The idea that social identities constitute the glue that links social groups to parties is central to cleavage theory. Yet efforts to measure the social identity component of cleavages are fairly recent. In this project, we use original survey data to tap into the shared understandings and group boundaries of the social groups situated on the two sides of the universalism-traditionalism/particularism cleavage. A pilot study was focused on the Swiss case and is published in CPS (link). In a comparative perspective extending the analysis to France, Germany, and the UK, we address the question to which extent has the new cultural divide that opposes the New Left and the far right evolved into a full-blown cleavage. We also explore whether the more fine-grained identities rooted in social structure coalesce into broader political identities. Furthermore, we study how closure at the level of social structure interacts with political agency in group identity formation and political polarization.