Short selling and short squeezes explained, and why they are so risky

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Short selling and short squeezes became household phrases after the meme stock frenzy of early 2021. They are often discussed as if they were a battle between heroes and villains, but the mechanics are simple and the risks are real for everyone involved. This guide is educational and is not financial advice.

What short selling is

Short selling is a way to profit from a fall in price. The basic steps are:

Borrow shares, usually arranged through a broker, and pay a fee for the loan.
Sell them at today's price.
Buy them back later, ideally at a lower price.
Return the shares to the lender and keep the difference, minus fees and costs.

If the price rises instead, the short seller has to buy back at a higher price and takes a loss. In the US, rules generally require a broker to have reasonable grounds to believe the shares can be borrowed before a short sale, and selling without that arrangement, known as naked short selling, is restricted.

Why short sellers matter

Short sellers are a normal part of markets. They can help prices reflect bad news, and some publish research that has exposed accounting problems and fraud. They also take on unusual risk, which is why regulators watch the practice closely and sometimes require large short positions to be disclosed.

Why the losses can be unlimited

When you buy a share, the most you can lose is what you paid, because a price cannot fall below zero. When you short a share, there is no ceiling on how high the price can go, so there is no theoretical limit to the loss. On top of that:

Borrowing costs can rise sharply when shares are hard to borrow.
The lender can recall the shares, forcing you to close at a bad moment.
Margin calls arrive as the price rises, because a short position is held in a margin account.
Dividends paid while you are short must be paid to the lender.

What a short squeeze is

A squeeze happens when a heavily shorted share starts to rise and short sellers rush to buy back to limit their losses. Their buying pushes the price higher, which forces more short sellers to buy, and the loop feeds itself for a while. Signs that commentators look at include a high proportion of shares sold short, called short interest, and a high days to cover figure, which estimates how many days of normal trading it would take for short sellers to buy back.

What the GameStop episode showed

In January 2021, GameStop shares rose dramatically amid heavy short interest, intense discussion on Reddit and wide media coverage. A US SEC staff report published later that year looked at the episode and pointed to a mix of factors, including large price and volume changes, short interest and social media attention, and it noted that some brokers restricted trading during the peak. The lesson many people missed is that squeezes are short-lived. Prices that rise on forced buying can fall just as quickly once the buying stops, and people who arrived late can be left holding large losses.

Lessons for ordinary investors

A high short interest figure is not a buy signal. Shares are often heavily shorted because the business is in genuine trouble.
Timing is almost impossible. Nobody reliably knows when a squeeze starts or ends.
Beware coordinated calls to act. Posts urging everyone to buy at once to hurt short sellers can cross into market manipulation, and they often benefit those who bought early.
Shorting is not a beginner's tool. Unlimited downside, margin and recall risk make it unsuitable for most people.

Check your national regulator's guidance, such as the FCA in the UK or the SEC in the US, and consider independent regulated advice before making decisions. Nothing here is a recommendation to buy, sell or short anything. Did you follow a squeeze as it happened? Share what you learned about the risks in the comments.