Diversification fails when the assets share a shock

explainerPublished 19 October 20262 min readmulti asset

Key points

• Correlation is a statistic about a chosen window. It is not a property of the assets forever.
• Weights decide how much a common factor matters. A small weight in a wild asset can still dominate.
• This note does not print a correlation matrix. None was estimated.

What is going on

Portfolio variance depends on each variance and on the covariances, scaled by weights. If two assets are perfectly correlated, mixing them does not cancel their common move. If they are uncorrelated, the mix cancels some of the idiosyncratic noise and leaves the average. In stress, correlations among risky assets have often risen. That sentence is a warning about the statistic, not a measured coefficient from a WiT Markets model. Inflation shocks can hurt nominal bonds and some equities together. A duration shock hurts long bonds and can reprice equities through the discount rate, which is a different channel.

Why it matters

The allocation calculator takes weights you type and shows a mix. It does not know the next covariance. The rebalancing calculator shows how trades restore weights you choose. Rebalancing is not a source of return by itself. It is a rule. A portfolio that is 90 percent in one equity index and 10 percent in a commodity is mostly the equity factor, whatever the number of tickers.

What to watch

Name the factor: equity, credit, duration, currency, liquidity. Ask which holdings share it. Read concentration inside the ‘diversified’ fund, which is the subject of the index-concentration note. In a drawdown, the recovery calculator shows the gain required after a loss you enter. It does not predict one.

Risks

A backtest of correlations will look kinder than the stress you will care about, because the stress is rare in the sample. Leverage makes a modest common shock into a large loss. Diversification is not a guarantee against loss, and this sentence is the limit of the idea rather than a slogan.