Get ready for dramatically lower stock market returns over the next decade

Enjoy the stock indexes riding at record highs for now, but get ready for much stingier markets in the years to come.

That's the message consistently conveyed these days by investment counselors and finance scholars, who argue that with today's starting equity valuations and low interest rates, the coming decade should produce dramatically lower returns than the historical average.

The leaders of Vanguard Group, overseers of some $4 trillion in client assets, have been advising investors to expect a typical 60 percent stocks/40 percent bonds portfolio to deliver two- to- three percentage points less in nominal annual returns than its long-term norm. (Since 1926, such an asset mix has returned better than 8.5 percent annualized.)

Other forecasts are even less generous. Research Affiliates, a quantitative and "smart beta" fund manager, projects that U.S. stocks might only offer one percent a year for the next decade, after inflation. This is based largely on the so-called Shiller P/E, a ratio of the S&P 500 index to its trailing ten-year average earnings, which is now above 29 and higher than any period aside from the run-up to the 1929 and 2000 market peaks.

Jeremy Grantham of institutional value manager GMO has, by his admission, been wrong for years in assuming that corporate profit margins and equity valuations would revert to their pre-1990s trend levels. Yet even accounting for some more permanent upward shift in these gauges, he sees real (after inflation) returns of 2-3 percent a year looking out two decades.

And a simple plot of the market's forward P/E ratio against subsequent market returns shows that, since 1978, when starting at today's multiple of around 17.5 forecast earnings, ensuing seven- and 15-year nominal returns (before inflation) have been clustered in the mid- to low-single digits.

These forward-return calculations vary in their approach and assumptions, but all are anchored on today's stock valuations, long-term norms in corporate-profit growth and current interest rates. Stocks, even during the depths of the last bear market, never got dramatically cheap compared to prior cycles and certainly didn't stay inexpensive for very long.

And with risk-free 10-year government debt yielding a skimpy 2.3 percent in the U.S. and far less elsewhere, all other financial assets have repriced for skimpier future returns as well.

The first thing to say about this chorus of sober forecasts is that no one truly knows. Valuations have certainly adjusted into a higher range the past two decades, and if one wants to argue that corporate profit margins can climb higher still from record levels due to technology or whatever other factor, it can't be refuted outright.