What Happens to Markets in September: The Data Behind the September Effect for Stocks, Crypto, Gold, and Forex
September is the only calendar month with a negative average return for the S&P 500 since 1928. The index has averaged a decline of 1.13 percent in September going back to 1928 per Yardeni Research and has ended lower 55 to 56 percent of the time. The Nasdaq 100 has experienced September declines of 11.8 percent in 2023, 19.5 percent in 2022, and 7.4 percent in 2021. Bitcoin has its own September pattern. Gold typically performs differently. Here is the complete data-driven guide.
TL;DR: September has been the worst calendar month for the S&P 500 since reliable market data began in 1928. The index has averaged a decline of 1.13 percent in September per Yardeni Research, the only month with a negative long-term average return, and has ended lower 55 to 56 percent of the time over that near-century window. The Nasdaq 100, which is more sensitive to valuation compression from the rate anxiety that often accompanies September Federal Reserve meetings, has experienced particularly sharp September declines in recent years: 11.8 percent in September 2023, 19.5 percent in September 2022, and 7.4 percent in September 2021. Bitcoin has its own September pattern that is distinct from equities but not reassuring: historically weak September performance with the notable exception of 2023 when Bitcoin gained approximately 4 percent. Gold and the US dollar often outperform in September as institutional investors reduce risk asset exposure and rotate toward safe-haven positions ahead of year-end. However, Fisher Investments analysis provides the critical counterpoint: if you remove big outlier years that skew the data, September's average returns are far less worrisome, with the month showing positive performance more than half the time in non-outlier years. The September Effect is not a law but a tendency. In 2026, the tendency faces an unusual set of circumstances: a Nvidia blowout earnings report that validated the AI infrastructure thesis on August 27, a hawkish Fed Chair Warsh whose Jackson Hole speech pushed rate hike probability to 55 to 60 percent, and the CLARITY Act September 15 cloture vote that represents the most significant regulatory catalyst for crypto since January 2024. Whether these 2026-specific forces override the historical September tendency is what every investor is calculating right now. MediaCrypto note: the September Effect is the most widely known and least actionable seasonal pattern in financial markets. It is real in the historical data. It is unreliable as a trading signal in any specific year. What it correctly identifies is the specific types of pressure that September consistently brings: institutional rebalancing, tax-loss harvesting, mutual fund fiscal year-end window dressing, and the Federal Reserve meeting. In 2026, all four of those forces are operating simultaneously alongside the most active crypto regulatory calendar in US history.
Every September, the same articles appear. The September Effect is coming. History says stocks fall. Reduce your risk. The advice arrives as reliably as the calendar turning.
The historical data that underlies it is real. The practical usefulness of acting on it is much more limited than the articles typically acknowledge.
The S&P 500 September Record Since 1928
The numbers are not ambiguous. Going back to 1928 per Dow Jones Market Data, the S&P 500 has declined an average of 1.2 percent in September, making it the weakest month of the year. The index has ended lower in September 56 percent of the time over that stretch, the only month to decline more than 50 percent of the time. The Motley Fool notes that from 1928 to 2025, the S&P 500 averaged a return of negative 1.13 percent in September per Yardeni Research, with February coming in second at negative 0.10 percent and no other month averaging a negative return.
The severity in specific years has been notable. September 1931 produced a 29.6 percent decline during the Great Depression. September 2022 produced the worst performing September since 1974 with a 9.34 percent decline per Stock Trader's Almanac data. September 2001 was devastated by the 9/11 attacks. The outlier years dominate the average and drive the narrative.
Fisher Investments' most important contribution to the September Effect debate is removing those outliers. When the catastrophic one-off events are excluded, September's average returns look far less alarming, with the month showing positive performance more than half the time. The decade-level data is also important: September was positive in 2019, 2017, 2016, and 2010 among other recent examples, demonstrating that the effect is a tendency rather than a law.
Three theories explain why September has historically been weak. Portfolio managers returning from summer vacations rebalance portfolios in September, creating a concentrated burst of selling pressure as they exit positions to make room for new holdings. Mutual funds and hedge funds whose fiscal years end September 30 engage in window dressing, selling underperformers before the year closes. And the Federal Reserve's mid-September interest rate decision exacerbates selling pressure if investors anticipate a hawkish outcome. In 2026, all three forces are present simultaneously: the September 17 Fed meeting follows Warsh's hawkish Jackson Hole speech, positioning the meeting as potentially hawkish, exactly the configuration that the historical data identifies as the worst September setup.
Algorithmic trading amplifies these seasonal patterns in ways that did not exist in the pre-quant era. Many trading algorithms are trained on historical market data that includes the September Effect. As a result, they trigger sell signals simultaneously as September approaches, creating a self-fulfilling cascade of selling pressure that amplifies human behavioral patterns through machine speed and scale.
