"Don't put all your eggs in one basket": the advice is as old as finance, and it is sound. But one still has to know what a basket is. Spreading holdings across several countries or sectors achieves nothing if they all depend, ultimately, on the same cause. This notion explains what a common risk factor is, and why it decides a portfolio's fate far more reliably than the number of lines it contains.
1 The definition
A shared cause that moves many holdings at once.
The definition
What explains common movements
A risk factor is an economic cause that simultaneously influences the price of many assets: global growth, interest rates, energy prices, or a theme such as artificial intelligence. We speak of a "common" factor when several holdings in a portfolio are exposed to it at once. A stock's return then splits in two: what is specific to it (the quality of its management, its niche market) and what it shares with the others. The first part dilutes as you add holdings; the second does not. It is the second that makes a portfolio's true risk.
2 Apparent or real
Counting holdings is not enough.
The distinction
A thousand holdings, one single bet
Apparent diversification is measured by what can be counted: the number of stocks, countries, sectors. It reassures because it is visible. Real diversification is measured by the independence of the risks: two investments diversify only if they do not react to the same causes. Holding a thousand stocks exposed to the same factor is holding the same bet a thousand times, with the illusion of prudence thrown in. Conversely, a few genuinely independent positions can protect better than a catalogue of hundreds of lines. It is not the count that dilutes risk, it is the variety of causes.
3 Hidden correlation
It usually reveals itself at the worst moment.
The trap
Correlations rise just when you need them lowest
The common factor has an unpleasant property: it shows little in calm times and a great deal in a crisis. While all goes well, holdings exposed to the same theme can move fairly dispersedly, which gives the impression of fine diversification. But when the factor turns, they fall together: correlations climb precisely when you were counting on them to cushion the shock. That is why diversification that has never been tested by a difficult episode should always be viewed cautiously: it has not yet shown whether it holds for real.
The key idea
Diversification that has not yet been tested by a crisis is only a hypothesis: the common factor reveals itself above all when it turns.
4 Spotting the factor
Three simple questions before believing you are diversified.
The method
Look for the cause, not the label
Spotting a common factor requires no elaborate model, just a little curiosity. Three questions are often enough. First: what are my largest positions on a look-through basis, once funds and indices are opened up? Next: what do those positions owe their recent growth to, distinct causes or one and the same theme? Finally: what would make them fall at the same time? If the same answer comes back three times, the common factor has been found. The exercise also applies beyond the stock market: a job, a home, a business can depend on the same engine as the portfolio.
5 Takeaways
Worth remembering.
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The common risk factor: an economic cause (rates, energy, a theme such as AI) that moves many holdings at the same time.
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Apparent versus real: the first is counted in holdings, the second is measured by the independence of risks; only the second protects.
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Hidden correlation: the common factor shows up mainly in crises, when holdings fall together and the cushion is missing.
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The right question: not "how many holdings do I have?" but "what would make them all fall at once?"
This notion illuminates an analysis
First published: 16 July 2026