NEW YORK (TheStreet) --The week following a U.S. presidential debate has been historically negative for the stock markets.

CNBC's business news reporter Deirdre Bosa appeared on Friday afternoon's "Closing Bell" to explain the story behind the weak performance.

"If history is any guide stay out of the markets after Clinton and Trump face off Monday. Going back to 1980, presidential debates have not been good for stock gains and beginning around 2000 that trend became even more pronounced," Bosa explained.

On average, the Dow is down one week following a debate 83% of the time, with an average loss of 1.8%. The S&P 500's average loss is 1.5%.

"The why of course is up for speculation. Perhaps as the presidential contest has become more publicly heated, and the media coverage greater volatility has gone up too. For whatever reason the week after the debates has historically been a bad time to be long," Bosa said.