Why diversification works
Spreading your money does not lower the expected return — it narrows the range of outcomes.
Two risks, not one
Every stock carries two kinds of risk. The first belongs to that company alone: a product recall, a weak CEO, a competitor inventing something better. The second is shared by all of them: a recession, a war, a big shift in interest rates.
Diversification removes the first and leaves the second untouched. If you hold a hundred companies, one going bankrupt costs you one percent. But if the whole market falls thirty percent, you fall thirty percent too — inside the stock market, there was nowhere to hide.
So diversification is not a shield against loss. It is a shield against losing for the wrong reason.
A worked example
A hundred thousand euros in one company
If that company goes bankrupt, you are left with zero. It does not matter how good your reasoning was.
A hundred thousand euros in a hundred companies
The same bankruptcy costs you a thousand euros. All hundred would have to fail for you to be left with zero.
The average return in both cases
The same. Diversification cost you nothing to remove that risk.
Why it costs nothing
This is the part that surprises people. If you hold a hundred companies instead of one, your expected return does not drop. You get the average of all hundred, and that average is exactly what the market delivers.
What drops is the spread around that average. With a single company, you might quadruple your money or lose it all. With a hundred, both extremes vanish and you stay close to the average.
It is a trade where one side is free: you give up the bad tail and the good tail, but the middle stays where it was. That is why it is sometimes called the only free lunch in finance.
When it stops working
Diversification rests on one assumption: that the things you hold do not all fall at once. When that assumption breaks, the protection disappears exactly when you need it most.
In major crises, correlations head towards one. Stocks that usually moved independently start falling together, because the cause of the fall has nothing to do with any of them — it is about money fleeing risk in general.
That does not make diversification worthless. It makes it a tool for everyday risk, not an umbrella for the storm.
Key terms in this lesson
- Correlation
- How often two investments move in the same direction. Low correlation is what makes diversification work.
- Idiosyncratic risk
- The risk that belongs to a single company, removed by holding many others.
- Systematic risk
- The risk that hits everything at once and that diversification cannot remove.
Quick check
During a severe crisis, your diversified portfolio falls almost as much as a single stock. Why?
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