Recursion

Unless you really stop and think about language—why and how we say what we say—it's easy to believe that language just happens. The truth is, though, that language follows very specific rules. Some rules you learn, while others you just pick up along the way. One rule that many take for granted because they use it, but don't always understand it, is recursion.

Recursion Explanation

Think of recursion like Russian nesting dolls—one doll fits inside another of a similar shape, which fits inside another, and the process repeats itself over and over again.

Fig. 1 - The concept of recursion is like a Russian nesting doll—something nested inside of something else with the same shape.

More simply, linguistic recursion is the ability to place a grammatical element inside an element of the same kind.

Recursion Properties

There are a few important properties of recursion that help set it apart from other linguistic phenomena.

The first key property of recursion is that the nested words or structure must be of the same kind. Sentences can be nested within other sentences, noun phrases within other noun phrases, and so on.

This example could continue as long as the structure stays the same. You wouldn’t add a verb phrase within this noun phrase: “Natalie’s sister’s dogs she can’t find them” doesn’t make sense. However, you could add another possessive to the string of possessives. The sentence could continue “Natalie’s sister’s dogs’ leashes…” with more room for further recursion or the conclusion to the sentence.

The second key recursion property is that the elements continue to repeat, increasing depth of meaning, until you stop. In other words, the addition of each grammatical element must add a new deeper level of meaning. If there is not a new layer of meaning added, then you’re repeating yourself—this is a different linguistic phenomenon known as iteration.

These two principles of recursion—that structures must be of the same kind and increase depth of meaning—are what led some linguists to believe that recursion is what separates human language from that of other animals.

Recursion and Chomsky

Noam Chomsky often called the father of modern linguistics, introduced the theory of Universal Grammar, which proposes that the human brain contains a set of innate skills necessary for the creation of language. Chomsky and others came to the conclusion that recursion is one of those innate skills and that it specifically is what gives human language the ability to create endless meaning.

Chomsky asserted that the essence of language is the ability to create an infinite number of sentences and meaningful utterances. There is no ceiling to cap the number of things we can say, even within a single sentence, thanks to recursion. In their article in Science (2002), Chomsky, Hauser, and Fitch¹ posit:

Chomsky called this concept discrete infinity and believed that it’s true of all human language (and only human language). Recursion is the necessary tool for this interpretation of language to be true.

Opposition to Chomsky

Not all linguists agree on the concept of Universal Grammar, or Chomsky’s conclusion about recursion as the key to human language. Daniel Everett, a missionary linguist, dwelt with an indigenous people group in the Amazon called the Pirahã for many years. Everett learned the native language and discovered that the language of Pirahã showed no signs of recursion.

This discovery stands in apparent opposition to Chomsky’s understanding of recursion as the only uniquely human feature of language. Today, linguists continue to debate Chomsky’s hypothesis and the implications of Everett's findings.

Recursion and Syntax

Recursion is also called syntactic recursion because the recursive structures can be words, phrases, or sentences. These are all components of syntax.

Because recursion appears at the syntactic level, it cannot apply to smaller elements of language, such as morphemes (the building blocks of words). The most common recursive structures involve noun phrases.

Recursion in Language Examples

Recursion may take many forms in language because, again, as long as the repeated element is the same as the element in which it is embedded and it deepens the meaning of the phrase or sentence, it is recursion.

Recursion can be a string of adjectives:

Fig. 2 - The description of the teacher could continue until the speaker runs out of things to describe them.

In theory, this list of adjectives describing the teacher could go on and on, but syntactic convention usually caps these strings at two or three.

Many literary geniuses use recursion as a hallmark of their writing style. Faulkner, Woolf, and Fitzgerald are just a few examples of authors that frequently used recursion.

There are six major syntactic movements in this single sentence, and, in theory, it could continue like this until Woolf ran out of words. Inside each movement is at least one example of recursion, where the phrase deepens in meaning.

The first unit of the sentence alone contains four recursive movements:

1. swooping, swerving, flinging themselves
2. in and out,
3. round and round,
4. yet always with perfect control, as if elastics held them

This single sentence carries on like this, describing the scene observed by the character in great detail.

^1. Noam Chomsky, Marc D. Hauser, and W. Tecumseh Fitch. 'The Faculty of Language: What Is It, Who Has It, and How Did It Evolve?' Science (2002).

^2. Virginia Woolf. 'Mrs. Dalloway'. 1925.