Fear rather than euphoria is currently driving global stock markets to new highs, Nobel Economics laureate Robert Shiller said in a German newspaper interview on Sunday.
The Yale University professor also warned of potential stock market bubbles and suggested it could be a good time to invest in oil, given the current record low prices.
"The stock market boom we're currently experiencing isn't driven by euphoria," Shiller told the Frankfurter Allgemeine Sonntagszeitung.
"I call it the 'new normal boom'. It is an investment boom... The mechanisms are all there, but the feeling is missing," Shiller said.
"We're not seeing any optimism. This boom is driven by fear," he argued.
European and US stocks ended the week on a high Friday with London's FTSE surging past the 7,000 mark for the first time in history, and other leading European indices — such as the DAX in Frankfurt — also at record highs.
Shiller said European stocks, including German stocks, were still a bargain, compared with US stocks.
"There aren't many alternatives to stocks at the moment. We're in a period of extremely low interest rates."
In a bid to kick-start the eurozone's moribund economy, the European Central Bank has slashed its interest rates to new all-time lows and pumped unprecedented amounts of liquidity into the financial system.
In face of the flood of cheap cash, some critics see a danger of asset bubbles forming, in stocks and real estate, which could burst soon.
"I believe there are bubbles," but central banks were not to blame, Shiller said.
The ECB "isn't the cause of low interest rates," but rather global pessimism, the expert argued.
Asked how he would invest his money, Shiller replied: "It's difficult. But I think now could be a good time to invest in oil or in a rise in oil prices," he said.
"Prices are very low and there are a lot of reasons to assume that they won't stay low. That's what I've bet on," Shiller said.
Shiller, 68, won the Nobel Economic Prize in 2013, sharing it with two other US academics for research on financial markets.