Self-Censorship Under Law: A Case Study of the Hong Kong National Security Law

Hong Kong Twitter users are 33% more likely than a random control sample to have protected their accounts, and over 247% more likely to have deleted past Tweets, after enactment of the June 2020 national security law (NSL). These differences are statistically significant at p ≤ 1.74e-48 for all account-protection and Tweet-deletion metrics.

§4.1 / Table 2 policy keyword-filteringmeasurement-platform cn

Hong Kong Twitter discussion of COVID-19 continued declining after mid-2020 and did not resurge during Hong Kong's large March 2022 COVID wave, unlike the control group whose COVID discussion tracked local transmission rates. The authors interpret this anomalous pattern as a generalized chilling effect: NSL legal risk suppressed even politically ambiguous health discussion that mainland China had censored but that may not clearly fall under the NSL.

§4.2 / Figure 2c policy keyword-filteringmeasurement-platform cn

Of inaccessible Tweets from 2019, those containing NSL-sensitive political keywords are disproportionately deleted or protected by both Hong Kong users (36.38% inaccessibility) and Taipei users (34.89%), compared to New York City (29.45%) and Tokyo (30.80%). This suggests that NSL legal exposure — which extends extraterritorially under Article 38 — may be chilling speech even among users outside Hong Kong who transit or have ties to mainland China.

§4.1 / Table 3 policy keyword-filteringmeasurement-platform cn

While Hong Kong users sharply reduced discussion of NSL-sensitive political topics after July 2020, their rate of Tweeting about non-sensitive topics (travel, food, art, media) remained stable and mirrored control-group trends. This targeted suppression — rather than a general withdrawal from Twitter — confirms the NSL produced specific self-censorship of covered speech rather than platform abandonment.

§4.2 / Figure 2b policy keyword-filteringmeasurement-platform cn

After the NSL entered into force in July 2020, the proportion of Hong Kong Tweets containing NSL-sensitive political keywords declined steadily and never returned to prior levels. By contrast, the control group's equivalent keyword usage rebounded (e.g., surging around the August 2021 Taliban takeover and March 2022 Ukraine invasion), indicating the Hong Kong decline is attributable to legal chilling rather than global topic cycles.

§4.2 / Figure 2a policy keyword-filteringmeasurement-platform cn