A clean quantitative framing is this: in a crisis, correlations converge — which means diversification fails exactly when you need it most. Three quick checks before you act: 1. Name the mechanism in plain English: Assets that appear uncorrelated in normal markets can move in lockstep during stress events. This makes naive diversification less protective than it appears. 2. Say why it matters for behavior or portfolio decisions: That is why stress-testing should use conditional correlations, not historical averages. 3. Set the review question: Ask whether the market is mispricing the mechanism or simply narrating it loudly. Market translation: In March 2020, equities, credit and even gold initially sold off together as liquidity evaporated. The diversification that "worked" in 2019 failed in the first weeks of the crisis. Failure mode: The mistake is building portfolio protection around average-condition math when tails are where the real damage happens. That is usually where the edge is: not in the vocabulary, but in the structure underneath it.