Many investors are entering this week with fresh hopes the worst is over, after last week’s sudden stock-market rebound. But history suggests that in times of market turmoil, there is a risk that big, sudden gains like last week’s will prove temporary respites before stocks fall again. Head-snapping volatility, both steep drops and sharp gains, most often comes in times of market trouble. It suggests that, despite the bounce last week, the market isn’t healthy, says economic historian Richard Sylla of New York University’s Stern School of Business. “Financial markets become more volatile in periods of stress. People don’t know which way things are going to go, so you get these big up and down movements as people pile in and get out,” he says.