Contemporary West European shantytowns have essentially been studied with qualitative methods. Questions related to their ethnic structure, homophily and interaction with local institutions have not been analysed through large samples and survey data. Based on the example of Romanian and Bulgarian Roma living in shantytowns in the Parisian metropolitan area, we analyse the ‘historical’ region of origin (autochthony), networks between individuals and households having lived together in a shantytown, as well as some of their attitudes, skills, and behaviour (i.e. expectations as to local welfare, French proficiency, children’s schooling). We used a database of slum-dwellers in Paris collected by social workers (N = 12,019). The paper looks at how autochthonies combine with socialisation in shantytowns and with territorial institutional effects of local policies. Evidence shows that while there are limited differences in the socio-professional backgrounds of slum-dwellers, there are several differences in attitudes, behaviour and skills. Moreover, social network analysis shows that these differences are only weakly related to the households’ region of origin (autochthony), while they are more correlated to emergent structural clusters of co-habitation connections, where individuals socialise in acting together to build and manage a shantytown. Moreover, local policies at the city level play a role in shaping shantytown dwellers expectations and skills for integration.