The 60/40 portfolio — 60% stocks, 40% bonds — has been the default "balanced" allocation for decades, and the reason isn't tradition, it's a specific historical relationship: when stocks fall, investors typically flee to the safety of government bonds, pushing bond prices up right when the stock side of the portfolio needs the cushion most. For most of the last few decades, that relationship held reliably enough that 60/40 became shorthand for "sensible."

What actually happened

A sharp rise in interest rates hits stocks and bonds through two different mechanisms that, for once, pointed the same direction. Rising rates directly reduce the market value of existing bonds — a bond paying a fixed rate becomes less attractive the moment newly issued bonds pay more, so its price falls to compensate. At the same time, rising rates raise the discount rate used to value future cash flows, which — as the arithmetic in how rate rises hit valuations lays out — pushes equity valuations down too. Same underlying cause, two separate transmission channels, both firing in the same direction. Stocks and bonds fell together, and the diversification the 60/40 portfolio is built around simply didn't show up when it was needed.

Seeing it in the data, not just the narrative

Our asset correlation tool computes the actual rolling correlation between US stocks and US bonds from historical data — you can watch the relationship drift and, in periods like this, turn sharply positive rather than staying reliably negative. It's a useful reminder that correlation is a statistical pattern, not a law of physics: it holds under certain conditions and can break under others, and interest-rate regime shifts are one of the more common triggers.

What this means for how you build a portfolio

It doesn't mean diversification is a myth — it means the specific diversification a 60/40 mix provides depends on a specific market condition (falling rates, or at least a "flight to safety" dynamic) that doesn't hold in every environment. Our portfolio backtest tool lets you test a given allocation across different historical periods precisely so you can see how it performed across more than one regime, not just the one everyone remembers. A portfolio that only looks diversified in the environment it was designed for isn't really diversified — it's environment-dependent, and it's worth knowing which one you're actually holding.