Mass selection into groups of like-minded individuals may be fragmenting and polarizing online society, particularly with respect to partisan differences^1,2,3,4. However, our ability to measure the social makeup of online communities and in turn, to understand the social organization of online platforms, is limited by the pseudonymous, unstructured and large-scale nature of digital discussion. Here we develop a neural-embedding methodology to quantify the positioning of online communities along social dimensions by leveraging large-scale patterns of aggregate behaviour. Applying our methodology to 5.1 billion comments made in 10,000 communities over 14 years on Reddit, we measure how the macroscale community structure is organized with respect to age, gender and US political partisanship. Examining political content, we find that Reddit underwent a significant polarization event around the 2016 US presidential election. Contrary to conventional wisdom, however, individual-level polarization is rare; the system-level shift in 2016 was disproportionately driven by the arrival of new users. Political polarization on Reddit is unrelated to previous activity on the platform and is instead temporally aligned with external events. We also observe a stark ideological asymmetry, with the sharp increase in polarization in 2016 being entirely attributable to changes in right-wing activity. This methodology is broadly applicable to the study of online interaction, and our findings have implications for the design of online platforms, understanding the social contexts of online behaviour, and quantifying the dynamics and mechanisms of online polarization.

大量选择志同道合的人可能会分裂和极化网络社会，特别是在党派差异方面^1,2,3,4. 然而，我们衡量在线社区的社会构成并进而了解在线平台的社会组织的能力受到数字讨论的匿名、非结构化和大规模性质的限制。在这里，我们开发了一种神经嵌入方法，通过利用大规模的聚合行为模式来量化在线社区在社会维度上的定位。将我们的方法应用于 14 年来在 Reddit 上 10,000 个社区中发表的 51 亿条评论，我们衡量了宏观社区结构在年龄、性别和美国政治党派方面的组织方式。检查政治内容，我们发现 Reddit 在 2016 年美国总统大选前后经历了重大的两极分化事件。然而，与传统观念相反，个人层面的两极分化是罕见的。2016 年的系统级转变主要是由新用户的到来推动的。Reddit 上的政治两极分化与平台上的先前活动无关，而是在时间上与外部事件保持一致。我们还观察到明显的意识形态不对称，2016 年两极分化的急剧增加完全归因于右翼活动的变化。这种方法广泛适用于在线互动的研究，我们的研究结果对在线平台的设计、理解在线行为的社会背景以及量化在线两极分化的动态和机制都有影响。Reddit 上的政治两极分化与平台上的先前活动无关，而是在时间上与外部事件保持一致。我们还观察到明显的意识形态不对称，2016 年两极分化的急剧增加完全归因于右翼活动的变化。这种方法广泛适用于在线互动的研究，我们的研究结果对在线平台的设计、理解在线行为的社会背景以及量化在线两极分化的动态和机制都有影响。Reddit 上的政治两极分化与平台上的先前活动无关，而是在时间上与外部事件保持一致。我们还观察到明显的意识形态不对称，2016 年两极分化的急剧增加完全归因于右翼活动的变化。这种方法广泛适用于在线互动的研究，我们的研究结果对在线平台的设计、理解在线行为的社会背景以及量化在线两极分化的动态和机制都有影响。