In March, Kalshi had Gavin Newsom’s odds of winning at 29 percent, but Polymarket had them at 24 percent. These disparities are good news, if you’re gambling.

Taking both sides of the same bet is usually a wash. But not when there’s a price disparity.

If this sounds like printing money, that’s because it basically is. It’s called “arbitrage,” long a favorite strategy of quantitative traders trying to juice profits from the stock market with minimal risk. You buy something at a cheap price, and simultaneously sell it at a more expensive price. It’s a win-win.

Some bettors are now using the same strategy to rake in thousands of dollars from online prediction sites. Moving quickly, they can take advantage of price gaps between exchanges like Polymarket and Kalshi, or even between the prediction sites and sports-betting sites like DraftKings and FanDuel. The wider the spread, the bigger the potential profit.

Ryan Noel, 25, has built a career arbitrage-betting (or “arbing,” as he calls it) during sports games. He regularly makes more than 1,000 arbitrage bets per week on prediction sites like Polymarket, Kalshi, Novig and ProphetX, in addition to online sportsbooks, he said.

“Software shows me the price of every sort of market at the same time,” said Mr. Noel, who started arbing in late 2023, while working as an actuary, before quitting his job last year. So far, the strategy has netted him more than $1 million, he said. “I don’t care about sports at all. I think watching sports is the most boring thing you can do with your time. I’m a mathematician.”

Math skills are essential — but so are the right tools, said Aidan Gawlowski, a Chicago-based college student who started arbing last year before coding his own software to hunt down prediction-market price discrepancies. Mr. Noel buys software from OddsJam, Pick the Odds and Bookie Beats that tracks price changes across thousands of markets, flagging the possible arbitrage.

“I figured out that there was this opportunity,” said Mr. Gawlowski, 21, who said he started betting when he was 14. “You’re mathematically guaranteed to make money.”

Some moneymaking opportunities last longer than others. The arbitrage with Mr. Newsom? It existed, unexploited, for weeks. During that period, you could’ve bought “Yes” on Polymarket and “No” on Kalshi, for a roughly 3 percent profit. (The probability spread of around five percentage points, minus Kalshi’s transaction fee.)

But there are a couple of reasons that opportunity was an anomaly. For one, the market doesn’t resolve for two years. That’s a long time to tie up money you could invest elsewhere, said Abraham Wyner, a professor of statistics and data science at the Wharton School at Penn. There’s also additional risk that some bets carry more than others: What if the election gets weird, and the sites don’t agree on what defines a Newsom nomination? Then, you might lose both sides of your bet.

That was enough to deter Mr. Noel and Mr. Gawlowski, who spend most of their time arbing on sports. There are loads of sites that let users bet on sports, meaning more chances for price discrepancies. And during games, odds must constantly update to keep up with live developments. That process takes time, which can translate into arbitrage opportunities.

“You can make a significant amount of money on a big N.B.A. day,” Mr. Gawlowski said. During sports games, Mr. Noel’s price-tracking programs catch an arbitrage opportunity every minute or so, he said.

These discrepancies often emerge when casual users, betting based on vibes, move a market just a hair out of alignment. Then arb bettors pounce, and their actions end up evening the odds across the sites again.

Taking advantage of these short-lived opportunities is hard enough for you and me. But the window is closing even for bettors like Mr. Noel and Mr. Gawlowski, as big financial institutions get in on the action with automated bots that can trade faster than any human.

Sophisticated bots compare prices across platforms and identify arbitrage opportunities — just like software Mr. Noel and Mr. Gawlowski use — but they also execute trades, fast. Many prediction platforms let computerized agents place orders without a human. That gives institutions with the wherewithal to deploy bots effectively, and at scale, a huge edge.

Wall Street quant firms like Susquehanna International Group have been recruiting algorithmic traders specifically for prediction markets.

“In the prediction-market space, arbitrage is being dominated by bots,” said Ron Yurko, director of the Carnegie Mellon Sports Analytics Center. “Kalshi and Polymarket encourage it.”

Unlike traditional sportsbooks, prediction markets make money mainly from transaction fees — more transactions, more money. And because bots facilitate speedier trading at higher volumes, the sites have a financial incentive to allow them.

