Language is endlessly variable. Each of us can come up with  an infinite number of sentences in our native language, and we’re able to do so from an early age— almost as soon as we start  to communicate in sentences. How is this possible? In the early 1950s, Noam Chomsky  proposed a theory based on the observation that the key  to this versatility seems to be grammar: the familiar grammatical structure  of an unfamiliar sentence points us toward its meaning. He suggested that there are  grammatical rules that apply to all languages, and that the rules are innate— the human brain is hardwired to process language according to these rules. He labelled this faculty  universal grammar, and it launched lines of inquiry  that shaped both the field of linguistics and the emerging field  of cognitive science for decades to come. Chomsky and other researchers  set out to investigate the two main components  of universal grammar: first, whether there are, in fact,  grammar rules that are universal to all languages, and, second, whether these rules  are hardwired in the brain. In attempts to establish  the universal rules of grammar, Chomsky developed an analytical tool known as generative syntax, which represents the order of words  in a sentence in hierarchical syntax trees that show what structures are possible. Based on this tree, we could suggest  a grammar rule that adverbs must occur in verb phrases. But with more data,  it quickly becomes clear that adverbs can appear  outside of verb phrases. This simplified example illustrates  a major problem: it takes a lot of data  from each individual language to establish the rules for that language, before we can even begin to determine which rules all languages  might have in common. When Chomsky proposed universal grammar, many languages lacked the volume  of recorded samples necessary to analyze them  using generative syntax. Even with lots of data, mapping the structure of a language is incredibly complex. After 50 years of analysis, we still haven’t completely figured out English. As more linguist data  was gathered and analyzed, it became clear that languages  around the world differ widely, challenging the theory that there were  universal grammar rules. In the 1980s, Chomsky revised his theory in an attempt to accommodate  this variation. According to his new hypothesis  of principles and parameters, all languages shared certain  grammatical principles, but could vary in their parameters,  or the application of these principles. For example, a principle is  “every sentence must have a subject," but the parameter of whether the subject must be explicitly stated could vary between languages. The hypothesis of principles  and parameters still didn’t answer the question of which grammatical principles are universal. In the early 2000s, Chomsky suggested  that there’s just one shared principle, called recursion, which means structures can be nested inside each other. Take this sentence, which embeds a sentence within a sentence  within a sentence. Or this sentence, which embeds  a noun phrase in a noun phrase in a noun phrase. Recursion was a good candidate  for a universal grammar rule because it can take many forms. However, in 2005 linguists  published findings on an Amazonian language called Piraha, which doesn’t appear to have  any recursive structures. So what about the other part  of Chomsky’s theory, that our language faculty is innate? When he first proposed universal grammar, the idea that there was a genetically  determined aspect of language acquisition had a profound, revolutionary impact. It challenged the dominant paradigm,  called behaviorism. Behaviorists argued that all animal  and human behaviors, including language, were acquired from the outside  by the mind, which starts out as a blank slate. Today, scientists agree that behaviorism  was wrong, and there is underlying,  genetically encoded biological machinery for language learning. Many think the same biology  responsible for language is also responsible for other  aspects of cognition. So they disagree with Chomsky’s idea that there is a specific, isolated, innate language faculty in the brain. The theory of universal grammar prompted the documentation and study of many languages  that hadn’t been studied before. It also caused an old idea to be  reevaluated and eventually overthrown to make room for our growing  understanding of the human brain.