The performance of stock markets, especially in the United States, during the coronavirus pandemic seems to defy logic. With cratering demand dragging down investment and employment, what could possibly be keeping share prices afloat?

The more economic fundamentals and market outcomes diverge, the deeper the mystery becomes - until one considers possible explanations based on crowd psychology, the virality of ideas, and the dynamics of narrative epidemics. After all, stock-market movements are driven largely by investors’ assessments of other investors’ evolving reaction to the news, rathe...