For more than a decade, the 60/40 portfolio ran on fumes from bonds. Today, yields north of 5% on parts of the Treasury curve are rebuilding the income engine—and reshaping the decisions behind a balanced mix. With long-end rates testing highs not seen since 2007 and stocks sometimes moving in the same direction as Treasuries, the math and the mechanics of a 60/40 look different. That affects expected returns, volatility, and how the “40” is built.