The Stock Market Is Doing Something It Hasn't Done in Over a Half-Century, and It Could Signal a Big Move in 2026

Group 1 - The S&P 500 index has reached new all-time highs in 2025, largely driven by high-performing artificial intelligence stocks, with some exceeding $1 trillion in market capitalization [1] - Many investors are turning bearish, with indicators suggesting the market may be overvalued, signaling potential significant movements in stocks for 2026 [2] - The S&P 500's largest constituents, particularly nine stocks, now account for over 38% of the index, indicating a lack of diversity [5][8] Group 2 - The concentration measure of the S&P 500, which compares the weighted average market cap to the unweighted average, has risen to 10.67, the highest level since 1970 [6][7] - This concentration level is even greater than during the dot-com bubble, highlighting the potential risks associated with the current market structure [7]