Does the enumeration of ethnic, racial, and/or religious categories on national household censuses increase the likelihood of conflict? The authors propose a theory of intergroup relations that emphasizes the conflictual effects of institutionalizing boundaries between social identity groups. The article investigates the relationship between counting and various forms of conflict with an original, global data set that classifies the type of enumeration used in more than one thousand census questionnaires in more than 150 countries spanning more than two centuries. Through a series of cross-national statistical analyses, the authors find a robust association between enumeration of ethnic cleavages on the census and various forms of competition and conflict, including violent ethnic civil war. The plausibility of the theory is further demonstrated through case study analysis of religious conflict in India.