The article analyses single-parenthood as a topic of discussion in the agenda-setting for diversity and anti-discrimination policy in the Brussels-capital region. Inspired by the founding works of Herbert Blumer (1971), we will retrace within a sociological perspective how various actors (public administrations, governments, unions, as well as the « civil society » and researchers) have participated to a long process of « collective definition » that constructed single-parenthood as a new category (Dubois, 2009) of regional diversity policy. Based on a qualitative survey (observation and interviews), as well as longitudinal statistics, we analyse how the various situations of single-parenthood are approached in a tension between the will to assert and recognize the specificities of these situations, and the choice to favour a universalistic approach. The notion of “adapted universalism” permits to explain, following our hypothesis, the connections carried out the last years with diversity policies.