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Universal grammar specifies the mechanism of language acquisition. It determines the range of grammatical hypothesis that children entertain during language learning and the procedure they use for evaluating input sentences. How universal grammar arose is a major challenge for evolutionary biology. We present a mathematical framework for the evolutionary dynamics of grammar learning. The central result is a coherence threshold, which specifies the condition for a universal grammar to induce coherent communication within a population. We study selection of grammars within the same universal grammar and competition between different universal grammars. We calculate the condition under which natural selection favors the emergence of rule-based, generative grammars that underlie complex language.
Language consists of words and rules. The finite ensemble of memorized words is called the mental lexicon, whereas the set of rules is called the mental grammar of a person (1, 2). Grammar is the computational system (3) that is essential for creating the infinite expressibility of human language. 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 (7).
Universal grammar consists of (i) a mechanism to generate a search space for all candidate mental grammars and (ii) a learning procedure that specifies how to evaluate the sample sentences (8-13). Universal grammar is not learned but is required for language learning. It is innate (14-16). Most linguists claim that the acquisition of language is done by specific neuronal circuitry within the brain and not by the general purpose problemsolving ability of the brain (17). Therefore, it should be possible to observe genetic defects of language acquisition (18).
References and Notes
1. S. Pinker, Words and Rules (Basic Books, New York, 1999).
2. G. A. Miller, The Science of Words (Scientific American Library, New York, 1996).
3. N. Chomsky, in The View from Building 20,...