We study how legislation that restricts speech can induce online self-censorship and alter online discourse, using the recent Hong Kong national security law as a case study. We collect a dataset of 7 million historical Tweets from Hong Kong users, supplemented with historical snapshots of Tweet streams collected by other researchers. We compare online activity before and after enactment of the national security law, and we find that Hong Kong users demonstrate two types of self-censorship. First, Hong Kong users are more likely than a control group, sampled randomly from historical snapshots of Tweet streams, to remove past online activity. Specifically, Hong Kong users are over a third more likely than the control group to delete or restrict their account and over twice as likely to delete past posts. Second, we find that Hong Kong users post less often about politically sensitive topics that have been censored on social media in mainland China. This trend continues to increase.

翻译：我们以近期香港国安法为案例，研究限制言论的立法如何引发网络自我审查并改变网络话语。我们收集了700万条香港用户的历史推文数据集，并辅以其他研究者收集的推文流历史快照。通过比较国安法实施前后的线上活动，发现香港用户表现出两种自我审查行为。首先，相较于从历史推文流快照中随机抽取的对照组，香港用户更倾向于删除过往网络活动——具体而言，其删除或限制账户的可能性比对照组高出三分之一以上，删除历史帖文的概率更是两倍以上。其次，我们发现香港用户对已被中国大陆社交媒体审查的政治敏感话题发帖频率显著降低，且该趋势持续加剧。