“The big institutions will take out a lot of the arbitrages,” said Nicholas Burgess, who builds and deploys bots for financial institutions, “but they’ll always leave the small ones for retail investors.”

Even so, what’s left is slim pickings. More bots mean the disparities between sites are smaller, and they vanish faster.

“Back in 2022, these arbitrage opportunities would last 30 seconds,” said Alex Llewellyn, 36, a professional sports bettor. “These days I execute bets in two to five seconds. And instead of 8 percent arbs, you generally see 4 to 5 percent.”

Prediction sites are also raising their fees, squeezing the tiny statistical edges that make arbitrage possible. When Polymarket added new fees in late March, Mr. Noel calculated that they would have cost him more than $30,000 a month, if he kept trading at his usual volume.

All this means that free money on prediction markets is probably out of reach now for many ordinary investors.

Prediction sites, awash in Wall Street money and bots, are heading toward the same fate as other major financial markets. One-tenth of the top one percent of accounts on Polymarket rake in more than two-thirds of the profits, a Wall Street Journal analysis found.

“You’re not betting against Joe Schmo anymore,” said Alex Monahan, the founder of OddsJam. “You’re betting against a quant firm with infinitely better technology than you.”

The U.S. Justice Department cleared the way for Paramount Skydance’s $111-billion purchase of Warner Bros. Discovery — a major milestone that moves David Ellison closer to his goal.

After a months-long review, Justice Department antitrust regulators on Friday concluded the combination would not violate federal anti-competition laws. Approval had been expected because President Trump — who has friendly ties with Ellison and his father, tech billionaire Larry Ellison — favors the deal.

The government stopped short of asking Paramount to make concessions or divestitures.

Antitrust regulators found that “based on the evidence received in its investigation … the transaction is not likely to result in harm to competition or American consumers,” the Justice Department division said in a statement.

They looked at whether the merger would give Paramount too much power in the streaming video on demand market; the linear television channel space and “studio development, production, or distribution of films for theatrical release” but did not find potential antitrust violations, the Justice Department said.

Ellison has promised to continue releasing 30 films a year with a combined Warner Bros.-Paramount studio.

“We are grateful for the Department of Justice’s thorough review of this transaction, as well as the work of the other agencies that have completed their reviews and provided clearance to date,” Paramount said in a statement.

“This deal is pro-competitive, resulting in a stronger company better positioned to compete against dominant technology platforms in an industry increasingly defined by intense competition for audiences, talent, technology, and investment. We remain focused on completing the transaction as soon as possible and delivering its benefits to consumers, creators, and the entertainment industry as a whole,” Paramount said.

Paramount wants to finalize its purchase by September.

With Friday’s victory, Paramount is staying on that timetable, but regulators in Europe and Britain have opened their own regulatory investigations and are expected to make their own determinations in the coming months.

Separately, California Atty. Gen. Rob Bonta and other state attorneys general have been scrutinizing the proposed merger, and are widely expected to file a lawsuit, perhaps as early as this month, to try to block it.

Paramount applied for the Justice Department’s approval in December — more than two months before it edged out Netflix in the Warner sweepstakes.

Buying Warner Bros. would allow Paramount — Hollywood’s smallest major company — to bulk up with such prestigious properties as HBO, CNN, HGTV and Food Network. Those would be combined with properties Paramount already owns, including CBS, Comedy Central, Nickelodeon and MTV.

The deal would put two historic film studios, and two prominent news organizations under the same roof. It would give Paramount four streaming services, including HBO Max, and dozens of cable channels.

Justice Department approval could complicate efforts by Bonta and other state attorneys general to block the deal. Should Bonta or others sue, they would have to convince a judge that the nation’s top antitrust regulators failed to make a proper finding.

That may pose a high bar for the state officials, who are facing political pressure to stop the deal.

“State AGs must block this merger,” U.S. Sen. Elizabeth Warren (D-Mass.) said in a statement Friday, adding that the Justice Department‘s approval was “terrible news for every American who doesn’t want Trump-aligned billionaires to control what they watch and how much they pay.”