Investors in ETFs and mutual funds should perhaps bring their expectations for stock market returns down a notch. Think: an annual average of 6% to 8% returns for global equities over the next 10 years, says Joe Davis, chief economist at Vanguard. The investment management firm’s GDP and market outlooks are the most guarded since 2006. That may be a tough pill to swallow for fund investors used to double-digit yearly returns for U.S. stocks.