Concentration Risk: Why a Single Stock Is a Risky Portfolio

A court ruling goes against the company. A product gets recalled. An auditor finds that revenue was booked that never existed. Any of these can knock a stock down hard in a single session, and none of them has anything to do with the economy, interest rates or the rest of the market. They belong to one company. If that company is most of what you own, they belong to you.

That is company-specific risk, sometimes called idiosyncratic or unsystematic risk. Every stock carries it. Careful research lowers the odds of walking into it, and the lesson on what you own when you buy a share covers what that research looks like. It can’t lower them to zero, because some of the worst surprises, fraud above all, are designed to get past exactly the kind of checking an outside investor can do.

The arithmetic of losses

Losses and gains aren’t symmetrical. A fall is measured on the bigger starting balance, and the climb back is measured on the smaller one that’s left.

Smaller losses work the same way, just less brutally. A 20% fall takes $10,000 to $8,000. Getting the $2,000 back is a 25% gain on $8,000.

The general rule is easy to keep in your head. Divide the loss by what’s left. A 20% loss divided by the remaining 80% is 25%, and a 50% loss divided by the remaining 50% is 100%, and the deeper the hole gets the faster the required gain grows, which is why one big loss on a concentrated position can take years of decent returns to repair.

What spreading out does

Now split the same $10,000 across 20 unrelated companies at $500 each. Suppose the worst happens to one of them and it goes to zero. You lose $500, or 5% of the portfolio. Recovering 5% takes a gain of a little over 5% on what remains.

Compare that with the whole $10,000 sitting in the one company that failed. Same event. The difference is entirely in how much of your money was exposed to it.

Size matters as much as count. Twenty holdings sounds spread out, yet if one of them is $5,000 and the other nineteen share the remaining $5,000, a collapse in the big one still costs you half the portfolio, so look at the weight of your largest position before you take any comfort from the number of lines on your statement.

That’s all diversification does at this level. The odds of bad news stay the same, and what shrinks is how much of your money any one piece of it can reach.

What it can’t fix

Market risk is the other kind. Recessions, rate changes and broad selloffs pull most stocks down together, and owning 20 or 200 companies won’t stop that, because the thing hurting them is common to all of them.

The concentration you may not see

Employer stock is the easy one to miss. If your workplace retirement plan lets you buy your employer’s shares, or pays a matching contribution in them, the balance can build up over the years until a large share of your retirement money sits in one company. That’s the same company paying your salary. A bad stretch for the business can then hit your job and your savings in the same year, which is about as concentrated as a position gets.

Check your plan statement for it. Stock options or restricted shares from your employer count too. So does a large inherited position you’ve never gotten around to trimming.

Fraud deserves its own attention, and the lesson on spotting investment fraud covers the warning signs. More holdings help, though only if they’re genuinely different, and the next lesson, on spreading across sectors, sizes and countries, shows how to tell whether yours are.