The S&P 500 is trading at about 15 times analysts’ forecasts for 2013 adjusted income, the highest since the second quarter of 2010. That compares with an average ratio of 14.3 since 2006, Bloomberg data show. The valuation has climbed 36 percent since March 2009, held back as company earnings surged, the data show.

For Glyn Owen, who helps oversee $4.6 billion at Momentum Global Investment Management Ltd. in London, central-bank stimulus is delaying the inevitable by allowing companies that would have gone bankrupt to keep trading.

“Companies are being kept alive when in previous cycles they would have gone to the wall,” Owen said in a phone interview on May 14. “It’s the opposite of the creative destruction that we have seen before. We are in a very big bull market for equities, but our strategy is to stick to quality and good balance sheets.”

Subprime Crisis

The index of companies with weaker finances tumbled 63 percent from the S&P 500’s previous peak in October 2007 through the low in March 2009. The less-risky firms fell 44 percent.

The crisis that started with U.S. subprime debt defaults wiped about $11 trillion from the value of American equities between July 2007 and March 2009. The total value of U.S. shares outstanding has now reached $19.6 trillion, Bloomberg data show, having recouped all losses. Even so, only $17.1 billion of the roughly $400 billion that was pulled from U.S. equity mutual funds over the last four years came back in 2013, according to data from Washington-based Investment Company Institute.

Companies in the stronger basket have average cash balances of $1.8 billion, 33 percent more than that of the indebted group relative to assets, the data show. While that provides a buffer should interest rates rise, it’s also a sign of caution among chief executive officers and may help explain why indebted companies are rallying more, according to Joerg de Vries-Hippen at Allianz Global Investors.

“To have cash in the balance sheet could be very crazy at the moment,” De Vries-Hippen, who helps oversee $392 billion in Frankfurt, said in a phone interview on May 15. “When we investors have to ask companies to do something with their cash, then it is often too late.”

The more indebted stocks posted some of the market’s worst returns during periods of economic weakness since the U.S. bull market began. They fell 3.9 percent in 2011, trailing the S&P 500, which was unchanged, and slipped 13 percent from April 2 to June 1 in 2012, when concern Greece and Spain would default sent the S&P 500 to a 9.9 percent loss.

That changed as the year progressed. From Nov. 15, when the benchmark gauge bottomed at an almost four-month low, the weak balance sheet measure has jumped 38 percent, compared with a 23 percent rally for the broader market.