As the S&P 500 delivered a 19.7% annualized gain over the past six years, talk of a market bubble has emerged.

Concerns of a bubble in prices are not just being voiced by perma-bears like Nouriel Roubini and "Dr. Doom" Marc Faber. Nobel laureate Robert Shiller also recently questioned asset prices in the second edition of his book, "Irrational Exuberance."

For many investors, it's tempting to think about locking in gains. While bull markets last an average 3.8 years, this one is already more than 50% longer. Selling stocks around their October 2007 highs would have protected a 119% gain since the 2002 low and avoided a 55% plunge in prices to 2009.

Where's The Next Market Bubble?




Trying to perfectly time the eventual correction would be a folly, but investors can start taking profits in bubbly assets while positioning in less expensive investments.

Citigroup Research recently published a report, "It's Bubble Time," that measured valuations in the market and across sectors. The report found four themes that are driving bubbles in different assets and investments.

First, they note the story of a "new normal" where low economic growth, inflation and interest rates continue to drive bond prices higher even as rates hover around historic lows. We have seen the new paradigm drive bond prices, as well as other cash yield investments like real estate investment trusts (REITs) and dividend-paying stocks, well into overvalued territory.

The report found that surplus liquidity, basically the wave of cheap money flooding the debt market and the economy through low rates, is also driving asset prices. The research noted that the weighted average global price of short-term money costs just 0.7%, the lowest in four decades.