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Correlation Breakdowns: When Diversification Fails

Diversification, spreading money across assets that don't move together, is one of the few ideas nearly every investor agrees on. But the relationships it relies on are not fixed. In a crisis, assets that normally move independently can fall together. This explainer covers what correlation measures, why it changes, and what recent crises showed.

4 min read · Figures checked October 2026

1. What correlation tells you

If two assets have a correlation near +1, owning both gives little protection: when one falls, so does the other. Near 0, their moves are unrelated. Below 0, one tends to rise when the other falls, which is what makes a mix of them steadier than either alone. The classic 60/40 portfolio, 60% shares and 40% government bonds, relies on shares and bonds having a low or negative correlation.

2. Why correlations change in a crisis

Everyone sells at once

When investors rush to cut risk, they sell shares, high-yield bonds, emerging markets and commodities together. Assets that normally follow different stories suddenly follow one: fear.

Forced selling

Investors who have borrowed to invest face margin calls when prices fall, so they sell whatever can be sold quickly, including assets that have nothing to do with the original problem.

The macro driver changes

The share–bond relationship depends on what the market fears most. When the main risk is a recession, bonds rally as shares fall. When the main risk is inflation, rising rates push both down.

3. Three crises compared

  • 2008: shares, corporate bonds, commodities and property all fell together. US Treasuries rose strongly: diversification within risky assets failed, but government bonds did their job.
  • March 2020: in a scramble for cash, investors briefly sold almost everything, including gold and even Treasuries, until the Fed stepped in to stabilise markets.
  • 2022: the 60/40 portfolio lost roughly 16–17%, one of its worst years in decades, because rising inflation and interest rates pushed shares and bonds down together for the whole year.

The long view

For most of the 20th century before 2000, shares and bonds tended to move together, because inflation was the main driver of both. The negative correlation investors relied on from 2000 to 2021 was itself unusual, a product of low, stable inflation. Many analysts now ask which world we are in.

4. What has held up in sell-offs

  • Long-dated government bonds in recession-driven crashes (2008, 2020 once the Fed acted), but not in inflation-driven ones (2022).
  • Cash and short-dated bonds, whose value barely moves.
  • Gold in many crises, though it was briefly sold in March 2020.
  • The US dollar and Swiss franc, which have tended to rise when investors seek safety.
  • Volatility itself, which rises when shares fall.

The lesson professionals draw is not that diversification fails, but that it depends on understanding why assets have moved differently in the past, and whether those reasons still hold.

Key takeaways

  1. 1Correlations are estimated from the past and change over time.
  2. 2In a sell-off, correlations between risky assets tend to rise as investors sell whatever they can.
  3. 3Shares and government bonds moved in opposite directions from about 2000 to 2021, mainly because inflation was low and stable.
  4. 4When inflation is the main risk, as in 2022 and the 1970s, shares and bonds can fall together.
  5. 5The assets that held up best in past crises depended on the cause: cash and short-dated bonds in inflation scares, long bonds in recessions.

Related

Educational content only. It describes how markets have behaved in the past and is not investment advice.

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