October Is the Stock Market's Most Volatile Month. Here's What 95 Years of Data Actually Shows

Markets can be volatile, and no month illustrates that more clearly than October. From the banking panic of 1907 to Black Monday in 1987 to the depths of the 2008 financial crisis, some of the most dramatic single-day collapses in US stock market history have one thing in common: they happened in October. 

Yet the data tells a more complicated story. Over 95 years of S&P 500 returns, October averages a slightly positive return and finishes higher roughly 59% of the time. The real story is not that October is the worst month, it is that October is the most volatile, with a standard deviation of monthly returns of 5.6%, the highest of any calendar month on record. For traders and investors, that distinction matters. Volatility creates both risk and opportunity, and understanding the historical pattern is the first step to navigating it. 

In this article, we break down the data behind October's reputation, examine what the numbers actually show, and outline what traders need to keep in mind heading into the most unpredictable month of the market year.

TL;DR

• Since 1928, the S&P 500 has averaged an October return of approximately +0.54%, despite the month's reputation for market crashes.

• October has historically recorded the highest monthly return volatility, with a standard deviation of approximately 5.6% in the dataset analysed.

• The VIX reached an intraday high of 89.53 on October 24, 2008, during the global financial crisis.

• October has accounted for 8 of the 20 largest single-day percentage declines in the historical S&P 500 series used in this analysis.

• September, not October, has historically been the weakest month, averaging approximately -1.17% since 1928.

• October has also frequently coincided with major market reversals, contributing to its reputation as a "bear killer."

• Key stats: October has averaged approximately +0.54%, finished positive about 59% of the time, and recorded monthly-return standard deviation of approximately 5.6%.

What Is the October Effect (Mark Twain Effect)?

The October Effect, also called the Mark Twain Effect, is the widely held belief that stock markets tend to decline during October. Despite its staying power, long-term market data does not show October as the weakest month for US stocks. In fact, the S&P 500 has historically generated a slightly positive average return during October.

The theory draws much of its credibility from a handful of high-profile crashes that occurred during the month, including the Panic of 1907, the 1929 crash and Black Monday in 1987. The global financial crisis in 2008 further reinforced October's association with extreme market volatility.

The "Mark Twain" label is a reference to a passage in Pudd'nhead Wilson in which Twain humorously describes October as one of the particularly dangerous months for stock speculation before listing every other month of the year.

In short, October's reputation is driven more by the magnitude of several historic market events than by consistently negative monthly performance.

US Stock Market Performance in October: The Full Historical Record

The table below shows the S&P 500’s average return for each calendar month since 1928, based on historical data through November 2024.

(Source: J.P.Morgan) 

Average Monthly Returns by Month (S&P 500, 1928-2024)

Despite its association with some of history's most famous market crashes, October has not historically been the weakest month for US equities.

S&P 500 data going back to 1928 show:

• Worst month by average return: September, approximately -1.17%

• September negative frequency: Approximately 56% of years

• October average return: Approximately +0.54%

• October positive frequency: Approximately 59% of years

(Source: SP Global)

October's reputation largely reflects the severity of several extraordinary crashes rather than consistently poor performance. The month hosted the 1929 crash, Black Monday in 1987 and extreme volatility during the 2008 financial crisis, but over the longer term, October has produced a positive average S&P 500 return.

Why October Has Historically Been a Volatile Month

October has developed a reputation as one of the more volatile months for US stock markets. This reputation is largely linked to several periods of extreme market turbulence that occurred during the month, including the 1929 Wall Street Crash, Black Monday in 1987 and the global financial crisis in 2008. 

Historical market data also shows that October has experienced substantial swings in both directions. Importantly, volatility does not necessarily mean negative performance: a volatile month can include sharp gains as well as steep declines. 

Several factors may contribute to October’s reputation, including the influence of unusually large historical market moves and heightened uncertainty during periods of financial or economic stress. 

These extreme events can also significantly affect long-term measures of volatility. While historical patterns can provide context for understanding October’s market behaviour, they do not predict future performance. Market conditions vary from year to year, and economic data, monetary policy, corporate earnings and geopolitical developments can all influence volatility during any given October.

S&P 500 Historical Chart 

(Source: MacroTrends. Accessed on September 2, 2026).

VIX Data: The Fear Gauge in October

The Cboe Volatility Index (VIX) measures the market's expectation of S&P 500 volatility over the next 30 days, based on prices of S&P 500 Index (SPX) options.

Historical October VIX data highlights several periods of elevated market uncertainty:

• October 2008: The VIX averaged approximately 61.2 based on daily closing values during the global financial crisis. It reached an intraday record of 89.53 on October 24, while closing that session at 79.13.

• October 2020: The VIX averaged approximately 29.4, as markets faced uncertainty surrounding the U.S. presidential election and the COVID-19 pandemic. It closed as high as 40.28 on October 28.

• October 2022: The VIX averaged approximately 30.0 amid high inflation, aggressive Federal Reserve interest-rate increases and concerns about the economic outlook.

• October 2018: The VIX rose from 12.00 on October 1 to 24.98 on October 11, effectively doubling in less than two weeks amid a sharp equity-market sell-off.

It is important to distinguish between intraday and closing VIX records. The VIX reached its historic intraday peak of 89.53 on October 24, 2008. It later closed at 80.86 on November 20, 2008, which was the highest closing level during the financial crisis. The October intraday figure is therefore the more relevant statistic for an article focused specifically on October.

October has historically recorded elevated average VIX readings, although extreme periods such as the 2008 financial crisis significantly influence long-term averages. Historical volatility should therefore not be interpreted as evidence that volatility will necessarily be elevated in any particular October.

Largest Single-Day Moves: October's Outsized Presence

October has featured disproportionately in some of the S&P 500's largest daily market moves.

