Some of the trends behind America’s earnings boom and stock market surge are about to change. Investors beware.

"So if earnings growth has been so anemic, why have stocks continued to soar over the past few years—with the S&P 500 rising 29% since September 2014? “It’s all multiple expansion,” says Silverblatt, noting that the price-to-earnings ratio for the 500 has jumped over those three-plus years from 18.9 to the current, super-rich 24.3. Let’s look at the S&P as one big company. Its current annualized earnings of $107 haven’t budged in three years, yet its “price” has risen from 2,018 to 2,602. Hence, investors who three years ago paid less than $19 for $1 of earnings now pay $24.30—an extra $5.30, or an almost 30% premium, for a dollar of earnings."

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