Since the launch of Robinhood in 2015 with its “zero, nada, zilch” commission-free trading, professional investors have got used to the fast-growing presence in their markets of amateurs and their gambling, thrill-seeking, Reddit-message-board-fuelled behaviour—and what they can do to prices.

The brokerage’s active users grew to 22.5 million by the end of 2021. And though the sway of retail traders in single-name options markets has faded from its peak during the meme-stock frenzies of early