This paper examines the proposition that the economic performance of advanced capitalist countries depends on their size and ethnic composition. As such it blends insights from two important literatures in comparative political economy. One is exemplified by the work of Peter Katzenstein, who wrote the classic treatise on the relationship between nation-state size and economic performance. Another is illustrated by the work of Ernest Gellner, whose work suggested that economic performance depends on the ethnic composition of the nation-state. The argument is tested on pooled data from 30 advanced capitalist countries for the 1985 through 2007 period. Regression analysis confirms that ethnically homogenous countries tend to have stronger rates of economic growth during this period than ethnically heterogeneous countries but that neither the size of countries nor the interaction of size and ethnic composition have significant effects. This points to the need for further exploration of these issues either with data covering a longer time frame or historical case studies.