Migration Space: Doubly Constrained Gravity Model Mapping of Relative Interstate Separation

Absolute physical distances are notoriously bad measures of the relative functional distances that influence the destination choices of U.S. interstate migrants. A novel calibration procedure for doubly constrained gravity models is proposed to map functional relative interstate separations. Doubly constrained models exactly reproduce observed migrant supplies and migrant demands. Using straight-line distances, however, they poorly reproduce the observed state-to-state patterns of flow. The proposed alternative calibration procedure leaves interstate separations as model outputs rather than model inputs. Distances are chosen to replicate observed flows. These distances define a “migration space” surrounding origin and destination states.