Morgan Stanley: S&P Equal-Weight Down 5%, AI Concentration Risk Builds Ahead of Year-End

According to Chaoxiang Research, Morgan Stanley's October 9, 2026 research report indicates that the S&P 500 index declined 0.5% on a market-cap-weighted basis and 5% on an equal-weighted basis in September; the top six contributing stocks accounted for a combined positive impact of approximately 190 basis points, while the remaining constituents dragged down performance by about 240 basis points combined; the median forward P/E ratio for AI and semiconductor stocks stood at approximately 32x, above the ten-year average of roughly 19x.