转自AQR：The short-term behavior goes beyond just rational choice. In times of stress, some investors may need cash to meet margin calls or fund other losses. When systemic events occur, they create cascading cash shortages – when there just isn’t enough to go around. Investors are forced to get out of their investments to get liquidity. The folks with cash don’t want to part with it so they require a healthy discount on the securities. At this point, the market becomes like a commodity market, in that it is entirely driven by supply and demand for short-term delivery.