In a recent article for MarketWatch, Yale professor and Nobel Laureate Robert Shiller shares insights on past bear markets in the U.S. and how the current market environment resembles the periods preceding them.

Citing the “conflicting messages” of “high valuations following a period of strong earnings growth and very low volatility,” Shiller shares data he collected by reviewing 13 U.S. bear markets since 1871 (a drop in the market by at least 20%) regarding each of these factors.

Valuations: Using his cyclically adjusted price-earnings ratio (CAPE), Shiller points out that the current level of 30 is “high,” which could imply “potential vulnerability to a bear market, though it is by no means a perfect predictor.”

Earnings: “To be sure,” writes Shiller, “there does seem to be some promising news” regarding earnings. But the growth, he argues, “does not reduce the likelihood of a bear market,” adding that the months preceding past bear markets have also tended to reflect high earnings.

Volatility: The currently low level of market volatility does not necessarily mean a bear market isn’t coming, says Shiller. “In fact,” he argues, “stock price volatility was lower than average in the year leading up to the peak month preceding the 13 previous U.S. bear markets, though today’s level is lower than the 3.1% average for those periods.”

Shiller concludes that, while today’s’ market resembles the peaks that preceded past bear markets, such events are difficult to anticipate and “the next one may still be a long way off.”