Nobel Laureate Robert Shiller writes in a recent New York Times article that “today’s CAPE is sending a troubling message,” noting that the market valuation ratio he developed (which now stands at nearly 30) was higher only in 1929 and around 2000 (when it hit 33 and 44, respectively). In both instances, Shiller writes, “market declines followed those very high readings.”

He qualifies his comments, however, by clarifying that the CAPE “suggests a dim outlook for the American stock market over the next 10 years or so, but it does not tell us for sure nor does it say when to expect a decline.” Shiller cites investor optimism that Trump’s focus on American businesses will support a continued stock market boom, but points out that stocks are relatively expensive and investor optimism is “tinged with plenty of worry.” He cites his Valuation Confidence Index (created using opinion surveys of both institutional and individual U.S. investors going back to 1989) which, in February, reached its lowest level since 2000.

The Yale professor clarifies that there is “no clear message from all of this,” and asserts that “long-term investors shouldn’t be alarmed and shouldn’t avoid stocks altogether.” However, he concludes, “my bottom line is that the high pricing of the market—and the public perception that the market is indeed highly priced—are the most important factors for the current market outlook. And those factors are negative.”