The one big thing economist Robert Shiller says is preventing a 1929-like stock market crash

It's a comparison no one wants to hear — that this stock market bears striking similarities to that of 1929.

The observation is coming from Nobel Prize-winning economist Robert Shiller, who's been arguing valuations are extremely expensive.

But instead of predicting an epic stock market crash, he's finding reasons to be optimistic.

"The market is about as highly priced as it was in 1929," said Shiller on Tuesday's "Trading Nation." "In 1929 from the peak to the bottom, it was 80 percent down. And the market really wasn't much higher than it is now in terms of my CAPE [cyclically adjusted price-to-earnings] ratio. So, you give pause when you notice that."

In his first interview since penning an op-ed on Sept. 15 in The New York Times, the Yale University economics professor reiterated to CNBC that there's one vital characteristic protecting investors from losing their nest eggs: Market psychology.

"It's not just a matter of low interest rates, it's something about the American atmosphere. It's partly the Trump atmosphere. Investors love this. I can't exactly explain – maybe it has something to do with prospective tax cuts. But I don't think it's just that. It's something deeper, and it's pushing the American market up," he added.

Unlike 1929, Shiller points out there's not much talk about people borrowing exorbitant amounts of money to buy stocks. Plus, he notes there's now more regulation.

But don't mistake the Yale University economics professor for a bull.

"I don't want to encourage people too much to put a lot into the most expensive market in the world," said Shiller. "The U.S. has the highest CAPE ratio of 26 countries. We are number one."

Shiller's latest thoughts came as stocks extended a record win streak.

The Dow hit both an all-time intraday high and close. It's now up 22 percent since Donald Trump won last November's presidential election.

The S&P 500 and Nasdaq both closed at fresh record highs on Tuesday, too.

Shiller may see red flags, but he isn't ruling out a market that continues to churn out fresh records for months, if not years.

"I wouldn't call it healthy, I'd call it obese. But you know, some of these obese people live to be 100 years, so you never know," said Shiller.

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Trading Nation is a multimedia financial news program that shows investors and traders how to use the news of the day to their advantage. This is where experts from across the financial world – including macro strategists, technical analysts, stock-pickers, and traders who specialize in options, currencies, and fixed income – come together to find the best ways to capitalize on recent developments in the market. Trading Nation: Where headlines become opportunities.

Michael Santoli

Michael Santoli joined CNBC in October 2015 as a Senior Markets Commentator, based at the network's Global Headquarters in Englewood Cliffs, N.J.  Santoli brings his extensive markets expertise to CNBC's Business Day programming, with a regular appearance on CNBC's “Closing Bell (M-F, 3PM-5PM ET).   In addition, he contributes to CNBCand CNBC PRO, writing regular articles and creating original digital videos.

Previously, Santoli was a Senior Columnist at Yahoo Finance, where he wrote analysis and commentary on the stock market, corporate news and the economy. He also appeared on Yahoo Finance video programs, where he offered insights on the most important business stories of the day, and was a regular contributor to CNBC and other networks.

Follow Michael Santoli on Twitter @michaelsantoli

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