A critique of the separation base method for genealogical subgrouping, with data from mixe-zoquean

Holm (2000 Holm, H. , 2000. Genealogy of the main Indo-European branches: Applying the separation base method , Journal of Quantitative Linguistics 7.2 (2000), pp. 73–95.Taylor & Francis Online , Google Scholar) proposes the “separation base” method for determining subgroup relationships in a language family. The method is claimed to be superior to most approaches to lexicostatistics because the latter falls victim to the “proportionality trap”, that is, the assumption that similarity is proportional to closeness of relationship. The principles underlying Holm's method are innovative and not obviously incorrect. However, his only demonstration of the method is with Indo-European. This makes it difficult to interpret the results, because higher-order Indo-European subgrouping remains controversial. In order to have some basis for verification, we have tested the method on Mixe-Zoquean, a well-studied family of Mesoamerica whose subgrouping has been established by two scholars working independently and using the traditional comparative method. The results of our application of Holm's method are significantly different from the currently accepted family tree of Mixe-Zoquean. We identify two basic sources of problems that arise when Holm's approach is applied to our data. The first is reliance on an etymological dictionary of the proto-language in question, which creates problems of circularity that cannot be overcome. The second is that the method is sensitive to the amount of documentation available for the daughter languages, which has a distorting effect on the computed relationships. We then compare the results of Holm's approach with lexicostatistics and show that the latter actually performs quite well, producing a family tree for Mixe-Zoquean very similar to the one arrived at through the comparative method.