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<Paper uid="W05-0502">
  <Title>Simulating Language Change in the Presence of Non-Idealized Syntax</Title>
  <Section position="2" start_page="0" end_page="11" type="intro">
    <SectionTitle>
1 Introduction
</SectionTitle>
    <Paragraph position="0"> The paradox of language change is that on the one hand, children seem to learn the language of their parents very robustly, and yet for example, the English spoken in 800 AD is foreign to speakers of Modern English, and Latin somehow diverged into numerous mutually foreign languages. A number of models and simulations have been studied using historical linguistics and acquisition studies to build on one another (Yang, 2002; Lightfoot, 1999; Niyogi and Berwick, 1996). This paper describes the initial stages of a long term project undertaken in consultation with Anthony Kroch, designed to integrate knowledge from these and other areas of linguistics into a mathematical model of the entire history of English. As a first step, this paper examines the verb-second phenomenon, which has caused some difficulty in other simulations. The history of English and other languages requires simulated populations to have certain long-term behaviors. Assuming that syntax can change without a non-syntactic driving force, these requirements place informative restrictions on the acquisition algorithm. Specifically, the behavior of this simulation suggests that children are aware of the topic of a sentence and use it during acquisition, that children take into account whether or not a sentence can be parsed by multiple hypothetical grammars, and that speakers are aware of variety in their linguistic environment but do not make as much use of it individually.</Paragraph>
    <Paragraph position="1"> As discussed in (Yang, 2002) and (Kroch, 1989), both Middle English and Old French had a syntactic rule, typical of Germanic languages, known as verb-second or V2, in which top-level sentences are re-organized: The finite verb moves to the front, and the topic moves in front of that. These two languages both lost V2 word order. Yang (2002) also states that other Romance languages once had V2 and lost it. However, Middle English is the only Germanic language to have lost V2.</Paragraph>
    <Paragraph position="2"> A current hypothesis for how V2 is acquired supposes that children listen for cue sentences that cannot be parsed without V2 (Lightfoot, 1999). Specifically, sentences with an initial non-subject topic and  finite verb are the cues for V2: (1) [CP TopicXP CV [IP Subject . . . ]] (2) [[On this gaer] wolde [the king Stephne taecen. . . ]]  [[in this year] wanted [the king Stephen seize. . . ]] 'During this year king Stephen wanted to seize. . . ' (Fischer et al., 2000, p. 130) This hypothesis suggests that the loss of V2 can be attributed to a decline in cue sentences in speech. Once the change is actuated, feedback from the learning process propels it to completion.</Paragraph>
    <Paragraph position="3"> Several questions immediately arise: Can the initial decline happen spontaneously, as a consequence of purely linguistic factors? Specifically, can a purely syntactic force cause the decline of cue sentences, or must it be driven by a phonological or morphological change? Alternatively, given the robustness of child language acquisition, must the initial decline be due to an external event, such as contact or social upheaval? Finally, why did Middle English and Old French lose V2, but not German, Yiddish, or Icelandic? And what can all of this say about the acquisition process? Yang and Kroch suggest the following hypothesis concerning why some V2 languages, but not all, are unstable. Middle English (specifically, the southern dialects) and Old French had particular features that obscured the evidence for V2 present in the primary linguistic data available for children: * Both had underlying subject-verb-object (SVO) word order. For a declarative sentence with topicalized subject, an SVO+V2 grammar generates the same surface word order as an SVO grammar without V2. Hence, such sentences are uninformative as to whether children should use V2 or not. According to estimates quoted in (Yang, 2002) and (Lightfoot, 1999), about 70% of sentences in modern V2 languages fall into this category.</Paragraph>
    <Paragraph position="4"> * Both allowed sentence-initial adjuncts, which came before the fronted topic and verb.</Paragraph>
    <Paragraph position="5"> * Subject pronouns were different from full NP subjects in both languages. In Middle English, subject pronouns had clitic-like properties that caused them to appear to the left of the finite verb, thereby placing the verb in third position.</Paragraph>
    <Paragraph position="6"> Old French was a pro-drop language, so subject pronouns could be omitted, leaving the verb first.</Paragraph>
    <Paragraph position="7"> The Middle English was even more complex due to its regional dialects. The northern dialect was heavily influenced by Scandinavian invaders: Sentence-initial adjuncts were not used, and subject pronouns were treated the same as full NP subjects.</Paragraph>
    <Paragraph position="8"> Other Germanic languages have some of these factors, but not all. For example, Icelandic has underlying SVO order but does not allow additional adjuncts. It is therefore reasonable to suppose that these confounds increase the probability that natural variation or an external influence might disturb the occurrence rate of cue sentences enough to actuate the loss of V2.</Paragraph>
    <Paragraph position="9"> An additional complication, exposed by manuscript data, is that the population seems to progress as a whole. There is no indication that some speakers use a V2 grammar exclusively and the rest never use V2, with the decline in V2 coming from a reduction in the number of exclusively V2 speakers. Instead, manuscripts show highly variable rates of use of unambiguously V2 sentences, suggesting that all individuals used V2 at varying rates, and that the overall rate decreased from generation to generation. Furthermore, children seem to use mixtures of adult grammars during acquisition (Yang, 2002). These features suggest that modeling only idealized adult speech may not be sufficient; rather, the mixed speech of children and adults in a transitional environment is crucial to formulating a model that can be compared to acquisition and manuscript data.</Paragraph>
    <Paragraph position="10"> A number of models and simulations of language learning and change have been formulated (Niyogi and Berwick, 1996; Niyogi and Berwick, 1997; Briscoe, 2000; Gibson and Wexler, 1994; Mitchener, 2003; Mitchener and Nowak, 2003; Mitchener and Nowak, 2004; Komarova et al., 2001) based on the simplifying assumption that speakers use one grammar exclusively. Frequently, V2 can never be lost in  such simulations, perhaps because the learning algorithm is highly sensitive to noise. For example, a simple batch learner that accumulates sample sentences and tries to pick a grammar consistent with all of them might end up with a V2 grammar on the basis of a single cue sentence.</Paragraph>
    <Paragraph position="11"> The present work is concerned with developing an improved simulation framework for investigating syntactic change. The simulated population consists of individual simulated people called agents that can use arbitrary mixtures of idealized grammars called fuzzy grammars. Fuzzy grammars enable the simulation to replicate smooth, population-wide transitions from one dominant idealized grammar to another. Fuzzy grammars require a more sophisticated learning algorithm than would be required for an agent to acquire a single idealized grammar: Agents must acquire usage rates for the different idealized grammars rather than a small set of discrete parameter values.</Paragraph>
  </Section>
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