Morphological complexity is widely believed to increase with sociolinguistic isolation, and to decrease with language spreads and absorption of L2 adult learner populations. However, this can be assessed only for communities with well-described histories. Morphological complexity has also been shown to be greater in higher-altitude languages, which are often sociolinguistically isolated, so we use altitude as an empirically determinable proxy for sociolinguistics. In past research, only a very few small locations have been surveyed and the measures of complexity used were family-specific and not easily generalizable. We apply several improved measures of complexity and show that the correlation holds, especially in the Andean regions of South America. We discuss the implications of the South American pattern for the settlement of the Americas and post-settlement prehistoric population formation.Peer reviewe
