While it may be reasonable to use long-term market data to help shape return projections, it should be a starting point which is then adjusted “up or down based on experts’ opinions about current valuations and the investor’s own time horizon,” writes Morningstar’s Christine Benz.
Benz offers insights from different market experts regarding return assumptions:
John Bogle, founder of Vanguard Group—in a September 2016 interview, Bogle said he expected “muted returns for both stocks and bonds” and projected equity returns of 4% to 5% over the next decade (less once inflation, taxes and investment fees are taken into account) and 2.5% bond returns “for investors who are willing to venture beyond Treasuries.”
Grantham Mayo Van Otterloo (GMO)—”Owing to low yields and high valuations, the firm is forecasting negative real returns from the U.S. stock and bond markets over the next seven years, but anticipates “mildly positive real returns from international equities and the strongest gains (4.4% real return) from emerging-markets equities.”
Morningstar—Like some others, Benz writes, “Morningstar Investment Management’s return expectations for U.S. stocks and bonds are low, if not downright discouraging, especially when you factor in inflation. But the outlook is more optimistic for foreign equities.”
Research Affiliates—while “even more downbeat about the prospect for U.S. stock and bond market returns over the next decade than it was a year ago, it’s still optimistic about investing overseas.”
Vanguard—The firm’s economic team “describes its outlook for global equities as ‘guarded but not bearish.'” Its research suggests that “investors in foreign stocks that are not hedged into the dollar could outperform the U.S. market in the next decade.”