Investing term
What is Single-stock risk?
The risk that one company you own goes to zero or close to it — independent of the broader market.
Single-stock risk is the danger that one company you own suffers a disaster — fraud, bankruptcy, a shattered business model, a failed drug trial — independent of the broader market. Unlike market risk, which affects everything at once, this is specific to one company, and it can send a stock to zero even while the rest of the market is fine.
The crucial point is that this risk is uncompensated: the market doesn't reward you for bearing the danger unique to a single company, because you can diversify it away for free. Holding one stock exposes you fully to its idiosyncratic catastrophes; holding a broad index spreads that risk across hundreds of companies, so any single blow-up is a rounding error. Concentrating in one or a few stocks — including an employer's stock — takes on single-stock risk the market won't pay you to hold, which is why diversification is the standard defence.
Single-stock risk is one company collapsing — fraud, bankruptcy — while the market is fine. It's a risk you aren't paid for, because diversification removes it for free. Watch employer-stock exposure.
For example
A company you own is exposed as a fraud and its stock collapses to near zero overnight, while the broader market barely moves — single-stock risk that diversification would have contained.
Learn it by doing
That's Single-stock risk in theory — it clicks when you use it. Practise it hands-on in a free, interactive lesson (Stage 17, Portfolio-Level Risk).
Try the free lesson →Why it matters to you
Single-stock risk matters because it's a danger you take on unnecessarily and aren't rewarded for. The market compensates you for bearing broad market risk, which can't be diversified away, but not for the idiosyncratic risk of a single company, which can. Concentrating in a few stocks — or piling into an employer's shares — exposes you to catastrophes that a broad index would render harmless. Understanding that this risk is both real and avoidable is the core argument for diversification.
⚠ Loading up on employer stock
Holding a large share of your wealth in your employer's stock doubles your exposure: if the company fails, you lose both your job and your investment at once. It's a common, dangerous form of single-stock risk, often accumulated through stock compensation without a deliberate decision. Diversifying out of concentrated employer stock is one of the most important risk-reduction moves many people can make.