You didn’t have to be a genius to make money in equities the last two years. Buy an index fund and let the bull market guide you to a 44 percent gain. Most money managers who actually pick stocks couldn’t match the Standard & Poor’s 500 Index, let alone beat it. Thanks to three weeks of volatility, that’s beginning to change. This year, U.S. stocks have started to go their own ways, rather than move in lockstep with each other and with little swings, as they did in 2014. That’s good news for active managers, who’ve seen investors pull money for years in favor of low-cost index and exchange-traded funds. “That’s a better environment for the way that we think of the world,” said Doug Burtnick, a Philadelphia-based senior investment manager at Aberdeen Asset Management, which oversees $525 billion. “Managers who focus on discriminating among companies, and to the extent they tend to get the fundamentals right, should be rewarded more richly.” Three weeks into 2015, stocks are get...