Other parts of the city were just as lively this past weekend. The regional magazine Washingtonian cataloged the crowds and long lines at a German beer bar, a cupcake shop, and upscale restaurants, among other establishments. Other media outlets and social-media posts documented the masses that showed up to bars in Los Angeles, Nashville, New Orleans, Chicago, and Boston. Even in Seattle, an epicenter of the outbreak, some bars have been packed.

Some people who carried on with their nonessential weekend outings shared their rationale with reporters. One 40-year-old who went with a friend to their favorite bar on Sunday explained to the Los Angeles Times, “This could be the last bar we go to in a long time.” In Boston, a man in line at a bar with an hour-long wait reasoned to a Boston Globe reporter that, as a pharmacist, he was already going to have a high risk of exposure at work anyway, so “there’s only so much I can do” to avoid the virus. And one compassionate, though still risk-taking, D.C. diner told Washingtonian, “As long as businesses are open and the condition doesn’t worsen, I want to support those folks depending on patrons to make their living.”

These are all understandable human reactions to an overwhelming, highly uncertain crisis. But they are also extremely weak justifications for a choice that ultimately puts one’s short-term social enjoyment ahead of the health—and maybe even lives—of countless people who are more vulnerable to the disease. Beyond lacking clear and forceful guidance from President Trump and his administration, why might people have failed to apprehend the gravity of the outbreak and the importance of staying in?

Baruch Fischhoff, a professor at Carnegie Mellon University who studies human judgment and decision making, had a handful of potential answers. First: “There are very few reported cases in most places, so maybe people [think], ‘This is still not here yet,’” he said. “If you haven’t been following the fact that we haven’t been testing [very much], you might not realize how deceiving the reported cases are.” He mentioned research suggesting that the human brain is well adapted to recording how often specific numbers are reported, but not as well adapted to understanding when those numbers might not be representative of reality.

Read: The dangerous delays in U.S. coronavirus testing haven’t stopped

Second, and relatedly, Fischhoff said, people tend to underestimate the speed at which exponential processes—such as a disease outbreak—unfold. “You really can’t trust your intuitions,” he said. “For anybody—whether it’s politicians or business leaders, or whoever—who’s been seeing the problem growing and relying on their intuitive feeling for how fast it’s going to grow, they’re going to be in trouble.”

For example, in the U.S., the number of confirmed cases of COVID-19, the disease caused by the coronavirus, was 213 on March 9, according to the World Health Organization. On Sunday, six days later, it was 1,678. Over the same period, the number of confirmed cases in France rose from 1,116 to 4,469, and the number in Italy rose from 7,375 to 21,157—and Italy had only 1,689 cases as of March 2. The human brain can have trouble keeping pace with such rapid growth (and a lack of widespread testing means the actual number of Americans with the disease is probably much higher than 1,678 anyway).