In a column for Kiplinger’s, the bullish Jeremy Siegel talks about what he sees as threats to his upbeat outlook.

Siegel cites a couple risks whose impacts would go far beyond the market: terrorism and pandemics. But another of his concerns is more economic. “One risk I did not note earlier was the country’s poor growth in productivity, which measures the output of the economy per hour worked and determines the standard of living,” Siegel says. “Productivity growth averaged only 0.6% in 2011 and 0.7% in 2012, with the first half of 2013 not much better. That’s considerably below the 2% average the economy has recorded since 1970.”

Siegel offers a couple possible explanations for the productivity slowdown. “The fall in U.S. educational achievement could be one cause, as well as the long periods of unemployment many Americans have experienced as a result of the recession,” he says, but adds, “I still believe we will emerge from the slow growth in productivity. Nevertheless, it’s crimping corporate profits.”