The difficulty of identifying susceptibility genes for common diseases has polarized geneticists' views on what disease models are appropriate and how best to proceed once high-density genome maps become available. Different disease models have different implications for using linkage or linkage-disequilibrium-based approaches for mapping complex disease genes. We argue that the choice of study population is a critical factor when designing a study, and that genetically simplified isolates are more useful than diverse continental populations under most assumptions.