The September Effect and Its Historical Reflections on Artificial Intelligence Stocks
The historical weakness trend observed in markets during September and the impact of artificial intelligence giants on portfolio allocation are examined.
In investment markets, the aspects of September that distinguish it from other months based on historical data and the weight of artificial intelligence stocks in market value are evaluated.
The September Effect and Historical Data
September is known as the only month in which the S&P 500 index has recorded a negative long-term average return between 1928 and 2025, with the index losing an average of 1.1 percent during this period.
The Weight of Artificial Intelligence Stocks
The Magnificent Seven, encompassing giants such as Nvidia, Apple, Alphabet, Microsoft, Amazon, Tesla, and Meta Platforms, account for approximately 34 percent of the S&P 500's value.
September Performances in Past Years
Market results varied across the three Septembers that have passed since ChatGPT was launched by OpenAI in November 2022.
In September 2023, the S&P 500 and Nasdaq-100 experienced declines of about 5 percent, while Nvidia dropped by 10 percent.
Increases Observed in Subsequent Years
In the Septembers of 2024 and 2025, interest rates, capital expenditures, and earnings growth overcame seasonal pressures.
In September 2024, the S&P 500 and Nasdaq-100 rose by around 2 percent, whereas in September 2025, the S&P 500 increased by 3.5 percent and the Nasdaq-100 by 5.4 percent.
Shift Toward Defensive Assets
When investors move away from growth and technology stocks, they may turn toward defensive assets such as gold, short-term bonds, utilities, and the healthcare sector.