What Next for the US Stock Market?

CAMBRIDGE – The US stock market achieved its longest rise in its history on August 22, with the Standard and Poor’s 500 index up by 230% since 2009. Although this wasn’t the biggest increase in a bull market, it marked the longest period of increasing share prices.

Several forces contributed to this impressive nine-year run. The primary driver has been the extremely low interest rates maintained by the Federal Reserve. The Fed cut its short-term federal funds rate to near-zero in 2008 and did not begin to increase it above 1% until 2017. Even now, the federal funds rate is lower than the annual inflation rate. The Fed also promised to keep the short rate low for a long period of time, causing long-term rates to remain low as well. With interest rates so low for so long, investors seeking higher returns bought shares, driving up their prices.

A rational model of share prices sets them equal to the present value of future profits. Low interest rates raised the present value of future profits, and the corporate tax reform enacted at the end of 2017, together with deregulation in several industries, has raised both current profits and expected future profits, contributing to the present value of future profits.

But even with rising profits, low interest rates have caused share prices to increase faster than profits. As a result, the S&P price-earnings ratio is now more than 50% higher than its historic average.

With real (inflation-adjusted) GDP rising at more than 3% this year, the strength of the US economy has induced foreign investors to shift their holdings to US equities. And in recent months, US households that had not owned stocks in the past, fearful of missing out on the bull market, have joined the equity bandwagon.

But what of the future? Stock-market booms don’t die of old age; they are generally killed by higher interest rates. That often happens when the Fed raises the short-term interest rate to stop or reverse rising inflation. Although the Fed’s preferred rate of inflation – the price of consumer expenditures – has just reached its target of 2%, other measures of price growth are rising more rapidly. The overall Consumer Price Index (CPI) is now 2.9% higher than it was a year ago. Even “core” consumer inflation, which strips out more volatile food and energy prices, has increased by 2.4% over the past year.