Nobel Prize-winning economist Robert Shiller told CNBC Tuesday that a market correction could come at any time and without warning…

“People ask ‘well what will trigger [a market correction]?’ But it doesn’t need a trigger, it’s the dynamics of bubbles inherently makes them come to a sudden end eventually…”

Shiller, who won the Nobel Prize for Economics in 2013 for his work on asset prices and inefficient markets, said that markets could “absolutely suddenly turn” and that he believed the bull market was hard to attribute totally to the U.S. political scene.

“The strong bull market in the U.S. is often attributed to the situation in the U.S. but it’s not unique to the U.S. anyway, so it’s hard to know what the world story is that’s driving markets up at this time, I think it’s more subtle than we recognize,”

Additionally Shiller writes in Project Syndicate that it is impossible to pin down the full cause of the high price of the US stock market, warning that this fact alone should remind all investors of the importance of diversification, and that the overall US stock market should not be given too much weight in a portfolio.

The level of stock markets differs widely across countries. And right now, the United States is leading the world. What everyone wants to know is why – and whether its stock market’s current level is justified.

We can get a simple intuitive measure of the differences between countries by looking at price-earnings ratios. I have long advocated the cyclically adjusted price-earnings (CAPE) ratio that John Campbell (now at Harvard University) and I developed 30 years ago.

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