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To date, comparative analyses of human migration and population structure have focused on a single explanatory variable—geographic distance. These models are expanded here by extensions of the gravity model developed by geographers. This model considers the number of mi­grants between populations to be a function of geographic distance and the population sizes of recipient and source populations. The generalized model allows detection of both negative size dependence and positive size depen­dence. Hypothetical examples are used to illustrate interpretations of the coefficients derived from the model. Applications of the model are present­ed using data from three culturally and ecologically distinct populations: horticultural (New Guinea tribes), continental agricultural (historical Mas­sachusetts), and island agricultural (historical Aland Islands). For all samples, the gravity model provides a better fit than simple distance decay models. The results suggest that the most common pattern of migration is from small to large populations.