Lesson 2 of 4 · 8 min read
Correlation & real diversification
Owning ten stocks isn't diversification if they all drop at the same time. Correlation explains why.
Diversification is about connectedness
Real diversification means holdings that don't all move the same way at the same time. The degree to which two investments move together is called correlation. Holdings with low or negative correlation absorb each other's blows.
Correlation from -1 to +1
Correlation +1: they move exactly alike. 0: no connection. -1: they move exactly opposite. For diversification you don't want a portfolio full of +1 holdings — that's essentially one big bet in ten different coats.
In practice
Ten different tech stocks look diversified, but they move strongly together (high correlation). In a tech crash they all drop at the same time — little real diversification.
Where diversification really comes from
You get better diversification by mixing across sectors, regions and categories that react differently to the economy. Stocks and bonds, or different parts of the world, often move less in sync than ten stocks from the same corner.
More assets, and above all less correlated ones, lower risk without costing you return. Assets that move together barely help.
Stocky says
Don't ask 'how much do I own?' but 'do my holdings move differently in a crisis?' That's diversification.
Watch out: correlations change
In calm times things move independently, but in a real panic many correlations shoot toward +1: almost everything drops at once. That's why a portion of safe, boring holdings (like quality bonds or cash) is valuable — precisely when things go wrong.
Did you know?
In the 2008 crash many 'unrelated' investments suddenly dropped together — 'correlations go to 1 in a crisis' became a well-known saying.
Your one-ETF foundation, made smarter
A broad world ETF already handles diversification within stocks for you. The next step is diversification between categories — exactly the asset allocation from the previous lesson. Correlation is the reason why that mix makes your portfolio more stable.
Remember this
- Real diversification is about connectedness: holdings that don't all move the same way at the same time.
- Correlation runs from -1 to +1; a portfolio full of +1 holdings is one bet in ten different coats.
- Diversification comes from mixing across sectors, regions and categories that react differently to the economy.
- In a real panic correlations shoot toward +1 — boring, safe holdings then prove their worth.
There is a quiz for this lesson
3 questions that check whether it stuck. They live in the app, because a quiz nobody grades teaches nothing.
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