If you see someone not wearing a mask, do you say something?

Stigma can, however, work both ways, either boosting or blocking behaviors that can be lifesaving for public health. Smoking went from being a “cool” norm to being widely frowned upon, though that took years of medical research and public health campaigns. Before 9/11, you could leave your suitcase in an airport terminal momentarily to go to the bathroom; now it triggers fear and police intervention, likewise reinforced by unceasing public messages: “If you see something, say something.”

In the mid-1990s, as a faculty member in Columbia’s School of Public Health, I engaged in fierce debates about whether to try to stigmatize people who didn’t wear condoms. Many advocates for AIDS patients argued that we would then be “blaming the victim,” since people living with H.I.V. would thereby be forced to reveal they had the virus. But public health experts persevered, arguing that anyone who is sexually active with multiple partners should wear a condom, not just those who were H.I.V.-positive. Celebrities like Magic Johnson reinforced the message by publicly disclosing their own infections and urging safer sex practices, helping to increase condom use.