Why The S&P 500 Could Drop 20%

S&P 500 could fall by as much as 20% in the coming weeks, Tim Hayes, senior strategist at Ned Davis Research, told Business Insider.

"I would expect a decline of 15-20% and then we get a rally that maybe leaves us back where we are around now." Last week all three major Wall Street indices fell due to a sell-off in big US tech stocks.

Hayes said the valuations are very optimistic.

But he expects stocks to pick up after November once the US election is over.