In a February interview with Bloomberg, Yale University professor Robert Shiller says, “I think the Trump effect is really important.”

While Shiller, winner of the Nobel Prize in Economics, says he can’t speak authoritatively on what’s ahead because that would be “guessing human psychology,” he says that the current Shiller P/E ratio (also referred to as the CAPE) of 29 is “very high” and could spell trouble. It’s not at the level it was in 1929, he says, but it’s close.

Shiller recalls that the CAPE was at a similar level in 1997 and “held on for 10 years.” While this could happen again, he says, “it doesn’t look good.” He argues, “There’s a Trump effect all over the world. He dominates our attention,” and this has bolstered hope to a level that didn’t exist back in 1997. He underscores the anxiety that has been generated during Trump’s first month in office, adding, “It’s a really uncertain time.” If Trump succeeds, he says, the market could rise significantly over the next ten years. In the short run, however, it’s “hard to know when to bail out” as that would constitute market timing.

“My advice for the typical investor,” says Shiller, “is to stay partly in the market, not to go overboard. Don’t assume that Trump is going to work miracles.”