Robert Shiller: Stock Market Ebola

Has a bear market begun? It’s a great question. The problem is that short-term market movements are extremely hard to forecast. But we live in the present and must try to understand what’s driving markets now, even if it’s much easier to predict their behavior over the long run.

Fundamentally, stock markets are driven by popular narratives, which don’t need basis in solid fact. True or not, such stories may be described as “thought viruses.” When they are pernicious, they are analogous to the Ebola virus: They spread by contagion.