Modeling German Word Order Acquisition via Bayesian Inference

The question of how children acquire the grammatical principles of their native language has been debated in cognitive science and linguistics for over 50 years (e.g., Chomsky, 1965; Xu and Tenenbaum, 2007). Perfors et al. (2011) suggest that children compare various syntactic hypotheses in order to determine which is most compatible with the socalled primary linguistic data (cf. Chomsky, 1965; Wexler and Culicover, 1980; Clark and Lappin, 2013; among many others), and that a Bayesian model selection approach might be able to do some of this work. More specifically, they demonstrate that a Bayesian inference system presented with child-directed input would prefer a probabilistic context free grammar (PCFG) that used hierarchical phrase structure over a regular grammar representing linear phrase structure, without any initial prior bias towards one grammar type or another. Therefore, they argued that correct linguistic generalizations can be achieved by a learner equipped with domain-general Bayesian inference capacities but without language-specific innate knowledge. In their work, the various syntactic hypotheses were represented by grammars that were all able to parse the entirety of a pre-processed English childdirected speech corpus. However, a computational model without the ability to compare competing hypotheses that are supported by different subsets of the data is severely limited due to the inevitability of noise. Children are also very likely to hear errors, actual or only perceived (Krentz and Corina, 2008; Friederici et al., 2011). Furthermore, hypotheses concerning typological tendencies might not warrant comparison over fully congruent data sets. The premise of this work is therefore to extend the Bayesian system designed by Perfors et al. (2011) to allow for comparing models supported by different data subsets. We evaluated this system in the context of word order acquisition in German. To this end, we designed PCFGs representing different word order hypotheses a child might entertain. Many linguists agree that German word order is Subject-Object-Verb (SOV) Verb-Second (V2) (henceforth: SOV+V2) (e.g., Bierwisch, 1963). Yet many simple German sentences also maintain Subject-Verb-Object (SVO) order. Because of the SVO word order of many simple German sentences, a child might initially consider a left-branching grammar only able to account for SVO sentences, only to reject it in order to account for embedded sentences and sentences with auxiliaries. Note that we do not commit to a specific theoretical position on the plausibility of humans performing verb movement operations, and merely lean on this theory to specify conceivable word order hypotheses, assuming that a PCFG representing SVO word order should be able to parse a decent number of German sentences, but fewer than the SOV+V2 PCFG. Additionally, our PCFGs are comparable in complexity in contrast to complexities differing by orders of magnitude for the grammars used by Perfors et al. (2011). Our PCFGs can thus serve as snapshots of different hypotheses comparable in a priori complexity that a child may at some point compare in order to determine the word order of their language, though they by no means exhaust the space of hypotheses that a child may consider.