Shieber, S. M. (1985). Evidence against the context-freeness of natural language. In Philosophy, language, and artificial intelligence (pp. 79-89). Springer, Dordrecht.
In searching for universal constraints on the class of natural languages, linquists have investigated a number of formal properties, including that of context-freeness. Soon after Chomsky’s categorization of languages into his well-known hierarchy [Chomsky, 1963], the common conception of the context-free class of languages as a tool for describing natural languages was that it was too restrictive a class–interpreted strongly (as a way of characterizing structure sets) and even weakly (as a way of characterizing string sets).