The images are everywhere: People crowded face-to-face in swimming pools, shoulder-to-shoulder in indoor bars, cheering without masks at a rally held by President Trump, who often downplays the global pandemic. Now, as many public health experts predicted, waves of new COVID-19 infections and deaths are rolling across the South and West. Many, still practicing social distancing, look at their fellow Americans and ask, "What are they thinking?" We turned to Steven Pinker for help with an answer. The professor of psychology at Harvard, author of widely discussed books, including How the Mind Works and most recently, Enlightenment Now, sees the deep-seated mindset, tribalism, at work in people's defiance of health recommendations.

From an office at Carnegie Mellon, my colleague John Miller and I had evolved a computer program with a taste for genocide. This was certainly not our intent. We were not scholars of race, or war. We were interested in the emergence of primitive cooperation. So we built machines that lived in an imaginary society, and made them play a game with each other--one known to engender complex social behavior just as surely as a mushy banana makes fruit flies.

Last month, I published an article on Nautilus called "Is Tribalism a Natural Malfunction?". It was a meditation on a series of computer experiments in the study of Prisoner's Dilemma, and a reflection on what these simulations, and more complex arguments from mathematical logic, might tell us about social life. The groups that formed in our simulation, shibboleth machines, were unable to tolerate others--and eventually became unable to tolerate the differences that emerged amongst themselves. There were some lovely comments on Nautilus and elsewhere, of course, and the usual rough-and-tumble of Internet argument. What I didn't expect was the robust defense of tribalism made by educated and apparently intelligent people writing on ostensibly science- and technology-focused sites.

From an office at Carnegie Mellon, my colleague John Miller and I had evolved a computer program with a taste for genocide. This was certainly not our intent. We were not scholars of race, or war. We were interested in the emergence of primitive cooperation. So we built machines that lived in an imaginary society, and made them play a game with each other--one known to engender complex social behavior just as surely as a mushy banana makes fruit flies.