The US stock market has never leaned so heavily on so few companies, which is bad news for anyone holding index funds.

• Nvidia and Apple alone now make up 15% of the S&P 500 — an all-time high.
• That beats the concentration seen before the 2000 dot-com bust and the 2011 peak, when the top two names topped out near 9%.
• Before 2020, the two biggest stocks never went past 10% of the index.
• Underneath the surface the market is weak: more stocks are hitting 52-week lows than at almost any point on record.
• South Korea is the cautionary tale — its market crashed after two giants swelled to half the exchange.

Outlook: If the big names stumble, the whole index goes with them, and the warning signs are already flashing.