In the middle of a prolonged period of negative real interest rates and loose monetary policy aimed at managing inflation and helping economies, fears are rising that asset bubbles are being created.
"We've lost our way so we look to central banks, who give us massively loose monetary policy and that's the little bubble we're living in," David Bloom, head of currency strategy at HSBC, told CNBC.
New records are constantly being set in the markets, with Thursday's close of the S&P, up 0.47 percent at 2,185.79, yet another new top. This is happening despite low productivity and growth in the U.S. economy.
Analysts at UBS see "scope for the markets to run further still over the near-term" because of central banks' policies in the developed world.