Nobel Prize-winning economist Robert Shiller believes investors should continue to own stocks because the bull market may continue for years.



CNBC's Mike Santoli spoke with Shiller in an exclusive interview for CNBC PRO. Santoli asked Shiller about his market outlook.



"I would say have some stocks in your portfolio. It could go up 50 percent from here. That's what it did around 2000, after it reached this level, it went up another 50 percent. So I'm not against investing in the stock market when you consider the alternatives. But I think if one wants to diversify, US is high in its CAPE ratio. You can go practically anywhere else in the world and it's lower," Shiller said. "We could even set a new another record high in CAPE, that's not a forecast."



Shiller developed the "cyclically adjusted price-to-earnings ratio" (CAPE) market valuation measure, which is calculated using price divided by the index's average historical 10-year earnings, adjusted for inflation. The economist's research found future 10-year stock market returns were negatively correlated to high CAPE ratio readings on a relative basis.

Shiller CAPE PE Ratio



Source: Multpl.com



However, even though the current CAPE ratio is at 29, which is above the 17 historical average, the economist is not calling for a market decline.



"I can see it as a real possibility that stocks prices and house prices would both keep going up for years, but I'm not forecasting that by any means," he added.



Shiller is Sterling Professor of Economics at Yale University and helped develop the widely followed S&P/Case-Shiller Home Price Indices. He was awarded the Nobel Prize in Economic Sciences with Eugene Fama and Lars Peter Hansen in 2013.



See here for the full CNBC PRO report and the interview video.