This chapter proposes a theoretical framework in which the absence or the presence of cross-fertilization depends on how a court and its judges strike the balance between the potential persuasive value of external citations and the potential concerns about deference that may arise from grounding decisions in external sources. It then tests observable implications from this theoretical perspective using a new data set of European Court of Human Rights (ECtHR) references to international court rulings and international legal materials. The findings generally support the theoretical framework. The ECtHR mentions external international law sources in 15 per cent of the judgments that engage in some form of new legal interpretation, but most of these references are to Council of Europe (CoE) documents or institutions, which are typically not controversial from a deference perspective. By contrast, the ECtHR cites international court judgments in only 3 per cent of its judgments and does so even more rarely in violation findings, which raise more deference concerns. The conclusion discusses the implications for other courts.