Molecular genetic studies of psychiatric disorders must face the possibility that despite the significant contribution of genetic factors to the expression of syndromes like schizophrenia, these syndromes may be a heterogeneous collection of genetic and non-genetic illnesses. These illnesses may be etiologically distinct from each other and still share many clinical features in common. Linkage studies of families with multiple affected members tend to favor the selection of genetic forms of a syndrome but can still represent a heterogeneous set of different genetic illnesses. To limit the potential genetic heterogeneity of a study sample, we selected a population that was geographically isolated and was historically relatively genetically homogeneous. We then assessed the relative level of homogeneity utilizing a surname analysis of the population of the Azores, mainland Portugal, rural USA, and urban USA. The average number of families with the same last name corrected for population size in the Azores is 30.88, in Coimbra it is 21.42, compared to 1.13 in a rural American population and 0.38 in an urban American population. The results of this analysis indicate that the Azores have the highest degree of homogeneity, and mainland Portugal has a high degree of homogeneity.