A tale of two hemispheres

specific type of gravity model when a homogeneous distribution of the populations (and thus opportunities) is assumed. This could explain why the gravity-theory analogy has been so persistent in mobility research, and also indicates that it is equivalent to an idealized scenario, one hardly ever encountered in real-life settings. The radiation model may provide a route to further exploration. It could be useful for researchers interested in understanding processes mediated by human mobility, such as the introduction of animals and plants into a new habitat and the spread of human infectious diseases. Until now, computational models in these areas relied on direct data implementation and gravity models to fill gaps in incomplete mobility data sets. A pertinent question for future work is why some societies are more mobile than others. A comparative analysis between the United States and European countries would be a promising starting point for addressing this issue. ■