Research on ethnolinguistic heterogeneity has so far mostly focused on domestic measures, while little attention has been paid to ethnolinguistic relations between nations. In this paper, I propose a way of measuring ethnolinguistic affinity between nations. This index measures the degree of similarity two randomly drawn individuals from two different populations are expected to display. I show that this measure has several attractive theoretical characteristics, which make it particularly useful. Subsequently, I construct the measure for all countries in Africa and use it to show that civil conflict in Africa is likely to spill over between contiguous ethnolinguistically similar countries.