An interesting paper: Political complexity predicts the spread of ethnolinguistic groups. Two basically unsurprising claims that it's good to have calculations supporting: "pastoralists were found to have larger language areas than agriculturalists" and "languages associated with more politically complex societies cover significantly larger areas than those of less complex societies". They also present arguments that "although regions of high biological and cultural diversity do overlap to a striking degree, it is unlikely that biological diversity has any direct effect on cultural diversity on a global scale." Surprisingly, mountainousness was found to correlate with larger language areas, not smaller ones - seems a little suspicious that, though some mountainous areas are pretty un-diverse. Flaws: well, it relies on Ethnologue data and GMI maps, both of which are often unreliable, and systematically more splittist in some areas than in others; but it's not obvious that that would substantially affect the result. Also, ethnic groups, languages, and political units very often don't match up, and their measure of political complexity is based on data for ethnic groups rather than for languages.