Shaping states into nations: The effects of ethnic geography on state borders

Abstract Borders define states, yet little systematic evidence explains where they are drawn. Putting current challenges to state borders into perspective and breaking new methodological ground, this paper analyzes how ethnic geography and nationalism have shaped European borders since the 19 th century. We argue that nationalism creates pressures to redraw political borders along ethnic lines, ultimately making states more congruent with ethnic groups. We introduce a Probabilistic Spatial Partition Model to test this argument, modeling state territories as partitions of a planar spatial graph. Using new data on Europe's ethnic geography since 1855, we find that ethnic boundaries increase the conditional probability that two locations they separate are, or will become, divided by a state border. Secession is an important mechanism driving this result. Similar dynamics characterize border change in Asia but not in Africa and the Americas. Our results highlight the endogenous formation of nation‐states in Europe and beyond.