The stock market has a long and well-documented history of underperforming in September more than in any other month of the calendar year.
Commonly referred to as the “September Effect,” this seasonal pattern has puzzled investors, financial analysts, and economists for decades without a single universally accepted explanation.
From 1928 to 2025, the S&P 500 averaged a return of negative 1.13% in September, according to Yardeni Research, making it by far the worst-performing month on record.
February ranks as the second weakest month, averaging a negative 0.10% return, while no other month across the entire calendar has averaged a negative return over that same period.
According to S&P Global, the S&P 500 has declined in September 55% of years since 1928, meaning the market falls more often than not during that particular stretch of trading days.
One of the most widely cited theories behind the effect points to investor psychology, particularly the emotional and behavioral shifts that accompany the transition from summer into fall.
“Psychologically, when the leaves turn in the fall, vacations end and the days are getting shorter, there is this kind of negative vibe out there that tends to accentuate any negative events,” said Dan Seiver, a finance professor at San Diego State University.
Another contributing factor may be end-of-year selling pressure, as investors approaching the close of the fiscal year often begin offloading underperforming positions, driving stock prices lower in the process.
Some market watchers also point to the self-fulfilling prophecy theory, suggesting that if enough investors believe September will be bad, they sell preemptively and effectively create the very decline they anticipated.
Corporate earnings revisions may also play a meaningful role, as many companies fall short of earlier guidance and are forced to revise their outlooks downward, further denting investor confidence during that period.
Taken together, these overlapping forces create a seasonal headwind that has proven remarkably consistent across nearly a century of market history, even as the broader economy and market structure have changed dramatically.
While no single cause has been universally accepted, the September Effect remains one of the most closely watched and stubbornly persistent anomalies in modern financial markets.
