This paper examines the partner selection of the lower classes during an urban crisis period in early industrial Belgian cities. It was found that in this period characterized by an economic transition, overpopulation, migration and a low standard of living, social heterogamy was high, whereas social homogamy increased, or was ‘restored', in the subsequent period. The urban crisis effect on partner selection contradicts the claims of modernization theory that there was a gradual increase in societal openness and that societal openness was typically modern, but it fits the idea of the informalization of marriage, a process marked by an increase in unmarried cohabitation and illegitimacy.