When it comes to the seasonal performance of the U.S. stock market, September is often labeled as a 'traditionally weak month'—historical data shows that the probability of decline during this period is indeed significantly higher than in other months. Taking the core U.S. stock market index, the S&P 500, as an example, from 1950 to 2023, there were 41 years in which the index fell in September out of 74 years, resulting in a historical decline probability of 55.4% (close to 56%), which is significantly higher than the average decline probability for the entire year (about 45%); more intuitively, the average return of the S&P 500 in September over these 74 years was -1.02%, making it the only month with an average negative return, while during the same period, the average return for the Dow Jones Industrial Average and the Nasdaq Composite Index in September was -0.73% and -1.35%, respectively, with the Nasdaq being more obviously pressured due to higher volatility in tech stocks.

I. The core logic of September's weakness in the U.S. stock market: It's not just 'historical coincidence'

The seasonal weakness of the U.S. stock market in September is not a coincidence, but rather a result of multiple market behaviors and macroeconomic environments overlapping:


  1. Institutional position adjustment window: The U.S. stock market often presents a 'quiet market' in summer (July-August) due to investor vacations and decreased trading activity. As September marks the end of the third quarter, institutional investors will concentrate on returning to the market for position rebalancing—such as liquidating profits from the first half of the year and hedging potential risks in the second half, which can easily trigger market volatility due to short-term selling pressure;

  2. Emotional dominance during the earnings report gap: The U.S. stock market earnings season mainly concentrates at the end of Q1 (April), end of Q2 (July), and end of Q3 (October). September is in the 'gap period' between the end of Q2 earnings and the disclosure of Q3 earnings. The market lacks substantial support from corporate earnings data and is easily influenced by macro policies (such as Federal Reserve statements), geopolitical news, and other short-term factors, leading to amplified volatility under emotional dominance;

  3. Psychological anchoring of historical risk events: Many landmark volatility events in U.S. stock history are related to September, such as the eve of the 1929 crash (the market began to anticipate a decline in September), the escalation of the 2008 subprime mortgage crisis (Lehman Brothers' bankruptcy in September), and the second adjustment after the 2020 pandemic (the S&P 500 fell 4.8% in September). These events have formed the market's psychological expectation of 'September risk,' further reinforcing the seasonal weakness characteristic.

II. The 'exceptions' of historical patterns and key variables for 2024

It is important to note that the '56% decline probability' is not an absolute conclusion—historically, there have also been cases of the U.S. stock market rising against the trend in September. For example, in September 2021, the S&P 500 rose 0.58% in a single month, and the Nasdaq rose 1.27%, primarily supported by the continuation of the Federal Reserve's accommodative policy and tech stocks outperforming expectations (Apple and Microsoft both had year-on-year revenue growth exceeding 20% in Q3); in September 2019, the S&P 500 even rose 1.9%, benefiting from the Federal Reserve restarting the interest rate cut cycle.


As for September 2024, market performance will depend more on the following three major variables, which may amplify or alleviate seasonal pressures:

  • Federal Reserve monetary policy path: As of May 2024, the Federal Reserve still maintains a high-interest rate range of 5.25%-5.5%. If inflation falls more than expected in the first half of the year (e.g., core PCE year-on-year drops below 2.5%), the market prices in interest rate cuts within the year, which would provide liquidity support for the U.S. stock market and hedge against September's seasonal weakness; conversely, if inflation remains sticky (e.g., a rebound in energy prices driving up short-term inflation), the Federal Reserve's hawkish stance of 'keeping high rates longer' will further suppress risk assets and intensify adjustment pressure in September;

  • Corporate earnings growth resilience: The S&P 500's earnings growth rate fell to 3.2% year-on-year in Q1 2024, significantly slowing from 8.5% in 2023. If Q3 corporate earnings (disclosed in October) release weak signals early (e.g., tech and consumer sectors downgrade performance guidance), the September market may react early to earnings pressure;

  • Policy uncertainty before the election: The U.S. presidential election will be held in November 2024, and September serves as a critical window before the election. The economic policies of the two party candidates (e.g., taxation, industrial policy) may raise concerns about policy continuity in the market, leading some funds to choose to 'hold cash and wait,' thereby reducing market risk appetite.


In summary, based on historical data, September is indeed a high-probability period for the U.S. stock market to face challenges of weakness, but the final performance still requires a comprehensive judgment that combines the macroeconomic fundamentals, policy direction, and market sentiment of the year. For investors in 2024, rather than solely relying on 'seasonal patterns,' it is better to focus on core variables such as Federal Reserve interest rate decisions and corporate earnings data, and respond rationally to potential market fluctuations.#美联储何时降息？