Positive Stock-Bond Correlation Lifts 60/40 Risk by Nearly 25%

What's New

A move in stock-bond correlation can raise a 60/40 portfolio's volatility by nearly 25% with no change in holdings. Apollo's Christopher Lvoff and Lucy Xin make the case in After 60/40: What if Traditional Equity Portfolio Diversifiers Are Falling Short? They assume equity volatility of 15%. Investment grade bonds get 6%. Their first case uses a correlation of -0.3. Portfolio volatility comes to about 8.6%. The second uses +0.6. Volatility rises to roughly 10.6%. Allocators who count on bonds to cushion equity drawdowns should retest that assumption against current correlations.

Why It Matters

Investors who hold long-duration bonds as their equity hedge are most exposed. That sleeve is meant to rally when equities fall. Apollo argues that heavy Treasury issuance and AI-driven corporate borrowing are lifting the term premium. When the long end reprices on supply, long bonds can sell off alongside equities. Buyout investors face a related problem. The authors' example starts with a 5-year hold. Delayed exits stretch it to 7 or 8 years, adding duration nobody chose.

By The Numbers

• 0.7: the approximate post-2022 peak in the 36-month rolling stock-bond correlation, after two decades of low to negative readings.

• 4.8%: hyperscalers' share of the US investment grade index by mid-2026, up from 2.7% in July 2025.

• Nearly 10%: Apollo's estimate of that share by 2030 as AI borrowing accelerates.

• 33%: technology's share of buyout deal value. Public equity runs at 45%.

Between The Lines

The paper ends with a product case. Apollo recommends a structural role for hybrid capital: preferred equity, convertibles and secured loans with warrants. Its evidence is a proxy built from public ETFs over June 2009 to June 2026. Convertibles take 30% of the proxy. Preferreds take 50%. Broad US equities take 20%. Apollo then adds a 3% annualized premium for illiquidity and complexity. The paper shows no results without that premium, so readers can't separate the benefit of the structure from the assumption. The authors' own tech chart also cuts against part of their argument. Buyout deal value is less tech-heavy than the S&P 500, though the paper presents private equity as a concentration risk. The demand case holds up better. Companies want growth capital without selling equity at low valuations or adding senior debt.

The Wrap

Allocators should treat the bond sleeve as a weaker hedge than their models assume while stock-bond correlation stays positive. The case strengthens if Treasury supply and AI-related issuance keep the term premium elevated. It weakens if long bonds again rally when equities fall. Hybrid capital may help, but its edge rests on a premium Apollo assumes. The next 12 to 18 months of correlation data will show which way it breaks.