Demography and socio-economic stratification are two crucial factors that determine language contact outcomes ranging from areal convergence in heteroglossic small-scale societies to structurally arrested standard languages in monoglossic nation-states. Between these idealized poles, creoles, koines, lingua francas, urban sociolects and other languages with large communities of later learners are sites of intense contact and change, particularly in the Global South. Next to core linguistics, the study of social factors in language contact draws on methods of historical research, political economic analysis, and increasingly, linguistic data sciences. Dramatic socio-economic and demographic transformations in the next few decades are likely to alter the presently still limited understanding of how social factors shape language contact and change.