Don't put it all in one

The problem with one

Suppose all your money is in the shares of one company. If it does well, you do well. If it has a bad year, so do you. And if it fails — which happens to companies, including large and famous ones — you can lose almost everything.

A company’s past success does not protect you. A business can lose its customers to a competitor, take on too much debt, or be hit by a problem nobody saw coming. When everything you own depends on one company, your whole result depends on that one story.

Many baskets

The answer is an old one: do not put everything in one place. Spread the money across many holdings, so that a single failure is a small part of the whole. This is called diversification.

The arithmetic is simple. The effect of one holding on your portfolio is its weight — its share of the total — times how much it moves.

The same rule works for gains. If that one company had doubled instead, the concentrated portfolio would be up 100% and the diversified one 5%. Diversification gives up the chance that one great pick makes you rich, in exchange for not letting one bad pick ruin you.

Set a holding’s weight and how far it moves, and see what it does to the whole portfolio. A small weight keeps even a large fall small.

Unrelated is the key word

Twenty holdings only help if they do not all fall for the same reason. Five banks from the same country are five holdings, but one banking crisis can hit them all at once. Five companies selling the same product share the same risks.

Diversification works best across things that behave differently: companies in different industries, of different sizes, in different countries — and across different asset classes, such as shares, bonds and cash. How a portfolio is divided between these is its allocation.

For most beginners, the simplest way to own many companies at once is a fund. One ETF or index fund can hold hundreds of companies — for example, all the companies in an index such as the S&P 500, which follows about 500 large US companies. The Funds path explains how they work and what they cost.

What diversification cannot remove

Some risks belong to one company: a failed product, a fraud, a lost court case. Diversification shrinks these until they barely matter.

Other risks hit almost everything at once: a recession, a financial crisis, a sharp rise in interest rates. When the whole stock market falls, a portfolio of many stocks falls with it, because nearly all of them are going down together. This is market risk, and no amount of spreading across stocks removes it.

How concentration creeps back

A portfolio that starts out diversified does not stay that way on its own. If one holding rises much faster than the rest, it becomes a larger and larger part of the whole. A stock that was 5% of your portfolio can become 30% after a few good years — and then a fall in it costs six times as much.

Concentration also creeps in through familiarity: owning only companies from your own country, only the brands you use, or only the industry you work in. Each feels safe because it is known, but it ties your money to one set of risks — sometimes the same one your salary depends on.

Looking at each holding’s weight from time to time, and asking “what happens if this one falls by half?”, is a simple way to see where you stand.

In short

Owning one stock ties your result to one company’s fate. Spreading money across many unrelated holdings makes any single failure small: its weight times its fall. Diversification cannot protect you from the whole market falling, and it keeps working only while no single holding, industry or country quietly takes over.