September has a reputation on Wall Street as it’s often called the worst month for stocks. While that claim is rooted in long-term historical averages, it can also be misleading if you treat it like a forecast. Some Septembers are down sharply, some are flat and others are positive. “Historically weak” performance does not guarantee that markets will fall in September.

So, what should investors do with this information? The useful takeaway isn’t to brace for a guaranteed market drop in September, but rather to understand what seasonal patterns can tell you – and to prepare for the possibility of higher volatility without making impulsive changes.

"Markets face a confluence of geopolitical, fiscal and monetary policy uncertainty – prime conditions for volatility. But volatility and seasonality aren't destiny. Stay disciplined: If September gets choppy, treat it like a pullback (opportunity) – not a prophecy – and stay anchored to your long-term plan," J.P. Morgan Wealth Management Global Investment Strategist Ajene Oden said.

Here's what the data behind the “September effect” actually shows, and why the month can be volatile for stocks. Learn some ways you can use a practical, long-term playbook for managing your portfolio in September – whether markets dip or not.

Is September the worst month for stocks? What the data shows

Historically, September has tended to be a weaker month for stocks on average, but outcomes vary widely and aren’t reliably predictable year to year. Being the worst month typically means having the lowest average monthly return for a benchmark index like the S&P 500, and often a higher frequency of finishing negative compared with other months.

According to historical data, the S&P 500 has fallen by an average of 0.6% in September since 1945, making it the worst-performing month of the year. For context, February is the only other month that typically finishes in the red, declining by an average of 0.2%. By contrast, November and December are among the strongest months of the year, rising by an average of 1.4% and 1.6%, respectively.¹

On top of having the weakest average return, September has also historically been among the months most likely to finish negative. The S&P 500 has been positive only 44% of the time in September since 1950; indeed, it’s the only month with a positivity rate below 50%, according to data analyzed by Reuters.²

Put differently, this trend suggests a historical tendency – not a rule – and it can be overwhelmed by macroeconomic news, earnings, investor positioning and risk sentiment in any given year. Seasonality describes a long-term pattern rather than a reliable short-term signal.

Why September can be volatile: A few common explanations

There is no single reason why September tends to be weaker on average, but a handful of recurring dynamics may make markets feel more reactive as summer ends. These are best viewed as plausible contributors that sometimes overlap – not as a set of rules that explain market performance every September.

One explanation may be post-summer repositioning and liquidity. As investors return from summer breaks and portfolios are refreshed, trading activity can pick up and positioning can change. If liquidity is uneven – especially around large reallocations – even modest shifts in demand can translate into sharper day-to-day moves.³

Another factor is institutional flows and rebalancing. Many large funds adjust exposures on regular schedules, and systematic strategies may change risk levels based on volatility, momentum or timing (such as month- or quarter-end). Certain tax-related decisions, such as managing gains or losses, may influence what gets bought or sold. These flows aren’t inherently bearish, but they can amplify market moves in either direction if multiple players are adjusting at the same time.⁴

September can also coincide with investors’ resetting of expectations going into the fall season. With more of the year’s economic data in hand, markets may reassess the outlook for growth, inflation and interest rates as year-end approaches. Attention also often shifts toward upcoming earnings and forward guidance as markets start looking ahead. Combined with a busier macroeconomic and policy calendar, market sensitivity to surprises may increase.⁵

The above are just a few reasonable explanations for why September is often volatile, but they do not guarantee that it will be negative. In any given year, the month’s performance is usually driven by specific catalysts, with seasonality mostly shaping the backdrop rather than the outcome for investors.

How investors can prepare for September volatility

If September turns out to be volatile, the most useful response usually isn’t trying to predict the next move – it’s making sure your financial plan is built to handle normal market swings.

A helpful starting point is separating your money by time horizon, because what’s appropriate for near-term goals can be very different from what works for the long term. In general, short-term funds (money you expect to use in the next year or two) tend to prioritize liquidity and capital preservation. Long-term funds, which include money for retirement or multiyear goals, are often invested with the expectation that markets will fluctuate along the way.

Next, perform a quick liquidity check. Looking ahead to the next six to 24 months, consider upcoming expenses such as taxes, tuition payments, home repairs or any major purchases. The purpose isn’t to make a call on where stocks are headed, but rather to reduce the risk of being forced to sell equities during a drawdown. If a goal has a hard deadline, aligning those funds to the right time frame can help keep market volatility from putting your objective at risk.

From there, it may help to review whether your portfolio has drifted from its intended mix and whether rules-based rebalancing makes sense. Rebalancing is best thought of as risk management – that is, bringing your portfolio back toward your target allocation after markets move. Many investors use a simple framework such as reviewing allocations on a set schedule (such as once or twice a year) or rebalancing when exposures move beyond preset levels (consider setting a threshold). The key is to have a method you can follow consistently, especially when market headlines are noisy.

It’s often worth reviewing your portfolio diversification, as well. After strong market rallies or sharp declines, it can be common for risk to become more concentrated than investors realize. That can happen from a single stock growing too large, a sector becoming dominant or a single investment theme taking over the portfolio. Even if your overall mix of stocks and bonds looks fine on the surface, unintended concentration can make volatility feel more extreme than what you expected.

If markets get choppy in September: A quick decision checklist to consider

Before making any changes, investors may want to ask themselves some questions, such as:

• Has my goal or time horizon changed?
• Do I need cash in the next 12–24 months?
• Am I off my target allocation enough to rebalance per my rules?
• Did I drift into concentration risk with a single stock, sector or theme?
• If stocks fell another 5% to 10%, what would I do – and am I comfortable with that?
• Would the move I’m considering be reversible without harming my long-term plan (considering taxes or missed rebound risk)?
• Do I have a clear decision process – such as a written plan, policy or advisor conversation – versus reacting in the moment?

Finally, consider building in behavioral guardrails for choppy markets. September’s reputation may amplify emotional decision-making, like panic-selling after a market pullback or making big portfolio moves on short-term news. A simple rule of thumb: If you can’t clearly explain how a change improves your financial plan based on your goals, timeline or cash needs, it may be a reaction to volatility rather than a strategy. And if you’re unsure how these decisions fit your goals, consider speaking to a qualified financial advisor for guidance.

The bottom line: What September seasonality means for investors

September’s reputation is grounded in historical averages, but that reputation isn’t a forecast. While the data suggests September has tended to be the weakest month on average for U.S. stocks, outcomes can still vary widely year to year, and any given September is often driven by specific catalysts like economic data, policy decisions and earnings expectations.

A useful way to think about the September effect is as a planning prompt, not a timing signal. If your strategy fits your goals, time horizon and risk level, the best response to seasonal narratives is often to stay disciplined – while making sure near-term cash needs are covered, your portfolio hasn’t drifted from its target mix and concentration risk hasn’t quietly built up.

When markets get choppy, try to focus on what you can control: process over prediction, diversification over concentration and behavioral guardrails over headline-driven moves. If you’re unsure how to apply those steps to your situation, a financial professional may be able to help align allocation, liquidity and risk to your timeline.

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The Standard and Poor’s 500 Index is a capitalization-weighted index of 500 stocks. The index is designed to measure performance of the broad domestic economy through changes in the aggregate market value of 500 stocks representing all major industries. The index was developed with a base level of 10 for the 1941–43 base period.

The price of equity securities may rise or fall due to the changes in the broad market or changes in a company's financial condition, sometimes rapidly or unpredictably. Equity securities are subject to 'stock market risk' meaning that stock prices in general may decline over short or extended periods of time. 

Diversification does not ensure a profit or protect against loss.