Jeremy Siegel says 2021 will be 'very good' for stocks, but long-term returns could be below average

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Wharton School's Jeremy Siegel told CNBC on Friday that he believes the stock market's strong rebound from the coronavirus-era bottom can continue into next year. But the finance professor added he believes long-term returns may be lower than the historical average.

Bullish factors for the market next year include the significant increase in money supply since March, as well as better-than-expected developments for a Covid-19 vaccine and the outcome of the November elections, Siegel said on "Squawk Box." "On all those three fronts, we have positive forces for the market that I think are going to cause 2021 to be a very good year," he said.