Yen strength & portfolio risk
14 September 2026 · Research demonstration
Could a stronger yen amplify a US equity selloff?
Working conclusion: A plausible stress mechanism, but insufficient evidence for a standalone trading signal. The useful task is to identify when funding pressure and forced deleveraging coincide.
Hypothesis and historical evidence
Rapid yen appreciation can worsen the yen-denominated value of unhedged overseas positions. If leveraged investors face losses or margin calls, reductions in risk may amplify selling. This is a conditional mechanism, not an inevitable sequence.
BIS Bulletin 90 describes leveraged unwinds amplifying the response to negative US macroeconomic news in August 2024; markets then stabilized. That supports investigating the mechanism, while limiting claims about lasting crashes. BIS Bulletin 90, 27 August 2024.
How I would challenge the thesis
Evidence that strengthens it
Yen appreciation alongside documented funding pressure, leveraged position reductions, and deteriorating liquidity. Seek positioning evidence rather than treating a price correlation as proof.
Evidence that weakens it
Orderly currency adjustment, resilient liquidity, and stable risk assets. US earnings, growth, or rate surprises may explain an equity decline independently of Japan.
Portfolio questions
Which exposures depend on cheap funding, contain unhedged currency risk, or could become difficult to exit together? Compare orderly adjustment and forced-selling scenarios before assigning a probability or proposing a hedge.
Illustration: A US asset falling 10% while the yen appreciates 5% produces approximately −14.3% in yen: 0.90 ÷ 1.05 − 1. Financing, leverage, hedging, and liquidation costs are excluded.
Evidence and next test
The linked chart matches USD/JPY and SPY dates, rebases both series, and exposes observations. Different closing times, omitted dividends, and unavailable leverage data prevent a causal conclusion. Test multiple independent episodes and alternative explanations before defining any trading rule.
Explore the interactive chart and scenarios · Inspect dated rates and releases
No current market level or live trade recommendation is asserted in this memo. Observation dates are shown beside the dashboard data; saved snapshots are labeled.