The S&P 500 index historically performs worst in September, marking the only month with a negative average return since 1928 [1, 2].
This seasonal trend often creates anxiety for investors who fear a monthly downturn. However, current market indicators suggest that the typical September slump may not repeat this year.
Historical data shows the S&P 500 has averaged a -1.17% return during September [1]. The index has ended the month lower in 56% of years since 1928 [1]. These figures establish September as a statistical outlier compared to other months of the year.
Despite these trends, several metrics indicate a stronger environment for 2026. The SPY has seen a year-to-date gain of 13% as of early September [3]. Additionally, the VIX level stood at 15 at the start of the month [3].
Recent performance already deviates from the historical average. The S&P 500 has advanced 1.9% month-to-date in September 2026 [4].
Some analysts believe the current momentum outweighs historical patterns. "The market's strong year-to-date trajectory suggests 2026 could defy those historical odds," Ryan Detrick said [5].
While some data suggests September provides a stronger entry point for investors than in other years [3], the historical average of losses remains a primary concern for cautious traders [1].
“September is historically the only month with a negative average return for the S&P 500.”
The conflict between historical seasonality and current momentum highlights a common tension in equity markets. While the 56% probability of a decline suggests a cautious approach, the 13% year-to-date gain and positive early September movement indicate that macroeconomic drivers in 2026 may be overriding the 'September Effect.'
