Nasdaq's September Weakness Looms Over AI Stocks Like Nvidia

The Nasdaq has experienced a historically weak September in nearly half of all instances since its creation in 1971, leading to a persistent calendar anomaly known as the 'September Effect'. This phenomenon is not merely a statistical oddity but rather a reflection of heightened risk perception among investors.

During September, portfolio managers often reassess positions and trim winners or dump laggards ahead of the fourth quarter. Some funds operate on fiscal years that end in September or October, creating a short window for tax-loss harvesting, which contributes to lower liquidity and increased scrutiny of valuations.

The combination of these factors historically produces an average decline in the Nasdaq, despite it finishing higher roughly 52% of the time. The apparent contradiction lies in the magnitude of the index's September declines: when it falls, the losses are often sharper and more concentrated, dragging the overall long-term average down.

For AI stocks like Nvidia (NVDA) and Micron Technology (MU), this seasonality can amplify volatility and make them convenient vehicles for selling. A modest rotation away from these names can reduce the Nasdaq's upward momentum, potentially leading to even more selling from retail investors.