Editors' pick: Originally published Oct. 7.

Before even the earliest polls showed Hillary Clinton beat Donald Trump in the first presidential debate, another indicator had already called her the winner: the prediction markets. Reacting in real time to the debate, question by question, beat by beat, they ticked in Clinton's direction throughout the night and by the second the event ended, they had shifted about six points in her favor. They will again be a measure to look to come the second debate on Sunday and the first indicator we'll have on who won.

Prediction markets -- exchange-traded markets that allow investors to trade on the outcome of events -- gave Trump a 31% chance of winning the election following the September 26 debate at Hofstra University on Long Island, down from 38% at the start. Clinton's odds climbed to 69% from 64%.

"It's a deep and liquid market. For it to move that significantly suggests that, collectively, traders think something important happened," said Andreas Katsouris, global political and elections specialist at PredictIt, at the time.

On Sunday, Clinton and Trump will face off again at Washington University in St. Louis in a town hall-style debate moderated by CNN's Anderson Cooper and ABC's Martha Raddatz. 

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Join TheStreet on Sunday at 8:30 p.m. Eastern Time and immediately following the second presidential debate for a special Facebook Live with PredictIt CEO John Phillips. Like TheStreet on Facebook for updates.

Follow TheStreet on Twitter for live updates on the prediction markets throughout the debate.

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Clinton's odds on PredictWise, a platform that aggregates information from several prediction markets, including PredictIt, saw a similar swing. The markets moved five points in the former secretary of state's direction during the debate and an additional seven points in the following week, climbing as high as 81% -- her strongest position yet this election cycle. They have currently settled back down to 80%.

According to David Rothschild, the Microsoft Research economist behind PredictWise, about three-quarters of the markets' movement in Clinton's direction can be attributed to her boost in opinion polls following the debate. The other quarter came from Trump's ongoing feud with former Miss Universe Alicia Machado and revelations tied to his tax practices. The real estate magnate's late-night tweets didn't help, either.

"You put it all together, it was a pretty big move," said Eric Zitzewitz, professor of economics at Dartmouth College. He added something similar happened in 2012. "The first debate last cycle for [Mitt] Romney, the prediction markets had moved heavily in his direction before the snap polls came out, and the same is true for the first Clinton-Trump debate."

It's not just the prediction markets that reacted.

Following the first Clinton-Trump debate, the overnight futures markets rallied, raising the value of the S&P 500 by two-thirds to three-quarters of a percentage point, noted Michigan University economist Justin Wolfers in a piece in The New York Times.

The move provides insight how the broader markets view the candidates.

"'It seems like there was a movement in financial markets that's big enough that it suggests that financial markets view trump as higher risk than Clinton," said Zitzewitz, who alongside Wolfers has researched how the stock market has reacted to presidential elections dating back to 1880.

VIX futures indices, indicators of future volatility, cratered on debate night as well, Zitzewitz added. High-interest rate currencies like the Mexican peso went up, while the dollar and euro dropped, and the yen declined even further.

"The movements during the debate are what's really convinced us that the market is worried about a Trump presidency," said Zitzewitz.

To be sure, not all debates are sure to sway investors. The vice-presidential debate between Mike Pence and Tim Kaine spurred no change in the prediction markets, even though the broadconsensus was that Pence had delivered the better performance.

"In the past, VP debates haven't moved things much," said Zitzewitz.

Join TheStreet on Sunday at 8:30 p.m. Eastern Time and immediately following the presidential debate for a special Facebook Live with PredictIt CEO John Phillips. Like TheStreet on Facebook for updates.

Follow TheStreet on Twitter for live updates on the prediction markets throughout the debate.