"A 10 to 25 percent further decline in real home prices over the next five years would not surprise me at all," Shiller said at a Standard & Poor's housing summit in New York.
Shiller, a Yale professor, said it's possible that the market will follow the grim path seen in Japan after a 1980s housing bubble. Property values there declined every year for about 15 years, dropping by two-thirds overall, he said.
But he cautioned that he was not making a forecast, saying that since the recent housing boom-and-bust cycle was the biggest in U.S. history, he can't use previous housing patterns to figure out where this market is headed.
"It's impossible for statisticians to forecast," he said. "I honestly don't know."
Other housing analysts have recently predicted that prices will continue to drift lower this year and "bounce along the bottom" for a while, but the loss mentioned by Shiller is larger than most experts have forecast. (read more)