Merge-based syntax is mediated by distinct neurocognitive mechanisms in 84,000 individuals with language deficits across nine languages.

In the modern language sciences, the core computational operation of syntax, 'Merge', is defined as an operation that combines two linguistic units (e.g., 'brown', 'cat') to form a categorized constituent structure ('brown cat', a Noun Phrase). This structure can be further combined with additional linguistic units based on this categorial information, respecting non-associativity such that abstract grouping is preserved. Some linguists have embraced the view that Merge is an elementary, indivisible operation that emerged in a single evolutionary step. From a neurocognitive standpoint, different mental objects constructed by Merge may be supported by distinct tiers of processing demands: (1) simple command constructions (e.g., "eat apples"); (2) the merging of adjectives and nouns ("red boat"); and (3) the merging of nouns with spatial prepositions ("laptop behind the sofa"). Here, we systematically investigate participants' comprehension of sentences with increasing levels of syntactic complexity. Clustering analyses revealed behavioral evidence for three distinct structural types, which we discuss as potentially emerging at different developmental stages and subject to selective impairment. While a Merge-based syntax may have emerged suddenly in evolutionary time, responsible for the structured symbolic turn our species took, different processing tiers seem to underwrite the comprehension of various types of Merge-based objects.