In human geography, gravity models evaluate, or forecast, the spatial interactions of goods, people, and so on, between origins and destinations. ‘The gravity model assumes that the probability of patronizing a facility (facility utility) is proportional to the attractiveness of the facility and inversely proportional to a function of the distance’ (Drezner and Drezner (2006) Geog. Analys. 38, 4). These interactions are expressed as:

where I_ij denotes the interaction between two locations i and j; M_i and M_j represent the ‘masses’ measuring the strength of i and j (usually the population numbers of two settlements); Dist_ij stands for the distance between i and j; and k and β‎ are constants (G. Robinson 1998). ‘While the gravity model is now accepted by economics as a useful tool (it is said to be all over the place at the International Monetary Fund), it is considered only in an incidental manner in economic analysis that employs it’ (Isard (2001) Int. Reg. Sci. Rev. 24, 3). See Rodríguez-Pose and Zademach (2006) Tijdschrift 97, 3 on the gravity model, and mergers and acquisitions.