Investors today have good reason to worry about stocks. Europe, the U.S. and emerging markets are facing real problems today — and economic recoveries after financial crises almost always take longer than recoveries after ordinary downturns. The global economy may take several more years to fully recover from the credit crunch, and so may the stock market. Both could weaken again before getting better.

Indeed, our research has found that after 15 systemic banking crises around the world, the stock market took nine years on average to regain its prior peak. We don’t know if the recovery from the recent crisis will take a longer or shorter time than average, but assuming it is average, we’re now about halfway through. ....