Global trends and central bank policy have fueled an environment in which investors should expect "lower than normal returns with greater than normal risk," hedge fund titan Ray Dalio wrote in an investor letter released Thursday. The Bridgewater Associates founder contended that central banks would "increasingly ease" through negative interest rates and bond-buying. But those methods would have limited effectiveness amid already easy policy, he argued. "As a result, central banks will increasingly be 'pushing on a string,'" Dalio wrote, adding that "QE will be less and less effective because...