks are attributed to verbal misunderstanding. A child who says that six pennies in a line next to one another is less than six pennies spaced apart simply does not understand the term less. Piaget's reply is that "verbal misunderstanding" begs the question. The real question that must be asked is why the preschool child does not understand the term less in the accepted sense while the older child does. The answer, according to Piaget, is that the older child now has the requisite mental abilities to understand the term while the young child does not.

A third position with respect to language development is that of Russian psychologists Lev Vygotsky. In his view, thought is determined by language—and not language by thought, as Piaget contends. According to Vygotsky, language is progressively internalized, and whispering is a stage along the way. When language is completely internalized, it is the tool of our thinking. Inasmuch as language is, in part at least, learned, so too must be our thinking, and because language varies from society to society, thought must vary from society to society as well.

Evidence for the language determination of mental development comes from observations of how children use language when they go about their everyday activities. Children often talk out loud as they are engaged in building or in making something. At such times, their language seems to be a necessary guide to their action. This directive use of language has been called verbal mediation. Children often use verbal mediation as a memory strategy. If, for example, something is hidden in front of them, they may say "It is under the blue cup" as a way of remembering where the hidden object is until they are permitted to actually look for it.

Language development is so complex that no one of these theories can encompass it all. In fact, there seems to be a little truth in all three. Language structure does seem to follow its own course, quite independent of experience. On the other hand, the use of conceptual terms does seem to be related to conceptual development. Finally, language does sometimes enable young children to bridge delays and regulate their actions. In fact we all use language in this way. If we are doing something very delicate, we may walk ourselves through the task: "Okay, now just move this lever a little to the left." Language is directed by thought, but thought is also directed by language.

Despite these differences, researchers generally agree that adults play a very important role in the child's language development. First, adults provide a linguistic model from which children learn vocabulary, expressions, articulation, and so on. Second, from the earliest days of life, adults and children engage in a variety of verbal dialogues that teach children not only language skills, but also social interaction patterns. Finally, by reading to children, parents give children some of the requisite skills and interests that promote their later literacy.

Before leaving this section, it might be well to highlight one practical point that recent research has made quite clear. I have already emphasized the importance of parents and other adults talking to and reading to young children as an essential part of their linguistic growth. Such interaction helps build the child/s auditory, or listening, skills. These are critical for learning to read. When young children watch too much television, more than an hour or two a day, they may not develop the necessary auditory discrimination skills that are critical to learning to read. This is true because when young children watch television, they often do not listen. It is easier for them to get the information visually. We all take the path of least effort, but this path will cost children dearly when they set out to attach printed words to sounds that they cannot clearly discriminate. Limiting young children/s television watching is essential if we wish them to develop the listening skills they need to become accomplished readers.

Young children, then, make enormous progress in intellectual and language development. Despite these enormous accomplishments, they still have a long way to go before they can fully comprehend the language and thought of the adult world. This is in no way a negative description of young children, but only a realistic statement regarding the extraordinary complexity of the cognitive and linguistic world that the child must master and a recognition of the large amount of time and effort that is required to get a better understanding of our world.