When asked for his outlook on the market in a recent CNBC interview, Nobel Laureate Robert Shiller says investors should keep some stocks in their portfolio because the market “could go up 50 percent from here.”

The economist, who helped develop the cyclically-adjusted price-to-earnings ratio (CAPE) market valuation measure, says that although the current CAPE (29) is above the 17-year historical average, he isn’t calling for a market decline. That said, he notes that diversification outside the U.S. would avail investors of lower valuations. “You can go practically anywhere else in the world and it’s lower,” he argues.

The CAPE ratio is calculated using price divided by the S&P 500’s average historical 10-year earnings, adjusted for inflation.

Shiller says, “I can see it as a real possibility that stock prices and house prices would both keep going up for years, but I’m not forecasting that by any means.”