The propinquity effect is a concept proposed by psychologists Leon Festinger, Stanley Schachter, and Kurt Back, referring to the apparent fact that the more frequently we interact with specific individuals, the more likely we are to form friendships and romantic relationships with them.
The hypothesis was based on a study carried out by the researchers in 1950 in the Westgate student apartments on the campus of MIT Massachusetts, USA.
Festinger, L., Schachter, S., Back, K., (1950) “The Spatial Ecology of Group Formation”, in L. Festinger, S. Schachter, & K. Back (eds.), Social Pressure in Informal Groups, 1950. Chapter 4.