Why are language families larger in some regions than in others?

For most of the world’s language families that are not included in yet larger families, published sources were surveyed to determine the number of languages in the family, and also the lexical diversity among those languages as measured by cognate percentages in lexicostatistical wordlists. In this database, lexical diversity tends to be lower in American families than elsewhere; this result is consistent with several alternative explanations, some methodological and some historical. At any given level of diversity, however, African and Eurasian families tend to contain more languages than elsewhere; this result suggests faster historical expansion of language families, relative to lexical replacement within languages, in Africa and Eurasia.