Case and Cause in Icelandic: Reconstructing Causal Networks of Cascaded Language Changes

Proceedings of the 54th Annual Meeting of the Association for Computational Linguistics, pages 2421–2430, Berlin, Germany, August 7-12, 2016. c⃝2016 Association for Computational Linguistics Case and Cause in Icelandic: Reconstructing Causal Networks of Cascaded Language Changes Fermín Moscoso del Prado Martín and Christian Brendel University of California, Santa Barbara Department of Linguistics, South Hall 3521 Santa Barbara, CA 93106, USA fmoscoso@linguistics.ucsb.edu and cdbrendel@gmail.com Abstract Linguistic drift is a process that produces slow irreversible changes in the grammar and function of a language’s construc- tions. Importantly, changes in a part of a language can have trickle down effects, triggering changes elsewhere in that lan- guage. Although such causally triggered chains of changes have long been hypoth- esized by historical linguists, no explicit demonstration of the actual causality has been provided. In this study, we use co- occurrence statistics and machine learning to demonstrate that the functions of mor- phological cases experience a slow, irre- versible drift along history, even in a lan- guage as conservative as is Icelandic. Cru- cially, we then move on to demonstrate –using the notion of Granger-causality– that there are explicit causal connections between the changes in the functions of the different cases, which are consistent with documented processes in the history of Icelandic. Our technique provides a means for the quantitative reconstruction of connected networks of subtle linguistic changes. 1 Introduction Sapir (1921/2014, p. 123) noticed that “Language moves down on a current of its own making. It has a drift” (emphasis added). In Sapir’s view, the formation of different dialects requires that the small changes constantly being introduced by the speakers are not just plain white noise, but rather random walks in which minute changes accumu- late over time. The very high dimensionality on which languages operate makes cumulative lin- guistic changes irreversible. Once a change has been effected there is very little chance that the language will ever return to its original state be- fore the change, in the same way that a diffu- sion process in a very high dimensional space is never going to return to the exact same point in the space. Drift in language is in this respect reminiscent of random genetic drift from evolu- tionary biology (Wright, 1955). However, Sapir’s idea of drift goes further in that he viewed it as a directional process, more similar to Wright’s (1929) concept of a directional drift related to se- lectional pressures. In Sapir’s view, “language has a ‘slope”’; the small changes that accumulate in linguistic drift are not fully random, but rather they reflect the speakers’ unconscious cognitive ten- dency to increase the consistency within their lan- guages. This idea is currently challenged by some researchers (Croft, 2000; Lupyan and Dale, 2015), who are of the opinion that purely random drift – of the same type as that found in genetics–, when coupled with adequate selection mechanisms, is sufficient to account for the diachronic changes observed in the world’s languages. Sapir moti- vated the need for directional change in what he …

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