In this sense, diversity in the urban public sphere must be regarded both from the perspective of the design and the social character of urban spaces. A diverse urban landscape inevitably has an impact on the development of social diversity. The implementation of an “open city” that not only makes social heterogeneity possible but also promotes it, depends, however, on many other factors. Social behaviour, belonging to a specific milieu, cultural practices and income or class have a major influence on the way diversity is practised.
The question to be answered is how diversity can be lived and which urban planning and socio-psychological insights are helpful. In the 1950s, American psychologist G.W. Allport postulated the so-called contact hypothesis, which says that contact can reduce prejudices (Allport, 1954). For his hypotheses, Allport proposed four key factors that enable contact across groups: an equal status in the situation, common goals, cooperation and support by an official authority (ibid.). These socio-psychological factors can and should serve as orientation points for urban development and planning.