This paper explores possibilities of cooperation between rationalist and sociological insights by examining the institutionalization of legislative powers in the European Parliament (EP). By this, we refer to the social processes unfolding as the EP goes from being a de jure legislator (in virtue of its constitutional empowerment in successive EU treaties), to becoming a living legislator (in virtue of its everyday engagement in EU ordinary legislation subject to the so-called co-decision procedure). Between 1994 and 2014, the number of policy areas constitutionally subject to co-decision was multiplied by five, putting the EP on par with the Council of Ministers ('Council') in a broad range of domains ranging from highly redistributive types of policies such as the Common Agricultural Policy to technically arcane policies in the domain of financial regulation and harmonization. During the course of the 7th term, the EP had passed 488 legislative acts under co-decision (European Parliament, 2014a). In legislating, Members of the European Parliament (MEPs) develop and mobilize a collection of norms, which regulate their collective behavior while infusing it with purpose. In this paper, we are interested in tracing and explaining this developing body of norms, drawing on the rich theoretical scholarship on norms
