The Evolution of Subjacency without Universal Grammar: Evidence from Artificial Language Learning

The acquisition and processing of language is governed by a number of universal constraints. Undoubtedly, many of these constraints derive from innate properties of the human brain. Theories of language evolution seek to explain how these constraints evolved in the hominid lineage. Some theories suggest that the evolution of a Chomskyan universal grammar (UG) underlies these universal constraints. More recently, an alternative perspective is gaining ground. This approach advocates a refocus in evolutionary thinking; stressing the adaptation of linguistic structures to the human brain rather than vice versa (e.g., Christiansen, 1994; Kirby, 1998). On this account, many language universals may reflect nonlinguistic, cognitive constraints on learning and processing of sequential structure rather than innate UG. If this is correct, it should be possible to uncover the source of some linguistic universal in human performance on sequential learning tasks. This prediction has been borne out in previous work by Christiansen (2000) in terms of an explanation of basic word order universals. In this paper, we take a similar approach to one of the classic linguistic universals: subjacency. Why Subjacency? According to Pinker and Bloom (1990), subjacency is one of the classic examples of an arbitrary linguistic constraint that makes sense only from a linguistic perspective. Informally, "Subjacency, in effect, keeps rules from relating elements that are ‘too far apart from each other’, where the distance apart is defined in term of the number of designated nodes that there are between them" (Newmeyer, 1991, p. 12). Consider the sentences in Table 1. According to the subjacency principle, sentences 3 and 6 are ungrammatical because too many boundary nodes are placed between the interrogative pronouns and their respective 'gaps'. In the remainder of this paper, we explore an alternative explanation which suggests that subjacency violations are avoided, not because of a biological adaptation incorporating the subjacency principle, but because language itself has undergone adaptations to root out such violations in response to non-linguistic constraints on sequential learning.