What does a toddler who is barely putting words together know about convoluted rules of grammar that govern our mother tongue? What does a two-year-old know about sentence structure?
A lot, as it turns out.
Pairing the fact that children use grammar correctly at a very early age with the knowledge that this usage unfolds at a predictable rate, Dr. Kenneth N. Wexler, professor of psychology and lingusitics, and his colleagues have come up with a way to diagnose a disorder called Specific Language Impairment (SLI) that affects as many as 5 percent of American children. Professor Wexler presented the research at the annual meeting of the American Association for the Advancement of Science on February 15.
SLI is a difficult-to-diagnose disorder in which children have trouble mastering certain aspects of adult grammar. Professor Wexler has identified a specific grammatical marker that may make it easier to identify and treat children with SLI.
At the basis of tracking down language disorders is a fundamental understanding of how we learn language, which is different from how we learn trigonometry or how to hit a tennis ball.
Hearing people speak doesn't provide a listener with every possible example of how to put a sentence together, and nobody sits preschoolers down with grammar textbooks. Yet toddlers and preschoolers are able to apply the rules of language to sentences they have never spoken or heard before.
Professor Wexler, who studies the nature of language, pyschology and the brain, found that at a very young age, children "know" whether they are speaking a language in which the direct object is to the left or right of the verb. In Dutch and German, for instance, it is to the left; in English and French, it is to the right.
From the time they are about 18 months old and say more than one word at a time, children correctly use the rules that govern the adult language they are hearing. Professor Wexler discovered that at an early age, children know a lot about inflection, clause structure and verb movement. Using an example in French (because the rule is more obvious than in English), very young children know that the negative "pas" ("not") falls in a different place in the sentence depending on the tense of the verb.
For instance, "John does not speak," is "Jean parle pas." Yet with the infinitive version of "parler" (to speak), the negative "pas" moves from the end of the sentence to in front of the verb, as in "pas parler."
The fact that most children can perform this linguistic trick easily and automatically is just one indication that human beings are genetically programmed to use language. While there are upwards of 5,000 languages spoken in the world, they all seem to adhere to the rules of universal grammar -- the set of principles of the underlying system in the brain that forms the fundamental basis for all language.
In other words, although there is little apparent similarity between Chinese and Swedish, all languages use things like verbs and objects. While there are variations in how those elements are put together, most languages use them in particular ways. And no language will use them in certain other ways. For example, no language will ever place the verb in a sentence between an article and a noun, i.e., "Mary the hit tennis ball."
Although born with a grounding in universal grammar, people gain the nuances of how a language works -- for instance, whether the object of a sentence comes before or after the verb -- through experience.
GROWING INTO LANGUAGE
During the years leading up to kindergarten, children move toward adult grammar at different rates. Professor Wexler believes that just as children lose baby teeth and grow permanent teeth, they mature in their use of universal grammar.
"Language unfolds according to a genetic program while interacting with environmental events," he said. Certain elements show up later than others, but by kindergarten, most children have arrived at near-adult use of grammatical morphemes, or combinations of sounds, he said.
"These developments take place according to a biological program, with somewhat varying times in the population," Professor Wexler said. One of the skills to surface later in the preschool years is use of the passive tense -- for instance, "the cake was eaten."
Although environment may guide the rate of language maturation -- comparable to how nutrition helps determine the body's rate of physical maturation -- "it is clear that the development of language is essentially guided by a biological, genetically determined program," he said.
It turns out that exploring the connection between how children use words and word order is a powerful tool to determine their knowledge of underlying grammar.
A DEVELOPMENTAL MARKER
Just as a doctor uses a child's ability to master skills such as walking as a basis for assessing developmental progress, Professor Wexler suggests that a consistent inability to correctly match tense and object could be a milestone for speech and language development.
Professor Wexler has found that preschool children in many languages do not use adult grammar the way their peers do. In particular, they do not mark tense in every subject in the language. Proceeding from his work with normal children in what he calls the Optional Infinitive Stage, Professor Wexler, in collaboration with Professor Mabel Rice at the University of Kansas, has found that preschool children with SLI do not start to move toward adult grammar. He is developing a test that can identify this disorder by around age five, so that teachers and parents can come up with intervention strategies.
While most children by age five master the plural (as in "these cats"); the progressive ("the cats are eating") and the prepositions "in" and "on" ("the cats are in the house"), children with SLI appear to have difficulty with those morphemes relating to tense. In some cases, children with impaired language skills may never acquire the ability to always mark tense.
Although children with SLI often perform below age expectation on their use of grammatical morphemes, and normal preschool children vary considerably in their progress toward using adult grammar, Professor Wexler theorizes that the misuse of a very specific set of morphemes would conclusively diagnose SLI.
"The most important thing, and possibly the most conceptually difficult thing, to understand about language acquisition is that it is not at all obvious how it works," he said. However, it clearly does work, because very early on, most children are able to set correctly the lingusitic parameters of the language or languages with which they grow up.
Professor Wexler's work is supported by the National Institute on Deafness and Other Communicative Disorders.
A version of this article appeared in MIT Tech Talk on February 25, 1998.