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Five Hundred Stocks, One Bet: Why Diversification Is Dead
TLDR
The hedge is broken. Since Iran closed the Strait of Hormuz in late February, stocks and long bonds have fallen together. The stock-bond correlation is positive. The 10-year yield closed Friday at 4.78 percent. The 2-year closed at 4.37 percent, its highest since January 2025. Bonds now fall with equities and add to the loss.
The index is ten stocks. The top 10 names are about 40 percent of the S&P 500. The Magnificent Seven alone are 33.9 percent, worth 23.7 trillion dollars. By the Herfindahl math, the 503-stock index behaves like 54 equal stocks. Ten names carry close to half of its volatility. Buying "the market" is buying one theme: AI.
The diversifiers failed the test. In March, the S&P 500 fell about 5 percent. MSCI EAFE fell 10.2 percent. Emerging markets fell 13.1 percent. Gold sold off. Bitcoin traded near half its pre-war peak. Semi-liquid private credit funds received 20.8 billion dollars in redemption requests and paid out about half.
One factor drives everything. US debt crossed 40 trillion dollars on August 19. The 30-year yield hit 5.34 percent the same day, a 19-year high. Treasury doubled its bond buybacks. Gold rose 14 percent in August. Bitcoin rose more than 20 percent. Every asset now prices one variable: what the dollar is worth.
Returns were never spread out. Since 1926, about 4 percent of listed US stocks created all of the market's net wealth. More than half underperformed Treasury bills over their lives. Diversification by count guarantees you own the losers in proportion.