In new interview with Money Magazine's Penelope Wang, Shiller offers some bad news for stock market investors.

Citing his 10-year cyclically-adjusted price-earnings ratio (aka CAPE ratio aka Shiller PE), Shiller estimates that the expected long-run inflation adjusted return on the stock market is just 4 percent.

But he warns that even he doubts the predictive power of his namesake ratio.

From Money Magazine:

A real 4% return seems like a worthwhile investment.

Then again, I don't know that I trust that number. It goes back to this whole academic literature on the outperformance of equities.

My old friend Jeremy Siegel [Wharton professor and author of "Stocks for the Long Run"] makes the strongest claim about this. He has data going back 200 years showing that the market has had a real 7% return over that period.