The S&P 500 and Nikkei 225 both hit all-time highs on May 7 while behaving like five-stock portfolios. Thirty-three trillion dollars in retirement savings is riding a concentration bet that exceeds the dot-com peak. The Nikkei 225 surged 5.6 percent on May 7, posting the largest single-day point gain in its history. The S&P 500 closed at a fresh all-time high. Both indexes are doing something that the numbers attached to them do not convey: they are concentrating into a handful of stocks while wearing the label of broad diversification. The S&P 500 has an effective number of 54 stocks. The Herfindahl-Hirschman Index sits at 185, thirty percent above the five-year average of 142. The top ten holdings account for 35.6 percent of the index. Nvidia alone carries 7.0 percent. Apple holds 6.3 percent. Microsoft sits at 4.6 percent. The top three names represent roughly eighteen percent of what is marketed as a five-hundred-stock portfolio. In Tokyo, the pattern is sharper.…