Say goodbye to the US stock market's secret sauce

For the first time in over two years, corporate profit margins are falling

It isn’t how much you make but how much you keep.

Most investors grasp that concept when it comes to their own finances, but they also need to consider how it applies to the companies they own. A long-term tailwind to stocks from expanding profit margins is at risk of flipping into reverse.

Steady increases in margins have been the stock market’s secret sauce since the 1980s, allowing earnings to grow at a much faster clip than sales and pushing share prices higher as a result. If profit margins were merely to return to their levels of 20 years ago, then earnings—and share prices—might