The stock market hasn't looked this top-heavy since the heyday of the so-called "Nifty Fifty" — a group of stocks that included Polaroid, Sears and Eastman-Kodak and that drove a bull market more than half a century ago.
By the numbers: The market value of the top 10 companies in the S&P 500 makes up nearly 37% of the index's total value, the highest since 1965.