Most investors make money when a stock goes up. Short sellers do the opposite: they aim to profit when a stock goes down. It is a legitimate and common part of how markets work, but it is also one of the riskier things an investor can do, for a reason worth understanding before going anywhere near it. This is general information, not investment advice.
How it works
The mechanics run in reverse from normal investing. A short seller borrows shares, usually from their broker, and sells them straight away at today's price. Later, they buy the same number of shares back and return them to the lender. If the price has fallen in between, they keep the difference; if it has risen, they take the loss. The trade is a bet that the shares are worth less than the market currently thinks, as the US Securities and Exchange Commission explains in its investor guidance.
Most short selling is transparent and rule-bound. Firms must mark orders as short sales and confirm the shares can actually be borrowed and delivered, under the SEC's Regulation SHO. What is not allowed is "naked" short selling, where a trader sells shares without having borrowed or arranged to borrow them first.
Why the risk is different
Here is the crucial asymmetry. When you buy a stock, the worst case is that it goes to zero and you lose what you put in: your loss is capped. When you short a stock, there is no such cap, because there is no limit to how high a share price can climb, the SEC notes. Short a stock at $50 and it doubles to $100, and you have lost your entire stake; if it reaches $150, you have lost twice what you first received. The loss grows as the price rises, without end.
That is why short positions come with margin calls. If the stock moves against the seller, the broker can demand more cash as collateral, and if the seller cannot provide it, the broker can close the position at the prevailing price, crystallizing the loss whether the seller likes it or not.
The costs that eat returns
Short selling is not free even when the bet is right. The borrower pays a fee to whoever lends the shares, and that fee rises sharply for stocks that are hard to borrow. Sellers also pay interest on the margin they use. Those running costs accumulate for as long as the position stays open, quietly reducing any eventual profit.
Short squeezes
The unlimited-loss problem can feed on itself in a "short squeeze." When a heavily shorted stock starts to rise, sellers rush to buy shares to close their positions and cap their losses. That buying pushes the price up further, forcing still more sellers to cover, in a self-reinforcing spiral.
The clearest recent example was GameStop in January 2021. Short interest in the stock had reached around 140% of its freely traded shares, and when a wave of retail buyers piled in, the price ran from roughly $17 at the start of the month to intraday highs near $480, as widely documented. Some brokers temporarily restricted buying, citing the collateral their own clearing houses demanded.
What short sellers are for
For all the controversy, short selling serves real functions. Because short sellers profit by spotting what is overvalued, they push prices toward more realistic levels and act as a counterweight to hype. They also have a financial incentive to dig for problems, and short sellers have helped surface accounting frauds that others missed, a point made by the CNBC coverage of the practice.
Critics counter that shorting can deepen sell-offs and that some short sellers talk their book aggressively. Both things can be true. For an ordinary investor, though, the practical takeaway is simpler: the mechanics that make shorting possible also make it uniquely dangerous, and the unlimited downside is the part to respect most.
