Imagine two well-off households, each with $100,000 in the stock market in 2007. A family that sold in 2009 after losing half its portfolio’s value may now have $50,000 in a savings account. A family that held on would now have about $130,000 in stocks. The inequality has yawned merely because of the investing decisions. In the long run, those savings accounts have a vanishingly small chance of outperforming stocks.