Although the majority of the population remained bilingual until the second half of the 20th century, the original Brabantian dialect was often no longer passed down from one generation to another, leading to an increase of monolingual French-speakers from 1910 onwards. From the mid-20th century, the number of monolingual French-speakers surpassed the number of mostly bilingual Flemish inhabitants. This process of assimilation weakened after the 1960s, as the language border was fixed, the status of Dutch as an official language of Belgium was reinforced, and the economic centre of gravity shifted northward to Flanders. However, with the continuing arrival of immigrants and the post-war emergence of Brussels as a centre of international politics, the relative position of Dutch continued to decline. Furthermore, as Brussels' urban area expanded, a further number of Dutch-speaking municipalities in the Brussels periphery also became predominantly French-speaking. This phenomenon of expanding Francisation — dubbed "oil slick" by its opponents — is, together with the future of Brussels, one of the most controversial topics in Belgian politics.