. But based on one particular measure of market valuation, the S&P 500 is overvalued by 72 percent.

That measure would be the comparison between the S&P's total market capitalization (that is, the value of all the shares of the companies in the S&P) and the U.S. gross domestic product (which measures the value of the economic activity that has occurred within America's borders within a certain time period).

Going back to 1964, the S&P 500's market cap has been 57 percent of annual US GDP on average. If one excludes the tech bubble, that number falls to 53 percent. As of the end of March, however, the stocks contained within the S&P are collectively worth 99 percent of GDP, more than 70 percent above the average level.

Bank of America Merrill Lynch's equity and quant strategy team, which presented the above numbers in a report Thursday, noted that the S&P/GDP is just one of a bevy of metrics suggesting that "the market is expensive vs. history." They show that other, better-known indicators such as the S&P's forward price-to-earnings ratio also suggest overvaluation, albeit to a lesser extent.