The yield gap between bonds and stocks has flipped to its most bond-favorable level in a quarter century, a structural shift that is compelling investors to rethink their asset allocation frameworks.

The 10-year US Treasury yield has broken above 5%, lifting bonds' appeal relative to equities to roughly the highest in about 25 years. The stock earnings yield, measured as the inverse of the S&P 500's price-to-earnings ratio, now sits below the 10-year US Treasury yield, meaning bonds have clearly gained the upper hand over stocks on an income basis.

This setup poses potential downward pressure on the stock market. Yale University economist Robert Shiller's cyclically adjusted excess CAPE yield model suggests the S&P 500 may outperform bonds by only about 1% per year over the next decade. Investors' margin for error on corporate earnings expectations has narrowed substantially.

Long-bond investors take heavy losses as bubble burst becomes evident

High yields are in some ways a sign of economic resilience — US stocks are near record highs, the US economy is performing better than expected, and the shock from rising long-end rates has yet to fully surface at the macro level.

Yet the first to feel the pain are investors who had bet on long-dated bonds. Using the TLT ETF, which tracks US Treasuries with maturities of 20 years and above, as a reference, these investors have suffered sizable losses.

Looking back, the bond bull market spawned over the decade after the financial crisis by low inflation and government intervention, along with the pandemic-era price surge, now appears in hindsight to have carried clear hallmarks of an asset bubble.

Market patterns suggest that when a bubble bursts, it often signals a buying opportunity. Today's 10-year Treasury yield above 5% is a level rarely seen in decades, and the allocation value of bonds has been substantially repriced.

The inverted-yield logic ends and the two-decade paradigm of stocks beating bonds is broken

The old market orthodoxy held that stocks, given their growth potential, deserve a valuation premium over bonds — meaning the stock earnings yield should naturally be lower than the bond yield.

Yet for roughly two decades after the global financial crisis, that logic was completely upended — the stock earnings yield stayed above the bond yield for an extended period, making stocks the undisputed favored asset.

Now the situation has reversed again. Comparing the stock earnings yield with the 10-year US Treasury yield, bonds' relative appeal has returned to levels last seen about 25 years ago.

This means that even though stock market valuations have already compressed somewhat to absorb higher bond yields, the equity outlook still faces certain pressure.

Shiller model issues a warning, though its historical limitations also deserve attention

Robert Shiller's excess CAPE yield indicator forecasts stocks' excess returns over the next decade by comparing the cyclically adjusted real earnings yield with the 10-year Treasury yield.

Historical data show the indicator has significant predictive power for stocks' excess returns over the following decade, and the current reading implies the S&P 500 will outperform bonds by only about 1% per year over the next ten years.

Still, the model's forecasting accuracy has declined in recent years — actual stock market performance has far exceeded its projections, possibly because the market has been continuously supported by policy intervention.

Even so, the core signal remains clear: bonds' substitution value is the highest in a generation. Against this backdrop, investors need to scrutinize S&P 500 earnings growth forecasts with far stricter standards, because if corporate profits fall short of expectations, the bond market will no longer provide any cushion.