Robinhood and others in the industry say commission-free trading has created huge savings for individual investors and opened up trading to the masses.

Yet critics worry that it is really the big players on Wall Street who are winning. Gensler’s comments suggest he sympathizes with this concern.

“There are costs. It’s like an iceberg: most of the iceberg is below the surface,” said the SEC chief. “The costs are below the surface.”

“ Dominant ” position held by Citadel Securities

Gensler has also pledged to address concerns about the broader structure of the modern stock market, including the “dominant” position held by Citadel Securities, the market maker owned by billionaire Ken Griffin.

The SEC chairman specifically pointed out that Citadel Securities executes 47% of all retail volume. The Chicago firm is Robinhood’s main source of income.

“Concentration could lead to more fragile markets, which means less orderly, but also more expensive or less efficient markets. That’s what history and economics tell us when we get the concentration,” said Gensler.

Additionally, Gensler expressed concern over information held by Citadel Securities due to its dominant position.

“There is an online search engine in America that most of us turn to. This business has data advantages,” he said. “Now, in the midst of our financial markets, a company in the retail order flow may have growing data advantages over other market participants.”

Isaac Boltansky, director of policy research at Compass Point Research & Trading, said discussions about market concentration “will be a concern for Citadel” in the future.

“The dial has been made stronger on the issue of focus,” Boltansky said in an email.

Citadel Securities has previously suggested that it does not care about the fate of the payment for the order flow.

“We’re just playing by the rules of the road,” Griffin said during a February hearing on GameStop. “If they choose to change the rules of the road, we have to drive on the left side rather than the right side, that’s great for us.”

A spokesperson for Citadel Securities declined to comment.

Game features in trading

Like other regulators, Gensler has expressed unease with the integration of game-like features into trading applications to increase client engagement. He asked SEC staff to look into these issues.

Gensler noted that this so-called gamification is not exclusive to trading apps, although the stakes are higher.

“Streaming apps figured out a long time ago that I’m a bit of a romantic comedy guy,” Gensler said. “You thought I was a thriller guy, but it’s a romantic comedy. If I waste an hour and a half and it was a lousy romantic comedy, okay.”

Still, gamification in commerce may have greater effects on investor finances, Gensler argued.

Robinhood representatives were not immediately available to comment on the hearing.

The trading app has repeatedly defended itself by showing how its platform has enabled countless new investors to participate in the financial markets.

“We look forward to working with the SEC as it examines issues involving retail investors and market structure,” Robinhood said in a statement. “To the extent the SEC decides that regulatory changes may be necessary, we intend to participate in the process of regulating public notices and comments.”

Despite the increased control, Robinhood continues to enjoy explosive user growth, especially in cryptocurrencies. The startup confidentially filed for its IPO in March, paving the way for a much-anticipated IPO.