In linguistics, the theory of universal grammar holds that there are certain basic structural rules that govern language that all humans know without having to learn them. This is one way to explain how humans acquire language — if the brain is already primed to understand certain sentence structures, it explains how children can understand and speak sentences that they've never heard before. Supporters of this theory point to the elements that are common in different languages as evidence.
Theory
The capacity for language — the ability of human beings to develop ways to communicate complicated information — is a complex phenomenon that researchers have tried to explain in many different ways. At the beginning of the 20th century, most experts believed that language was learned like any other process, through imitation, trial, and error. In contrast, the theory of universal grammar holds that there are deeper, physical processes at work: that the brain itself is designed to allow humans to use grammatical language. Assuming this is true, humans could theoretically develop language without being taught by other people.
This approach is contextual, meaning that, although there are believed to be similarities among all languages, not all languages have the same grammar. It does not attempt to determine independent facts that hold true for every single language on Earth. These rules outline how human languages develop when faced with these basic principles, however. By combining the rules with observations about a language, linguists can often determine a language's word order, phonemes, and other foundational traits.
History
The observation that there seem to be rules common to all human languages has been around at least since the 13th century. Historically, philosophers believed these features came from the language of the Garden of Eden, which was thought to be the original language of mankind. This theory has been largely abandoned in favor of alternatives that attribute these common elements to the evolution of the human mind and the way it processes language.
The most famous aspect of this line of thought was put forth by the linguist Noam Chomsky in the 1950s. Chomsky proposed a universal grammar hardwired into the brain of all humans underlying all languages. By his reasoning, children learn their native languages using this hardwired grammar as a support structure. Even so, the child must still be taught the specific features of his or her language through social interaction.
Poverty of Stimulus
One of the major assertions that underlies the theory of universal grammar is called the Poverty of Stimulus Argument. This claim states that children are not exposed to enough stimulus — people speaking the native language — to be able to learn language correctly. There are a huge number of ways that words can be put together, and no rule for doing so is obviously more correct than any other. In addition, this argument states, children are usually given positive evidence for how to speak correctly, but rarely provided with negative evidence, or correction when they speak ungrammatically. Yet children, despite a relatively limited amount of input, do reliably learn the grammatical structures of their language. This, the argument asserts, must mean that there is some innate capacity for the structures of language.
This argument is very controversial, and has many critics. Some argue that the amount of stimulus that a child receives from listening to other speakers is actually enough information for him or her to learn the language's basic grammar, and that the brain can recognize patterns in the language to fill in what's missing. Others assert that children are corrected and are told when a sentence is grammatically incorrect, and the fact that they are rarely (or never) exposed to ungrammatical sentences teaches them that those grammatical structures are wrong.
Other Criticisms
Some evolutionary biologists have argued that the physicality of speech cannot be ignored when considering how language developed. Language doesn't involve just one part of the brain, but involves a combination of neural structures. The anatomy of the mouth, tongue, and throat is important, they say, as is the need for all of these parts to move together to produce speech. It is the development of motor control that provides the basis for language development.
Another argument leveled against universal grammar is that the theory itself is not falsifiable; in other words, there's no way to prove it wrong. It claims to be able to predict what new languages will be like, but the sample size is small enough that when new languages are discovered, the rules laid out are sometimes adapted to fit the new data. If a theory cannot be tested, it can be argued, then it cannot be scientific. This may undermine its validity as a strong predictive theory, but may leave intact its descriptive accuracy.