Language contact has been invoked with increasing frequency over the past two or three decades as a, or the, cause of a wide range of linguistic changes. Historical linguists have (of course) mainly addressed these changes from a diachronic perspective -that is, analyzing ways in which language contact has influenced lexical and/or structural developments over time. But sociolinguists, and many or most of the scholars who would characterize their specialty as contact linguistics, have focused on processes involving contact in analyzing synchronic variation. A few scholars have even argued that contact is the sole source of lan guage variation and change; this extreme position is a neat counterpoint to an older position in historical linguistics, namely, that language contact is responsible only for lexical changes and quite minor structural changes. (This long-superseded older position is echoed, surpris ingly, in Labov's recent claim that 'structural borrowing is rare ' and superficial (Labov 2007: 349, 383).) In this chapter I will argue that neither extreme position is viable. This argument will be developed through a survey of general types of contact explanations, especially explanations for changes over time, juxtaposed with a comparative survey of major causal factors in internally-motivated language change. My goal is to show that both internal and external motivations are needed in any full account of language history (Hickey 2012) and, by implication, of synchronic variation. Progress in contact linguistics depends, in my opinion, on recognizing the complexity of change processes -on resisting the urge to offer a single simple explanation for all types of structural change.The structure of the chapter is as follows. Section 1 provides some background concepts and definitions, and sections 2 and 3 compare and contrast contact explanations with internal explanations of change. Section 4 discusses a much-cited typology-based theory about contact vs. internal explanations. Section 5, finally, is a brief conclusion that includes a warning about the need to be cautious in making claims about the causes of change -both because in most cases no cause can be firmly established and because of the real possibility that multiple causes are responsible for a particular change.
Some Background ConceptsFirst, what counts as language contact? The mere juxtaposition of two speakers of different languages, or two texts in different languages, is too trivial to count: unless the speakers or the texts interact in some way, there can be no transfer of linguistic features in either direction. Only when there is some interaction does the possibility of a contact explanation for