Why can the yen affect US stocks?
The yen has often been used as a low-cost funding currency. An investor can borrow yen, convert it to dollars and buy Treasuries, equities or other higher-yielding assets. When Japanese rates rise, the yen appreciates or the market turns defensive, both funding costs and currency losses can increase. Investors may then sell overseas assets to repay the yen borrowing.
This does not mean every yen rally causes a US-stock decline. It means some market liquidity comes from financing and exchange-rate conditions, not only from company fundamentals.
A simple carry-trade example
Suppose an investor borrows yen at 1%, converts it to dollars and buys an asset expected to return 5%. If the exchange rate does not change, the theoretical interest spread is about 4%. That is not a guaranteed return: the spread can shrink, the asset can fall, and the yen can appreciate.
If the yen appreciates 10%, the investor needs more dollars to repay the same yen loan. The currency loss can overwhelm the original spread. If many investors unwind at once, selling can move from currencies into equities and credit markets as a deleveraging event.
What I watch
On Rates & Purchasing Power, I can compare Japan's 2Y, 10Y and 30Y government-bond yields with US yields over three years. I watch whether Japanese yields rise quickly, whether the US-Japan rate gap narrows, and whether the yen appreciates at the same time. Exchange rates still require a reliable external live source; the Japanese yield curve alone cannot prove that a carry trade is reversing.
I then return to the home page for index averages and breadth, and use RS Ranking to find stocks that are still resisting the market. If rates, currency and broad-market risk all worsen together, I reduce leverage and chasing instead of increasing total exposure because of one company's short-term catalyst.
The yen carry trade is a capital-flow framework, not a daily US-stock forecast. Its value is reminding me to ask not only “what happened to this company?” during a sudden selloff, but also “are financing and currency conditions forcing investors to reduce risk together?”
This is an educational macro research framework, not investment advice.