Mega-Cap Tech Fuels Index Concentration Gains

America’s benchmark large-cap stock index is reaching fresh highs as Microsoft, Apple, Nvidia, Alphabet, Amazon, Meta and Tesla surge, with investors betting on artificial intelligence, cloud computing and digital advertising to drive profits into next year. Analysts say the resulting concentration — with a handful of mega-cap names accounting for an unusually large share of index value — boosts headline returns for passive investors but raises volatility if any leader falters.