“…The resulting policy mixes are thus seen as having developed unsystematically over time and more or less at random, certainly not as a result of conscious design choices by policy-makers (Van der Heijden, 2011;Howlett and Rayner, 2007;Thelen, 2003). The layering or accumulation of policies over time is generally being seen as impeding the functionality of policy mixes and therefore as a persistent challenge for researchers interested in the effectiveness of policies as such (Adam et al, 2018; see Chapter 16 in this volume). Interestingly, despite the prevalence of the term in the literature, 'layering' as an empirical phenomenon is not clearly defined (Burns et al, 2018b;Van der Heijden, 2011), making it hard to apply in empirical research and thus limiting its value as a conceptual tool to understand policy mixes.…”