Evolutionary Game Theory and Linguistic Typology: a Case Study

The paper deals with the typology of the case marking of semantic core roles. The competing economy considerations of hearer (disambiguation) and speaker (minimal effort) are formalized in terms of evolutionary game theory. It will be shown that the case marking patterns that are attested in the languages of the world are those that are evolutionary stable for different relative weightings of speaker economy and hearer economy, given the statistical patterns of language use that were extracted from corpora of naturally occurring conversations. 1 The frequencies of clause types Consider all (logically) possible case marking types that only use case splits induced by the contrast between pronouns and full NPs. I will restrict attention to possible grammars where the morphological form of the intransitive subject (nominative/absolutive) is less complex than ergative and accusative (if present). Which language types are functional and which aren’t? The main function of case marking is of course to disambiguate, i.e. to enable the hearer to identify the semantic role of the denotation of an NP. More particular, case should uniquely identify the argument roles “A” (agent, i.e. the transitive subject) and “O” (the direct object). We can assume without loss of generality that the hearer always interprets an ergative morpheme as A if there is one, and likewise an accusative morpheme as O, so ambiguity can safely be avoided if at least one NP per clause is case marked. For the sake of brevity, I will denote case marking patterns from now on as a quadruple of case forms, in the order: case of 1. pronominal agents, 2. non-pronominal agents, 3. pronominal objects, and 4. non-pronominal objects. Ergative marking is abbreviated as “e”, accusative as “a”, and zero marking (i.e. nominative/absolutive) as “z”. For instance, a language like English where only pronominal objects are case marked would thus follow the pattern zzaz, while a language like Basque with obligatory ergative marking of all agents is eezz. Ambiguity will only arise if a grammar admits clause types without any case marking. However, this need not lead to ambiguity if one of the two unmarked arguments is prominent and the other isn’t. Then the hearer may em∗ Institute for Linguistics, University of Potsdam, e-mail: [email protected] ploy a default rule to the effect that in such a case the more prominent NP is A (or vice versa). This taken into account, the speaker strategies zeaz and ezza also avoid ambiguity in the sense that there is a corresponding hearer strategy that always correctly identifies semantic roles. One might assume that word order is a good predictor of syntactic roles too, but even in languages with fixed word order there may occur elliptical expressions which are, without the aid of case morphology, ambiguous. Let us assume that disambiguation is the main priority of the speaker, but he has the secondary priority to use as few case morphemes as possible. It depends on the relative frequencies of clause types which patterns minimize the average number of case morphemes per clause. We only have to consider four clause types – both A and O may be p (pronominal) or n (non-pronominal). The percentages in figure 1 are extracted from Geoffrey Sampson’s CHRISTINE corpus of spoken English, and I took pronouns to be prominent and full NPs to be non-prominent. The set of all clauses comprising a subject and a direct object amount to 100%.