(Bloomberg) -- Retail investors may finally be losing the will to backstop the U.S. equity bull market.

The day-trading crowd has stepped in during practically every pullback this year, but the scale of their interventions is getting smaller, according to data compiled by Vanda Research, which tracks traffic on retail trading platforms and industry-wide order flows.

This raises the chances of more serious declines if big investors continue to retreat, the firm’s strategists warn in their weekly note.

“While we have seen a pick-up in ETF buying this week, the magnitude has been a little underwhelming relative to previous selloffs,” Ben Onatibia and Giacomo Pierantoni wrote. “This diminishing appetite to support the equity rally raises the odds of a larger selloff if institutional investors continue to sell.”

Empowered by a boom in apps like Robinhood and drawn to trading during the pandemic, an army of retail traders has reliably shown up to buy broad exchange-traded funds at various points in 2021. 

They did so again in the five days through Tuesday, Vanda says, with $657 million of purchases. Yet such buying was between 35% and 100% larger during similar-sized drawdowns in July and August, Onatibia and Pierantoni note.

While seasonal factors could be at play -- as vacations end and a new school year begins in the U.S. -- there’s another worrying sign, Vanda says: Retail investors have been concentrating their buying in tech stocks.

“There are two distinct phases in the outperformance of technology shares,” the strategists wrote. “The first one is usually driven by institutional investors, who enter the trade when valuations are attractive. The second leg of the outperformance happens when FOMO-driven retail investors join the trade.”

It all hints at “an imminent reversal” in market leadership, they said. 

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