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Every time we speak, we’re improvising. "Humans possess a remarkable ability to talk about almost anything, sometimes putting words together into never-before-spoken or -written sentences," said Morten H. Christiansen, the William R. Kenan, Jr. Professor of Psychology in the College of Arts and Sciences. We can improvise new sentences so readily, language scientists believe, because we have acquired mental representations of the patterns of language that allow us to combine words into sentences. The nature of those patterns and how they work, however, remains a puzzle in cognitive science, Christiansen said.

Challenging traditional views of grammar

In new research, Christiansen and co-author Yngwie A. Nielsen of Aarhus University offer a new perspective on those mental representations, challenging the long-standing assumption in the language sciences that they consist of highly complex syntactic structures.

The study focused on English, but the researchers are optimistic that its findings hold across languages, and could reshape how we understand language evolution, language development, and second-language education.

For decades, scientists have believed we rely on a complex mental grammar to build sentences that have hierarchically organized structure—like a branching tree. But Christiansen and Nielsen suggest that our mental representations might be more like snapping together pre-assembled LEGO pieces (such as a door frame or a wheel set) into a complete model.

Evidence for non-hierarchical language structures

Instead of intricate hierarchies, they propose, we use small, linear chunks of word classes like nouns and verbs—including short sequences that can’t be formed by way of grammar, such as "in the middle of the" or "wondered if you."

Their study, "Evidence for the Representation of Non-Hierarchical Structures in Language," is published in Nature Human Behaviour.

The prevailing theory since at least the 1950s is based on hierarchical, tree-like mental representations, setting humans apart from other animals, Christiansen said.

In this view, words and phrases combine according to the principles of grammar into larger units called constituents. For example, in the sentence "She ate the cake," "the" and "cake" combine into a noun phrase "the cake," which then combines with "ate" into the verb phrase "ate the cake," and finally with "she" to make the sentence.

"But not all sequences of words form constituents," Christiansen and Nielsen wrote in a summary of their paper. "In fact, the most common three- or four-word sequences in language are often nonconstituents, such as ‘can I have a’ or ‘it was in the.’"

Implications for language understanding

Because they don’t conform to grammar, nonconstituent sequences have been overlooked. But they do play a role in a speaker’s knowledge of their language, the researchers found.

In experiments, an eye-tracking study and an analysis of phone conversations, they discovered that linear sequences of word classes can be "primed," meaning when we hear or read them once, we process them faster the next time. That’s compelling evidence they’re part of our mental representation of language, Christiansen said. In other words, they’re a key part of our mental representation of language that goes beyond the rules of grammar.

"I think the main contribution is showing that traditional rules of grammar cannot capture all of the mental representations of language structure," Nielsen said.

"It might even be possible to account for how we use language in general with flatter structure," Christiansen said.

"Importantly, if you don’t need the more complex machinery of hierarchical syntax, then this could mean that the gulf between human language and other animal communication systems is much smaller than previously thought."

Publication details

Yngwie A. Nielsen et al, Evidence for the representation of non-hierarchical structures in language, Nature Human Behaviour (2026). DOI: 10.1038/s41562-025-02387-z

Citation: Study challenges long-held theory that language is built on grammar trees (2026, January 22) retrieved 22 January 2026 from https://phys.org/news/2026-01-held-theory-language-built-grammar.html

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