Trimming Phonetic Alignments Improves the Inference of Sound Correspondence Patterns from Multilingual Wordlists (2023)

Abstract: Sound correspondence patterns form the basis of cognate detection and phonological reconstruction in historical language comparison. Methods for the automatic inference of correspondence patterns from phonetically aligned cognate sets have been proposed, but their application to multilingual wordlists requires extremely well annotated datasets. Since annotation is tedious and time consuming, it would be desirable to find ways to improve aligned cognate data automatically. Taking inspiration from trimming techniques in evolutionary biology, which improve alignments by excluding problematic sites, we propose a workflow that trims phonetic alignments in comparative linguistics prior to the inference of correspondence patterns. Testing these techniques on a large standardized collection of ten datasets with expert annotations from different language families, we find that the best trimming technique substantially improves the overall consistency of the alignments, showing a clear increase in the proportion of frequent correspondence patterns and words exhibiting regular cognate relations.

## introduction with the introduction of automated methods for the inference of correspondence patterns from multilingual wordlists @xcite , computational historical linguistics has acquired a new technique with multiple applications in the field. correspondence patterns have been used to identify problematic cognate judgments in individual datasets @xcite or to assess their general characteristics @xcite , they have been used as the basis to predict cognate reflexes @xcite @xcite or to reconstruct protoforms @xcite . they have also shown to be useful to compare different cognate judgments with respect to the overall regularity they introduce in a multilingual dataset @xcite . while machine-readable correspondence patterns have already shown to be useful for various tasks in historical linguistics, their basic properties have so far not yet been thoroughly investigated. thus, although we can easily see that correspondence patterns show long-tail distributions with respect to the number of alignment sites that individual patterns reflect in multilingual datasets, no closer investigations of these patterns have been carried out so far. here, historical linguistics can learn from evolutionary biology, where specific characteristics of alignments of dna or protein sequences have been investigated for several decades now. scholars have also looked into the characteristics of those alignment sites that turn out to be problematic when it comes to phylogenetic reconstruction and similar secondary tasks @xcite . in order to handle these "irregular" sites, biologists have proposed methods to trim alignments by removing sites that contradict more general evolutionary tendencies. this allows scholars to reduce the amount of artifacts in the data and retrieve more accurate information about the evolutionary processes behind the alignments. in computational historical linguistics, trimming of alignments has so far been ignored. in classical historical language comparison, however, the practice of ignoring specific sites in the alignment of cognate words has been practiced for a long time. when arguing for particular sound changes or correspondence patterns, scholars routinely consider only the supposed root of a cognate set @xcite , ignoring inflectional and derivational markers or irregular parts of individual cognate reflexes. while this is a common procedure for the comparative method, it is seldom made explicit. one of the few cases where this process is made explicit is offered by @xcite . here, the author provides an alignment matrix where all the non-cognate material is set into brackets, distin-guishing them from the true alignment sites. this step is accompanied by a detailed discussion of the morphemic elements and its implication for reconstructing the proto-forms, a step that is rarely put into such detail. the importance of this practice is also reflected in tools that allow for the manual correction of alignments, like edictor @xcite and reflex @xcite which offer options to flag alignment sites as problematic (or important). specifically the trimming facility of the edictor tool has also been used to increase the transparency of cognate sets in studies devoted to phylogenetic reconstruction @xcite . given the highly skewed distributions of alignment sites over correspondence patterns in computational comparative linguistics and the practice of human annotators to regularly ignore certain parts of phonetically aligned cognate sets in historical linguistics, it would be beneficial to find automated ways to trim phonetic alignments in multilingual wordlists. trimmed alignments could either form the basis of a more extensive annotation of phonetic alignments in a computer-assisted setting @xcite , or they could serve as the basis of extensive cross-linguistic, typologically oriented studies devoted to the regularity of sound change and sound correspondence patterns. for example, correspondence patterns have already been used in typological studies investigating the history of pronoun systems in