Five years ago, the United States was in the midst of its worst recession in seven decades, and stocks were feeling it.
On this day in 2009, the S&P 500 hit its nadir, closing at 676.53. That low marked a climax of a 16-month selloff that took more than half the S&P 500's value.
Since that day, the Standard & Poor's 500 index has gained more than 177 percent, the best rolling five-year performance since the June 1996 to June 2000 period that covers the dot-com bubble.
Naturally, some investors are questioning whether the bull run is nearing an end. Investors cited a number of reasons to be nervous. Though a number of these factors have been present for some time, the following stand out as concerns:
* Valuations: Profit growth, and especially revenue growth, may not be strong enough to support current price levels. Profit growth has slowed considerably from the peaks of this earnings cycle. There are concerns that revenue growth will be lackluster while economic growth remains mediocre.
The S&P 500's forward price-to-earnings ratio, at 15.8, is its highest since the fourth quarter of 2008, Thomson Reuters data showed. It comes as revenue growth has slowed, eating into profits, and productivity growth declined in the first quarter, suggesting slimmer margins in the next earnings period as well.
S&P 500 revenue growth has averaged just 3.2 percent since March 2009, while earnings growth has averaged 16.2 percent, Thomson Reuters data showed. For the most recent reporting period, revenue growth is estimated at 1 percent while profit growth is forecast at 9.8 percent.
The stock market has seen higher price-to-earnings ratios - notably during the technology bubble and the end of the 2007-2008 run, but to some, that's not comforting.
"A lot of people say the multiple is not that high, compared to history, but in no time in history, did you have these types of tailwinds pushing asset prices for five years," said Michael O'Rourke, chief market strategist at JonesTrading in Greenwich, Connecticut. Speaking of which ...
* The Punch Bowl: The Federal Reserve is steadily removing its bond-buying accommodation that took its balance sheet from about $900 billion to more than $4 trillion in the last several years. The central bank had been buying $85 billion monthly in U.S. Treasuries and mortgage-backed securities. But the Fed announced in December that it would begin reducing that amount. At its current pace, purchases will end by year-end,