Measuring the Similarity of Grammatical Gender Systems by Comparing Partitions

Proceedings of the 2020 Conference on Empirical Methods in Natural Language Processing, pages 5664–5675, November 16–20, 2020. c⃝2020 Association for Computational Linguistics 5664 Measuring the Similarity of Grammatical Gender Systems by Comparing Partitions Arya D. McCarthy Adina Williams Shijia Liu David Yarowsky Johns Hopkins University, Facebook AI Research, ETH ZurichRyan Cotterell arya@jhu.edu, adinawilliams@fb.com, liu.shij@northeastern.edu, yarowsky@jhu.edu, ryan.cotterell@inf.ethz.ch Abstract A grammatical gender system divides a lex- icon into a small number of relatively fixed grammatical categories. How similar are these gender systems across languages? To quantify the similarity, we define gender systems ex- tensionally, thereby reducing the problem of comparisons between languages’ gender sys- tems to cluster evaluation. We borrow a rich inventory of statistical tools for cluster evalu- ation from the field of community detection (Driver and Kroeber, 1932; Cattell, 1945), that enable us to craft novel information-theoretic metrics for measuring similarity between gen- der systems. We first validate our metrics, then use them to measure gender system similarity in 20 languages. Finally, we ask whether our gender system similarities alone are sufficient to reconstruct historical relationships between languages. Towards this end, we make phylo- genetic predictions on the popular, but thorny, problem from historical linguistics of inducing a phylogenetic tree over extant Indo-European languages. Languages on the same branch of our phylogenetic tree are notably similar, whereas languages from separate branches are no more similar than chance. 1 Introduction As many as half the world’s languages carve nouns up into classes (Corbett, 2013). In these languages, nouns are subdivided into gender categories, which together comprise the language’s grammatical gender system. A gender system tends to use a small, fixed number of categories with fixed usage across speakers. Such categories, like ‘feminine’, can be defined extensionally,1 and are reflected by agreement with other words within the noun phrase (i.e., concord). Gender 1When we talk about the extension of a gender system, we refer to the set of nouns that belong to each gender. This stands in contrast to the intension of that gender system, which would be the governing dynamics that gave rise to the particular partitions observed. See §3. (a) German, K = 3 (b) Spanish, K = 2 Figure 1: Two gender systems partitioning N = 6 con- cepts. German (a) has three communities: Obst (fruit) and Gras (grass) are neuter, Mond (moon) and Baum (tree) are masculine, Blume (flower) and Sonne (sun) are feminine. Spanish (b) has two communities: fruta (fruit), luna (moon), and flor are feminine, and cesped (grass), arbol (tree), and sol (sun) are masculine. exhaustively divides up the language’s nouns; that is, the union of gender categories is the entire nominal lexicon. Taken this way, a gender system can be viewed as a partition of the lexicon into communities of same-gendered nouns. Given this, a lexical typologist might naturally wish to ask: how similar are two languages’ gender systems? Using modern statistical and information- theoretic tools from the community detection liter- ature, we offer the first cluster evaluation (Jardine et al., 1971) perspective on grammatical …

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