south america (rojas-berscia and roberts, 2020), or for studies with simulated data that use phonetic alignments to construct artificial cognate sets @xcite . in the following, we will provide a first framework for the trimming of phonetic alignments and test it on ten datasets from typologically diverse language families. our experiments show that trimming increases the overall regularity of the correspondence patterns -even when using very rudimentary strategies-and thus shrinks the long tail of their distributions over alignment sites. the closer inspection of individual trimmed alignments, however, also shows that our methods still have a lot of room for improvement. we conclude by pointing to various techniques that could enhance the trimming of phonetic alignments in the future. ## background sound correspondences are the core of the comparative method. they form the basis for proving ge-netic relationship between languages, for establishing the internal classification of language families, as well as for the reconstruction of proto-languages. sets of sound correspondences are commonly analyzed as correspondence patterns. a crucial component of correspondence patterns in contrast to sound correspondences is that the correspondence set is not defined on the basis of language pairs, but rather as a pattern shared between several languages @xcite . in other words, a correspondence pattern is defined as the set of sounds in any number of daughter languages that derive from the same phoneme of the ancestral language in a specific environment @xcite . in order to qualify as a pattern, sound correspondences must be backed by many examples. examples are drawn from concrete cognate sets that need to be phonetically aligned in order to reveal which sounds correspond with each other. in order to constitute a valid pattern that would be accepted as a regular or systematic sound correspondence @xcite , a considerably large amount of examples backing a certain pattern must be assembled from the data. this step is necessary to avoid chance similarities resulting from erroneous cognate judgments or undetected scarce borrowings. while the minimum number of examples is not universally agreed upon, most scholars tend to accept two or three examples as sufficient to consider a pattern as regular. correspondence patterns are typically represented with the help of a matrix, in which the rows correspond to individual languages and the columns correspond to patterns, with cell values indicating the sounds (the reflexes) of individual language varieties in individual patterns @xcite . correspondence patterns are traditionally inferred by manually inspecting phonetic alignments of cognate sets, trying to identify individual columns (alignment sites) in the alignments that are compatible with each other @xcite . figure 1 illustrates this process with phonetic alignments of fictitious words from fictitious languages. in order to reconstruct the ancestral form underlying a cognate set, it is common to ignore certain sites in the alignment that are considered as difficult to align. problems of alignability @xcite usually result from the fact that words in a cognate set are not entirely, but only partially cognate. this can be due to processes of word formation or inflection in individual language varieties @xcite , as illustrated in figure 2 with data from quechua @xcite . ## conclusion we introduce the concept of trimming multiple sequence alignments, originally developed for applications in evolutionary biology, to the field of historical linguistics. trimming as such is already practiced implicitly in the comparative method, but as of yet, there are no computational implementations for the procedure. our trimming algorithms provide considerable improvements compared to state-of-the-art alignment methods. by trimming the alignment sites down to a subsequence without gaps, we achieve a higher number of regular correspondence patterns and cognate sets than without trimming. even though our technique is merely a very preliminary approximation to the classical workflow of the comparative method, the average regularity of correspondence patterns across data sets is improved in all settings analyzed. our study thus shows that automated trimming is both achievable and worthwhile in computational historical linguistics. the main target of our trimming-strategies were alignment sites that included more gaps than defined in a certain threshold. our model comparison shows that the best results are achieved when all such sites are trimmed, rather than only those at the periphery of stable alignment sites. similar to biology, we find that alignment sites with many gaps contain divergent information, and trimming them improves the accuracy of our methods. it is also not sufficient to trim sites at random, since in that case we lose correspondence patterns that explain the data well. the examples we provide show both the potential of trimming alignment sites and their methodological limitations. the success of our strategy varies considerably between the datasets. a closer analysis of those cases where improvements are considerably small could provide valuable information for improved trimming strategies to be implemented in the future.