Alphabet (GOOGL)—accounted for nearly a quarter of the index, the highest percentage in at least two decades. That concentration had fallen to 21% by the end of the year. As of September, those five stocks were up an average of nearly 60% for the year, and due to their large sizes, contributed to more than 80% of the S&P 500’s gains. But half of the stocks in the S&P 500 were actually down for the year, and more than a quarter were trading in bearish territory, 20% or more below where they started in 2020. That means the index was disproportionately lifted by just a few names. Investors in S&P 500 funds are making bigger bets on fewer stocks than they likely realize.