The Nasdaq 100 and Nvidia September Pattern
The Nasdaq 100 has its own September problem that is more severe than the broader S&P 500 pattern. Technology stocks, which dominate the Nasdaq's composition, are more sensitive to rate anxiety because their valuations depend heavily on discounted future cash flows. When rate hike expectations rise in September, as they did in 2022, 2023, and now in 2026 following Warsh's speech, the multiple compression hits growth stocks harder than value stocks, producing Nasdaq September declines that exceed the S&P 500 average.
The Nvidia-specific September pattern is notable given the stock's dominance of the current AI investment narrative: Nvidia fell 11.8 percent in September 2023, 19.5 percent in September 2022, and 7.4 percent in September 2021. Motley Fool's August 28, 2026 historical analysis specifically highlights this three-year declining September streak before noting that Nvidia has averaged more than 5 percent gains in both October and November from 2017 to 2025, suggesting the post-September setup may be more favorable than September itself.
Bitcoin and September: The Crypto Pattern
Bitcoin has a separate September seasonal pattern from equities. Historically, Bitcoin has experienced some of its weakest monthly returns in September, with 2022 producing a sharp decline alongside equities. The Bitcoin-equity correlation that has strengthened since the introduction of Bitcoin ETFs means that the equity September Effect now transmits more directly to Bitcoin than in earlier cycles when Bitcoin traded more independently. In 2026 specifically, the September 15 CLARITY Act cloture vote creates a crypto-specific event that could override the seasonal pattern in either direction: a successful vote would produce a significant positive catalyst, while a failed vote would compound the seasonal headwind.
Gold and September: The Safe-Haven Pattern
Gold's September pattern differs from equities because the same risk-off institutional rebalancing that depresses stocks often benefits gold. Institutional investors reducing equity risk in September often rotate into safe-haven assets including gold and Treasuries, creating a September tailwind for gold that partially offsets the weakness in risk assets. This pattern has not been consistent enough to be called reliable, but it provides the directional framework for understanding why gold sometimes strengthens in September while equities weaken.
The 2026 Verdict: Pattern Versus Catalyst
In 2026, September's three specific catalysts either confirm or override the historical pattern. If Nvidia's August 27 blowout earnings sustain momentum into September, the AI infrastructure thesis may override the seasonal selling pressure. If the CLARITY Act fails on September 15, it adds a specific negative catalyst to the seasonal headwind. If the September Fed meeting delivers a rate hike as Warsh's speech implied it might, the valuation compression that has historically driven September Nasdaq weakness arrives with specific institutional backing.
The honest assessment per Fisher Investments' framework: do not let the calendar determine your investment decisions. The September Effect is a tendency, not a prediction. The specific catalysts of 2026 matter more than the historical average.
About the Author
This article was researched and written by the MediaCrypto editorial team. Follow us on X at https://x.com/MediaCrypto_AI and Instagram at https://www.instagram.com/mediacrypto.ai/
FAQ — September Effect Markets 2026
What is the September Effect? The September Effect refers to the historical tendency for the S&P 500 to underperform in September. The index has averaged a decline of 1.13 percent in September since 1928 per Yardeni Research, the only calendar month with a negative long-term average. The index has ended lower in September 55 to 56 percent of years.
Is September really bad for stocks? The data shows September is the weakest month on average. However, Fisher Investments analysis shows that removing outlier years (Great Depression, 9/11, 2022) leaves September showing positive performance more than half the time in non-outlier years. The September Effect is a tendency, not a law, and has been positive in multiple recent years including 2019, 2017, and 2016.
What are the three causes of the September Effect? Portfolio managers returning from summer vacations rebalance portfolios, creating concentrated selling. Mutual funds with September 30 fiscal year-ends engage in window dressing, selling underperformers before close. The Federal Reserve's mid-September interest rate decision exacerbates selling if investors anticipate hawkish outcomes. Algorithmic trading trained on historical patterns amplifies all three forces simultaneously.
How does September affect Bitcoin? Bitcoin's correlation with equities has strengthened since the introduction of Bitcoin ETFs, meaning the equity September Effect transmits more directly to Bitcoin than in earlier cycles. In 2026 specifically, the September 15 CLARITY Act cloture vote creates a crypto-specific catalyst that could override the seasonal pattern in either direction.
What makes September 2026 different from historical averages? Three 2026-specific factors: Nvidia's August 27 blowout earnings that validated AI infrastructure demand, Fed Chair Warsh's hawkish Jackson Hole speech that pushed September rate hike probability to 55 to 60 percent, and the CLARITY Act September 15 cloture vote representing the most significant regulatory crypto catalyst since January 2024.
For live market data see https://mediacrypto.ai/market
Read also: S&P 500 September 2026 Outlook — https://mediacrypto.ai/news/sp-500-september-2026-outlook-trading-at-7678-ai-earnings-season-ahead-and-wheth
Read also: Bitcoin Price Prediction September 2026 — https://mediacrypto.ai/news/bitcoin-price-prediction-september-2026-after-breaking-the-200-day-moving-averag
This article is for informational purposes only. Always do your own research before making investment decisions.
