How a feedback loop in market structure became the most discussed trade of 2021
How Stock Trading Works
Buying a Stock: The Long Position
When you buy a stock, you are buying a small ownership stake in a company. If you buy 100 shares of Coca-Cola at $60 per share, you spend $6,000. You now own those shares. If the price rises to $84, you can sell for $8,400, making a $2,400 profit. If the price falls to $36, you can sell for $3,600, losing $2,400. The most you can ever lose is what you paid: $6,000. Your shares cannot go below $0.
This is called a long position: you buy first, hope the price goes up, and sell later. Most investors are long most of the time.
Short Selling: Betting a Stock Will Fall
Short selling is the opposite. Instead of buying a stock and hoping it goes up, a short seller bets the stock will go down. The mechanics are unusual: you cannot sell something you do not own, so a short seller has to borrow shares from someone who does own them, sell those borrowed shares on the open market, and then buy shares back later to return to the lender.
If the price falls between the time you sell and the time you buy back, you profit. If the price rises, you lose money. And unlike owning a stock (where the worst case is $0), a stock's price can rise without limit, which means short selling losses are theoretically unlimited.
Short selling plays a useful role in financial markets. It gives investors a direct financial incentive to do the hard work of identifying companies that are overvalued, declining, or engaged in fraud. If a short seller discovers that a company is misleading investors about its finances, they can profit by shorting the stock and publishing their findings. Some of the largest corporate frauds in history, including Enron and Wirecard, were first exposed not by regulators but by short sellers who had done the research and had money on the line. Without short selling, stock prices would only reflect the views of optimists.
The step-through below walks through the mechanics one step at a time. Click through both tracks: how a short sale works when the seller is right, and what happens when the seller is wrong.
Step Through the Mechanics
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Key Terms
Long position: Buying a stock and holding it, hoping the price rises. Your maximum loss is what you paid.
Short position: Borrowing and selling a stock you do not own, hoping to buy it back cheaper. Your potential loss is unlimited.
Covering: When a short seller buys shares to return to the lender, closing out their position.
Margin call: When your broker demands you deposit more cash because your losses are growing. If you cannot meet it, the broker can force-close your position.
Short interest: The total number of shares currently sold short, shown as a percentage of the float (shares available for public trading). Normal short interest is 2–5%. Above 20% is high.
The GameStop Squeeze
Why GameStop's Short Interest Was Extraordinary
Normal short interest on a stock is 2–5%. GameStop's short interest in January 2021 was approximately 140%. More shares had been sold short than actually existed in the float.
How is that possible? The same share can be borrowed and sold more than once:
Investor A owns 100 shares and lends them to Short Seller B.
Short Seller B sells those 100 shares. Investor C buys them.
Investor C's broker lends those same 100 shares to Short Seller D.
Short Seller D sells those 100 shares on the open market.
The original 100 shares have now been sold short twice. Short interest: 200%. This is legal, but at extreme levels it creates a dangerous situation: if both short sellers need to buy shares at the same time to close their positions, they need 200 shares but only 100 exist. They are competing with each other for a limited supply.
The short squeeze feedback loop
When short interest is very high and the price starts rising, short sellers lose money. Their brokers issue margin calls, forcing them to either add cash or close their positions by buying shares. But their buying pushes the price up further, which triggers margin calls for other short sellers, who also have to buy. Each round of forced buying triggers the next. This self-reinforcing cycle is a short squeeze.
The Options Effect
There is one more amplifier in the GameStop story, and it comes from options. You do not need to understand options in depth, but you need one key fact.
A call option gives you the right to buy a stock at a set price. If you buy a call option on GameStop at $50, and GameStop rises to $300, you can buy at $50 and immediately have a share worth $300.
When someone sells a call option, they take on risk. To manage it, the seller (usually a large firm called a market maker) buys shares of the stock as a hedge. As the price rises, the market maker buys more shares. This creates a second feedback loop running alongside the short squeeze: retail investors buy options, market makers hedge by buying shares, their buying pushes the price up, which forces them to buy even more. This roughly doubled the magnitude of the squeeze.
Step Through the Squeeze
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Who Did What
The GameStop squeeze involved several groups whose actions are a matter of public record. Here are the key players, followed by a timeline of what happened.
Melvin Capital
Hedge fund (~$12.5B in assets) with a large short position in GameStop. Reported a 53% loss for January 2021.
Citron Research
Short-selling research firm. Published reports arguing stocks were overvalued, profiting when prices declined.
r/WallStreetBets
Reddit community. ~2M members in early January 2021, growing to 9M+ by month's end. Known for high-risk trades and irreverent culture.
Robinhood
Commission-free trading app (~13M users). Makes money through payment for order flow: routing your trades to firms like Citadel Securities, which pay Robinhood for the right to execute them.
Citadel Securities
Executed ~47% of all U.S. retail stock trades as a market maker (a firm that stands ready to buy or sell, earning a small spread on each trade). Separately, Citadel LLC (same parent company) invested $2B in Melvin Capital during the squeeze.
