Why can two different holdings still behave like one concentrated position?

Count exposures as well as labels

Investor.gov describes diversification as spreading investments to reduce risk. The ECB’s November 2022 analysis discusses why less-than-perfect co-movement matters and how correlations can change across conditions. That historical analysis is background for the concept, not evidence about today’s correlation or any recommended allocation.

Use a worksheet listing each holding’s underlying exposures, not just its product name. If two invented funds both hold the same companies in the same proportions, two line items may still represent substantially the same underlying bet. Verify actual holdings before drawing that conclusion about real funds.

Compare two invented pairs

Start with $1,000 in fictional Holding A and $1,000 in B. In Scenario 1, both rise 10%, so the portfolio becomes $2,200. In Scenario 2, both fall 10%, leaving $1,800. Each position has a different name, but the stipulated movements provide no offset in either scenario.

Now replace B with invented Holding C. Stipulate that when A rises 10%, C falls 10%, and vice versa. Starting with $1,000 each, the ending total is $1,100 + $900 = $2,000 in either toy scenario. This constructed offset is not a claim that such a reliable pair exists in markets.

Do not estimate correlation from a story

The two scenarios are arithmetic illustrations, not a historical sample or a correlation estimate. A real estimate needs the return definition, matched dates, observation frequency, sample window, and treatment of missing data. Record those choices with the result.

One observed period in which two assets moved in opposite directions cannot establish a dependable hedge. Nor does a low co-movement estimate alone explain how much each holding can lose. The size and path of losses still need separate attention.

Add a common stress scenario

In a third invented scenario, A falls 20% and C falls 15%. The two $1,000 positions become $800 and $850, for a total of $1,650, down 17.5%. The earlier perfect offset has disappeared because the stipulated relationship changed.

This scenario does not predict a crisis. It is a question to ask of the exposure story: what shared event could affect both holdings? Keep proposed stress assumptions separate from probabilities. A table can help identify a concentration while still being unable to quantify its likelihood.

For each proposed offset, write the assumed mechanism next to the numbers. That makes a claim about a shared exposure easier to challenge than a bare statement that two labels are diversified.

Method, sources, and limits

All holdings, scenarios, and starting amounts are invented. The sums and percentages were checked with decimal arithmetic. Investor.gov and ECB sources were opened on 26 October 2026. No real fund, correlation series, forecast, or optimal portfolio was evaluated. This worksheet explains how to inspect a diversification claim; it does not eliminate loss risk or specify a suitable allocation.