Universal Grammar (UG) is a theoretical concept proposed by Noam Chomsky (not without criticism or controversy from scholars in the scientific community) that the human brain contains an innate mental grammar that helps humans acquire language. Chomsky theorized that the brain contains a mechanism he referred to as a language acquisition device (LAD), which is “separate from other faculties of cognitive activity….Input is needed, but only to ‘trigger’ the operation of the language acquisition device” (Ellis 32). Without this LAD, according to Chomsky, children would never be able to learn language from the input they receive.
Nowak et al. summarizes the theory in this way:
“Children acquire their mental grammar spontaneously and without formal training. Children of the same speech community reliably learn the same grammar. Exactly how the mental grammar comes into a child’s mind is a puzzle. Children have to deduce the rules of their native language from sample sentences they receive from their parents and others. This information is insufficient for uniquely determining the underlying grammatical principles (4). Linguists call this phenomenon the “poverty of stimulus” (5) or the “paradox of language acquisition” (6). The proposed solution is universal grammar” (114).
Poverty of stimulus is the ability of the human brain to recognize correct and incorrect grammar even in novel sentences. Vivian Cook writes,
“A second example from English is the well-known pair, ‘John is eager to please’ and ‘John is easy to please’, taken from the earlier ‘Aspects’ model (Chomsky 1965)….Conceivably an adult might explain the difference to the child, or some feature of the particular situation might make it obvious; such accidental and improbable occurrences cannot explain why children go through the same stages in acquiring ‘eager/easy to please’ and are successful at about the same age (Cromer 1970). If the child has not learnt the distinction from the input, he must have done so from some property of his own mind. Both examples therefore exploit the same argument, known as ‘the poverty of the stimulus’, to show that the child knows things about language he could not have learnt from outside, that important aspects of language are not strictly speaking learnable” (“Chomsky’s Universal Grammar”).
Cook, Vivian J. “Chomsky’s Universal Grammar and Second Language Learning.” Applied Linguistics 6.1 (1985): 2-18. http://www.viviancook.uk/Writings/Papers/AL85.htm.
Nowak, Martin A., et al. “Evolution of Universal Grammar.” Science, vol. 291, no. 5501, 2001, pp. 114–118. JSTOR, www.jstor.org/stable/3082186.
Ellis, Rod. Second Language Acquisition. Oxford, 1997.