The gravity model for social systems

Gravity is one of the most prominent models used across various social areas, including economics, demography, mobility, politics, and other systems where spatial interactions are relevant. The model represents a flexible approach that captures important regularities that might be detected when thousands or millions of people are observed, i.e. bigger origins and bigger destinations increase the intensity of interactions, but it tends to decay with longer distances between them. Therefore, the number of interactions between two locations is frequently modelled with an equation that resembles Newton's Law of Gravitation (also known as the Law of Gravity). Here, I explore different aspects, i.e. essential components, techniques for estimating the parameters, interpretation of results, and settings where gravity is a helpful tool, including mobility and migration.