They say nobody rings a bell at the top of the market. But whether this is the top or not, two prominent market observers and historians, Robert Shiller and Edward Chancellor, are expressing concern.

First, Shiller warns readers not to take big increases in earnings too seriously because earnings are volatile.

Everyone knows that stock prices have risen dramatically since 2009. A $100 investment in the S&P 500 in 2009 has grown to nearly $400 at the end of August 2018. But Shiller reminds us that earnings have grown dramatically too. In fact, “real quarterly S&P 500 reported earnings per share rose 3.8-fold over essentially the same period, from the first quarter of 2009 to the second quarter of 2018,” according to Shiller. Prices, in fact, grew a bit more slowly than earnings since the end of the crisis.

So should we think stocks are reasonably priced since earnings have grown at the same pace as prices? Not so fast, Shiller says. Earnings, the difference between two other data sets — revenues and expenses, are volatile, and cyclical. Rapid rises in earning are often followed by a return to long term trends or subpar levels. Such episodes have occurred more than a dozen times in U.S. stock market history.

Earnings can grow dramatically from things like “panicky demand” for U.S. goods from Europeans at the beginning of World War I. This led to political calls for “wealth conscription” or a heavy taxation on war-related profits. At that time stock prices didn’t follow profit advances as investors seemed to realize those gains would be short-lived.

In the “Roaring ‘20s,” however, emergence from a “war to end all wars” and a spirit of freedom and individual fulfillment spurred stock prices by Shiller’s lights. And this, of course, led to a crash at the end of the decade.

Another period where price gains outstripped earnings gains was 1982-2000. Real stock prices increased 7.5-fold, while real annual earnings only doubled, according to Shiller. Indeed the S&P 500 Index delivered an eye-watering 17% compounded annual return from 1982 through 2000, mostly on the back of multiple expansion (the increased price investors are willing to pay for underlying earnings).