The Gravity Model

Predicting US Migration with the Gravity Model

The gravity model of migration is a model, derived from Newton’s law of gravity, is used to predict the degree of interaction between two places (Rodrigue et al. 2009, 216). Newton’s law states that: “Any two bodies attract one another with a force that is proportional to the product of their masses and inversely proportional to the square of the distance between them.”

When used geographically, the words ‘bodies’ and ‘masses’ are replaced by ‘locations’ and ‘importance’ respectively, where importance can be measured in terms of population numbers, gross domestic product, or another appropriate variables. The gravity model of migration is therefore based upon the idea that as the importance of one or both of the location increases, there will also be an increase in movement between them. The farther apart the two locations are, however, the movement between them will be less. This phenomenon is known as distance decay.

The basic gravity model is as follows:

 Mij=k Pi*Pj dij2
 where: Mij = gravity model prediction of migration between origin i and destination j Pi = population of origin State i Pi = population of destination State j dij = distance from origin i to destination j k = a constant that adjusts the gravity model estimates so that the total numbers of actual and estimated migrants are approximately equal

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