What Does the Atlas Narodov Mira Measure?

The Atlas Narodov Mira has been used extensively to investigate the effects of ethnic divisions, but little is known about how it defines ethnicity. Most theories of ethnicity emphasize the tendency of group members to marry within the group (endogamy). This note surveys studies of ethnic intermarriage to evaluate whether the groups in the Atlas: (1) are endogamous and (2) have no endogamous subgroups. I find that the groups are generally endogamous but there are a number of cases of groups with endogamous subgroups. Therefore, the data tend to underestimate ethnic divisions.