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Millions of amateur investors got into the stock market during the pandemic — some gingerly, some aggressively, some determined to teach Wall Street bigwigs a lesson — and almost couldn't help but make money, riding a bull market for the better part of two years.

Now they may have to wrestle with a bear.

"It definitely isn't as easy to trade in this market," said Shelley Hellmann, a 47-year-old former optometrist in Texas who began actively investing in April 2020 while isolating from her family.

Tracking stock movements on an iPad Mini in her bedroom, she banked big gains as the market soared. Within a couple of months, she was considering making day trading a full-time gig. But since the S&P 500 peaked on Jan. 3, profits have been harder to come by.

"Sometimes I am glad to not be red for the year," she said.

Five months of bumpy declines have put the S&P 500 into a bear market — a drop of 20% or more from its most recent high.

In response, many of the estimated 20 million amateurs who started trading in the past two years — whether bored sports bettors or meme-stock aficionados who piled into GameStop — have tapped the brakes, or scrambled to shuffle their portfolios into more defensive positions.

S&P Global Market Intelligence, which analyzed April data from Charles Schwab and Interactive Brokers, said retail trading activity was down 20% compared with the meme-stock frenzy of January and February 2021. Popular retail brokerages report fewer active users: Robinhood, the choice of many amateurs who jumped in early in the pandemic, said last month that it had 15.9 million active users in March, down 10% from a year earlier and off 8% from the end of last year.

The recent decline, the company said, was tied to "users with lower balances, who are engaging less in the current market environment."

The mood has even cooled on Reddit forums like WallStreetBets. In the heat of the rising market, invincible traders congregated there to joke that stocks only went up. But the irrational exuberance has given way to darker humor: One recent post included an image of the grim reaper slaying low interest rates and stock market bulls.

Jonathan Colon got out as the market began its retreat. He put $3,000 into a Robinhood account last June and sold everything early this year as stocks slid in January. He cashed out with a $100 loss.

"It was like when you get smacked on the hand a few times as a kid and you learn not to go here or there," he said.

Though the stampede to open new brokerage accounts has abated, retail trading activity remains well above prepandemic levels — a testament to the sheer number of people who took up stock trading as the coronavirus upended normal life.

Even if their tastes have changed, they are a slice of the trading population that's still showing an appetite: As of the end of April, TD Ameritrade, part of Charles Schwab, said its retail customers were still buying more stocks than they were selling, according to its Investor Movement Index, which measures retail investors' behavior and sentiment, based on a sample of accounts that completed trades in the past month.

Hellmann, who started actively trading in the early days of the pandemic, said she was sticking with it, learning more and refining her approach as she goes along.

She often rises at 3 a.m. and turns on CNBC to begin plotting her strategy for the day, which involves studying stocks' price movements, a process she compared to learning to catch a softball — watching its arc, then trying to figure out the physics of where it will land. "That is what I'm doing with price and volume," she said.