U.S. single-family home prices rose in September and posted their strongest annualized gain in 7-1/2 years, a closely watched survey said on Tuesday.

Robert Shiller, a professor of economics at Yale and a 2013 Nobel laureate, told CNBC he felt the influence of investors in the real estate market played a larger role in the home price increase than the attitude of home-buyers, which Shiller tracks using surveys. A pull-back from the foreclosure crisis could also be driving home prices upward, he said.