It is a remarkable fact that the size of slums is similar across the globe, regardless of city, country, or culture [Friesen et al., Habitat Int. 73, 79 (2018)0197-397510.1016/j.habitatint.2018.02.002]. The main thesis of this paper is that this universal scale is intrinsic to the slum-city system and is independent from external factors. By interpreting reaction and diffusion as long- and short-distance migration, our paper explains this universal length scale as resulting from a Turing instability of the interaction of two social groups: poor and rich.