Polarisation research has demonstrated how people cluster in homogeneous groups with opposing opinions. However, this effect emerges not only through interaction between people, limiting communication between groups, but also between narratives, shaping opinions and partisan identities. Yet, how polarised groups collectively construct and negotiate opposing interpretations of reality, and whether narratives move between groups despite limited interactions, remains unexplored. To address this gap, we formalise the concept of narrative polarisation and demonstrate its measurement in 212 YouTube videos and 90,029 comments on the Israeli-Palestinian conflict. Based on structural narrative theory and implemented through a large language model, we extract the narrative roles assigned to central actors in two partisan information environments. We find that while videos produce highly polarised narratives, comments significantly reduce narrative polarisation, harmonising discourse on the surface level. However, on a deeper narrative level, recurring narrative motifs reveal additional differences between partisan groups.

翻译：极化研究已揭示人们如何聚集于持有对立观点的同质化群体中。然而，这种效应不仅通过人际互动（限制群体间交流）产生，亦通过叙事互动——塑造观点与党派认同——形成。尽管如此，极化群体如何集体构建并协商对现实的对立阐释，以及叙事是否能在有限互动下跨越群体传播，仍未得到充分探索。为填补这一空白，我们形式化定义了叙事极化的概念，并通过对212个YouTube视频及90,029条关于巴以冲突的评论进行测量予以验证。基于结构叙事理论并借助大语言模型实现，我们从两个党派性信息环境中提取出核心行动者被赋予的叙事角色。研究发现：虽然视频内容产生高度极化的叙事，但评论显著降低了叙事极化程度，在表层实现了话语调和；然而在深层叙事层面，反复出现的叙事母题揭示了党派群体间更多差异。