DTCC
The organization that settles U.S. stock trades. When you buy a stock, the transfer takes days to finalize. During that period, the DTCC requires brokers to post cash deposits as a guarantee.
Keith Gill ("Roaring Kitty")
Retail investor and financial educator. Posted analysis of GameStop's deep value on YouTube and Reddit (as u/DeepFuckingValue) starting in 2019, long before the squeeze.
SEC
Federal securities regulator. Published a staff report in October 2021 analyzing the squeeze.
Timeline
September – December 2020
Short interest climbs above 100%
GameStop trades between $9 and $20. Multiple hedge funds build short positions. WSB discussion of the stock grows steadily.
Price: $9 – $20 · Short Interest: ~120–140%
January 11, 2021
Ryan Cohen joins GameStop board
The Chewy co-founder and two former executives join the board. Stock rises on the news.
Close: $19.94
January 19, 2021
Citron announces "5 reasons GameStop goes back to $20"
Andrew Left tweets a planned livestream arguing GameStop is overvalued. WSB treats it as a challenge.
Close: $39.36
January 22, 2021
Momentum accelerates
GME closes at $65. Call option volume surges. Short sellers report significant losses.
Close: $65.01 · Volume: 197M shares
January 26, 2021
The squeeze goes mainstream
Elon Musk tweets about GameStop to his 42 million followers. Chamath Palihapitiya publicly buys GME call options.
Citadel LLC and Point72 invest $2.75 billion in Melvin Capital to prevent its collapse. GME hits ~$483 intraday. WSB is the most active community on Reddit.
Intraday High: ~$483 · Close: $347.51
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r/wallstreetbets · u/DeepFuckingValue · Jan 27
GME YOLO update — Jan 27 2021
50,000 shares · 500 call options Position value: $47,970,388.84 Total gain: +$45,577,903.76 (+5,843%)
January 28, 2021
Robinhood restricts buying
Robinhood and other brokers block purchases of GME. Users can only sell. Robinhood CEO Vlad Tenev says the DTCC demanded $3.7 billion in cash deposits (up from $26 million the day before). GME drops sharply.
Open: ~$265 · Low: $112 · Close: $193.60
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r/wallstreetbets · u/Sdevil88 · Jan 28
Robinhood is restricting purchases of $GME
You can sell your shares but you cannot buy more. They literally changed the rules in the middle of the game. This is what we're up against.
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r/wallstreetbets · u/ssauronn · Jan 28
Don't you dare satisfice. Don't you dare give in.
I'm not selling. This isn't about the money anymore. Exposed the entire system. They turned off the buy button and told you it was for your protection.
February 1 – 19, 2021
Price falls as squeeze unwinds
Without continued buying pressure, GME falls to ~$40. Short interest drops from 140% to ~25% as shorts cover during the volatility.
Feb 19 Close: $40.59 · Short Interest: ~25%
February 18, 2021
Congressional hearing
House Financial Services Committee holds hearing: "Game Stopped?" Witnesses include Vlad Tenev (Robinhood), Ken Griffin (Citadel), Gabe Plotkin (Melvin Capital), and Keith Gill.
"I am not a cat"
Keith Gill, testifying to Congress as both "Roaring Kitty" (YouTube) and u/DeepFuckingValue (Reddit), opened by clarifying he was not a financial advisor and "not a cat" (referencing a viral courtroom video). He then said simply: "I like the stock." Asked if he would buy GameStop at its current price, he said yes. He doubled his position the next day.
The Regulatory Aftermath
The squeeze triggered congressional hearings, an SEC staff report, and proposed rule changes. Here is what existed, what was proposed, and what actually changed.
Rules That Existed Before
Regulation SHO (2005): Requires brokers to locate shares before shorting. Exempts market makers.
Short interest reporting: Firms report short positions twice monthly. Data is published with an 11-day lag. No real-time reporting.
Payment for order flow: Legal. Brokers can route retail orders to market makers for payment, provided they meet "best execution" obligations.
Market manipulation (Rule 10b-5): Prohibits fraud. No specific rules address coordination via social media. The line between "sharing ideas" and "coordinating a pump" is undefined.
What Was Proposed
Real-time short interest disclosure: Replace the twice-monthly system with daily or real-time reporting. Opponents argued this would expose strategies and reduce the incentive to short.
PFOF ban or reform: SEC Chair Gary Gensler raised conflict-of-interest concerns. A full ban was discussed but not enacted.
T+1 settlement: Reduce the settlement period from two business days to one, decreasing the cash deposits brokers must post. This was the most broadly supported reform.
Gamification restrictions: Regulate app features encouraging frequent trading. Limited traction due to definitional and free-speech challenges.
What Actually Changed
The short answer: almost nothing
The SEC finalized T+1 settlement in February 2023 (implemented May 2024). That was the only significant structural change. PFOF remains legal. Short interest reporting is still twice-monthly. No new rules address social media coordination.