Procedures
As previously described, participants who began the study in Qualtrics completed sociodemographic questions intermixed with quality control checks (see our rationale for the importance of such checks in [Intervention image.Control image.Once participants advanced, they were shown a smaller copy of the image they had just viewed along with 1 (control) or 2 (intervention) comprehension questions. Participants who responded correctly to all questions advanced in the study, and those who did not respond correctly were returned to the larger image and shown the message, “At least one of your answers was incorrect. Please re-read the post carefully and then answer the following (one/two) questions to verify your understanding of the post.” Such participants then moved forward to the comprehension question page again. The comprehension questions were prespecified in our protocol [Both arms: (true/false) “In the social media post you read about a scientific study from April 2020, the study authors found that face masks could reduce the spread of COVID-19 as well as lowering hospitalizations and deaths.”Intervention arm only: (true/false) “In the social media post you read about a scientific study from April 2020, the study authors recommended that everyone in the US should start wearing masks immediately.”Comprehension was generally good. For the intervention group, 38/778 (4.9%) responded incorrectly to 1 or both questions the first time, and only 5 participants did the second time (0.6%). In the control arm, 14/748 (1.9%) responded incorrectly to the question the first time, and none did the second time.Finally, participants completed all prespecified outcome measures and covariates (see the “Outcomes” section and the protocol [