ETHNICITY IS THE primary unit of social organization in some areas of the world. In parts of Asia and Africa, ethnicity is more important than other measures of difference, such as class and nationality. However, in much of the Western world, nationality, class, politics, and religion have emerged as the main societal divisions. These divisions, though based on the same underlying principle of group difference as ethnicity, are cross-cutting divisions. For example, Western political parties incorporate people of many different ethnicities; party affiliation has more to do with philosophical and economic allegiances than ethnic ones.

The forces of European history are responsible for this structuring of social divisions along national, religious, political, and class lines, rather than ethnic ones. In the 1500s, the Protestant Reformation split European Christians into Catholic and Protestant camps that did not correspond to, for example...