Price gains of stocks in the Standard & Poor's 500 Index are outpacing profits by the fastest rate in 14 years as the bull market extends beyond the average length of rallies since Harry S. Truman was president.

The benchmark gauge for U.S. equities has risen 14 percent relative to income over the past 12 months—to 16 times earnings—according to data compiled by Bloomberg. Valuations last climbed this fast in the final year of the 1990s technology bubble, just before the index began a 49 percent tumble. The rally that started in March 2009 has now outlasted the average gain since 1946, the data show.

Bears say the failure of earnings to keep up with prices signals the bull market is in its last stages, as companies from Caterpillar Inc. and Danaher Corp. forecast slower profit growth and the Federal Reserve prepares to reduce stimulus. Optimists point to expanding multiples as proof individual investors are growing confident enough in the economy to return to stocks. History shows the final phases of rallies have provided some of the biggest gains.