October has featured disproportionately in some of the S&P 500's largest daily market moves.

Using the historical S&P 500 series applied in this analysis, including reconstructed data from before the index's formal 1957 launch, 8 of the 20 largest single-day percentage declines occurred in October:

• October 19, 1987: -20.47%

• October 28, 1929: -12.34%

• October 29, 1929: -10.16%

• October 18, 1937: -9.27%

• October 15, 2008: -9.04%

• October 26, 1987: -8.28%

• October 5, 1932: -8.20%

• October 9, 2008: -7.62%

October has also produced several exceptionally large rebounds. Notable examples include October 13, 2008 (+11.58%), October 28, 2008 (+10.79%) and October 21, 1987 (+9.10%).

This history illustrates an important feature of October volatility: extreme moves have occurred in both directions, with sharp declines sometimes followed by significant rebounds.

This history illustrates an important feature of October volatility: Extreme moves have occurred in both directions, with sharp declines sometimes followed by equally significant rebounds.

October as a Bear Market Killer: The Reversal Data

Despite its reputation for market crashes, October has historically also been associated with major market reversals. Stock Trader's Almanac describes October as a "bear killer", noting that the month has turned the tide in 13 post-World War II bear markets or major downturns, including 1957, 1966, 1974, 1987, 1990, 2002 and 2022.

Using conventional S&P 500 bear-market chronology, several major market troughs have occurred in October:

*The 1990 decline is reported at approximately 19.9%-20.0% depending on the dataset and rounding convention.

October's "bear-killer" reputation is broader than these final S&P 500 troughs. Stock Trader's Almanac identifies 13 post-WWII downturns in which October helped turn the market, including cases where the final low occurred in another month or the decline did not meet the conventional 20% bear-market threshold.

This history suggests that October has frequently been an important reversal period, although historical seasonal patterns do not guarantee future market performance.

*These are S&P 500 price-index returns, not total returns including dividends

Average October return 2019-2024: +1.8% | Median: +0.5%

October 2026: What Should Traders Keep in Mind?

Several macro factors may shape the setup in October 2026:

• Q3 2026 earnings season: October marks the start of Q3 corporate earnings reports. In years where S&P 500 earnings growth has exceeded 10% year-over-year, October has historically delivered above-average returns. Consensus Q3 2026 earnings estimates will be a key variable to monitor. Companies like Tesla, JPMorgan, Wells Fargo, CitiGroup, Delta Air Lines, Johnson and Johnson, Netflix, Alphabet, Microsoft, Amazon, and Apple are all expected to report earnings in October. 

• Federal Reserve policy: Any shift in the Fed's rate trajectory between now and October, particularly around the September FOMC meeting, will set the volatility baseline heading into the month. The penultimate Fed meeting of 2026 is scheduled for 27-28 October. 

• Seasonal positioning: Institutional year-end rebalancing typically begins in October. In years where equities have significantly outperformed bonds year-to-date, this rebalancing creates structural selling pressure in stocks

How to Trade During Market Volatility

October's volatility is not a reason to exit markets, it is a reason to be prepared. Here are the core principles traders apply during high-volatility periods:

1. Define your risk before the trade

Volatile markets move fast. Setting stop-loss levels and maximum position sizes before entering a trade removes emotion from the equation when prices spike unexpectedly. In October, the average daily range on the S&P 500 has historically been approximately 30-40% wider than the annual average.

2. Reduce position size, not necessarily exposure

Rather than exiting entirely, experienced traders often reduce position sizes during high-volatility periods. This keeps them in the market for potential upside while limiting downside exposure.

3. Monitor the VIX as a real-time risk gauge

• VIX below 20: Normal volatility environment

• VIX 20-30: Elevated uncertainty, tighten risk management

• VIX above 30: Historically associated with panic or crisis conditions; also historically associated with forward 12-month returns above the long-run average

• VIX above 40: extreme fear; all 5 instances since 1990 have been followed by positive 12-month S&P 500 returns (CBOE data)

4. Watch the earnings calendar

October's earnings season creates stock-specific volatility independent of broader market moves. S&P 500 companies reporting in October account for approximately 35-40% of total index market capitalization. Individual stock moves of -10% or more on earnings day are not uncommon during volatile Octobers.

5. Avoid over-trading

High volatility increases transaction costs and the risk of being stopped out of good positions by short-term noise. Data from DALBAR's annual Quantitative Analysis of Investor Behavior consistently shows that investors who trade most frequently during high-volatility periods underperform those who hold positions, largely due to poor market timing.

6. Keep liquidity available

October's historical reversals have created some of the strongest forward return setups in market history. Traders who maintain some cash heading into the month are positioned to act when dislocations appear.

Conclusion

October's reputation as a dangerous month for stocks is built on real events, but its statistical reality is more complex. It is the most volatile month by every measurable metric, standard deviation of returns (5.6%), average VIX readings (21.3), intra-month drawdown magnitude (-4.6% average), but not the worst-returning. September has consistently been harder on portfolios over the long run, finishing negative in approximately 56% of all years since 1928.

The crashes that defined October's mythology, 1907, 1929, 1987, 2008, were genuine and severe. But they represent fewer than 5% of all Octobers on record. The other 95% produced an average return of approximately +0.54%, with markets finishing positive roughly 59% of the time.

For traders, the practical implication is straightforward: October demands preparation, not avoidance. Understanding the historical pattern, monitoring volatility indicators, and managing risk proactively gives traders the tools to navigate whatever the month brings, rather than being caught off guard by a reputation that is, at least partly, a matter of market psychology.

*Past performance does not guarantee future results. The above is for marketing and general informational purposes only, and these are only projections and should not be taken as investment research, investment advice or a personal recommendation.