The four leaders — Jair Bolsonaro, Donald J. Trump, Vladimir V. Putin and Boris Johnson — also have a lot of differences, of course, as do their countries. Yet all four subscribe to versions of what Daniel Ziblatt, a government professor at Harvard and co-author of the book “How Democracies Die,” calls “radical right illiberal populism.”

This pattern isn’t a coincidence, many political scientists believe. Illiberal populists tend to reject the opinions of scientists and promote conspiracy theories.

“Very often they rail against intellectuals and experts of nearly all types,” Steven Levitsky, Mr. Ziblatt’s co-author, said. The leaders, he said, “claim to have a kind of common-sense wisdom that the experts lack. This doesn’t work very well versus Covid-19.”…

All four leaders also flouted guidance on personal protective measures early on, refusing to wear a mask or continuing to shake hands.