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：我们研究限制言论的立法如何能引致网上自我审查并改变在线对话,利用最近的香港国家安全法作为案例研究。我们从香港用户收集700万个历史Tweets数据集,辅之以其他研究人员收集的Tweet流历史照片。我们在颁布国家安全法前后比较在线活动,发现香港用户展示了两种自我审查。第一,香港用户比控制组更有可能,随机抽查Tweet流的历史照片,删除过去的在线活动。具体地说,香港用户删除或限制其账户的可能性比控制组高出三分之一以上,删除过去职位的可能性也高出一倍以上。第二,我们发现香港用户很少发表对中国大陆社会媒体已受到审查的政治敏感话题。 这一趋势在继续增加。