The biggest question faced by U.S. investors, according to a recent Bloomberg article by columnist Nir Kaissar, is whether “the stock market can maintain its historically high valuation, a perch it has commanded for most of the past three decades.”

Kaissar offers the follow visual representations of historical data:

“The difference between the two periods is striking,” Kaissar writes, noting that from 1871 to 1989, the market “traded below its long-term average valuation 47% of the time” but that since 1990 that percentage has dipped to 10%.

Echoing the commonly delivered warning that past performance is no indication of future results, Kaissar notes that it explains “why reputable money manager spend considerable time and energy honing their estimates of future returns.” He adds, however, that “every model must grapple with where valuations will land,” posing the question of whether “it is safe to assume the market will revert to its long-term average valuation?”

Given today’s high market valuation, Kaissar argues that “it’s reasonable to assume the valuation will be lower in 10 years.” How much lower, however, remains a mystery. And while the current lofty valuation levels make it “easier to suspect that something has indeed changed for good,” he stipulates, “that doesn’t mean the market won’t crater occasionally, as it did during the financial crisis.”

Still, Kaissar concludes, “there’s a growing chance that past 30 years haven’t been an anomaly and that forecasting models need to lift their valuation targets.”