In particular we have shown that the largest single factor predicting the area over which a language is spoken is the degree of political complexity exhibited by the society speaking that language. This is consistent with the hypothesis that more complex societies replace or incorporate less complex groups and thus spread their languages over larger areas. As political complexity is a property of groups, andPNAS doi:10.1073/pnas.0804698106
competition often occurs between groups, rather than just between individuals, if more politically complex groups tend to replace or incorporate others, then the proportion of more politically complex societies will tend to increase over time. Such a mechanism represents a process of cultural group selection (21, 48). An interesting area for future research will be to assess the impact this process has on the biological fitness of individuals within groups (49). Increasing political complexity is almost
always associated with greater degrees of social stratification, and wealth in the form of tax or tribute is often extracted by political elites from those lower down the social order (24), which could clearly have significant reproductive consequences for individuals at different levels in such societies. It will be important to assess empirically whether these costs are outweighed by benefits gained from being a member of such a group and from the advantage held in competition between groups.
Political complexity predicts the spread of ethnolinguistic groups
Thomas E. Currie and Ruth Mace
Human languages show a remarkable degree of variation in the area they cover. However, the factors governing the distribution of human cultural groups such as languages are not well understood. While previous studies have examined the role of a number of environmental variables the importance of cultural factors has not been systematically addressed. Here we use a geographical information system (GIS) to integrate information about languages with environmental, ecological, and ethnographic data to test a number of hypotheses that have been proposed to explain the global distribution of languages. We show that the degree of political complexity and type of subsistence strategy exhibited by societies are important predictors of the area covered by a language. Political complexity is also strongly associated with the latitudinal gradient in language area, whereas subsistence strategy is not. We argue that a process of cultural group selection favoring more complex societies may have been important in shaping the present-day global distribution of language diversity.