Via Goldman:
…the S&P is now just a handful of mega stocks, because as the chart below shows the largest 5 stocks in S&P500 now account for 22% of market cap, even higher than during the dot com bubble.
…the gap between the three month returns of the S&P 500 stock one standard deviation above the average vs. one standard deviation below the average has registered 40%, nearly twice the 10-year average of 23%.