October Effect

What Is the October Effect?

The October effect is a perceived market anomaly that stocks tend to decline during the month of October. The October effect is considered to be more of a psychological expectation than an actual phenomenon, as most statistics go against the theory. Some investors may be nervous during October because some large historical market crashes occurred during this month.

The events that have given October the reputation for stock losses have happened over decades, but they include:

  • The Panic of 1907
  • Black Tuesday (1929)
  • Black Thursday (1929)
  • Black Monday (1929)
  • Black Monday (1987)

Black Monday, the great crash of 1987 that occurred on Oct. 19 and saw the Dow plummet 22.6% in a single day, is arguably the worst single-day decline. The other black days, of course, were part of the process that led to the Great Depression—an economic disaster that stood unrivaled until the mortgage meltdown nearly took out the whole global economy with it.

Key Takeaways

  • The October effect is the perception that stock markets decline during the month of October, and it is classified as a market anomaly.
  • The October effect is considered to be more of a psychological expectation than an actual phenomenon, as most statistics go against the theory.
  • The October effect, as well as other calendar anomalies, have seemed to largely disappear over the past decades.

Understanding the October Effect

Proponents of the October effect, one of the most popular of the so-called calendar effects, argue that October is when some of the greatest crashes in stock market history, including the 1929 Black Tuesday and Black Thursday and the 1987 stock market crash, occurred. While statistical evidence doesn’t support the phenomenon that stocks trade lower in October, the psychological expectations of the October effect still exist.

The October effect, however, tends to be overrated. Despite the dark titles, this seeming concentration of days is not statistically significant. In fact, September has more historical down months than October. From a historical perspective, October has marked the end of more bear markets than the beginning. This puts October in an interesting perspective for contrarian buying. If investors tend to see a month negatively, it will create opportunities to buy during that month. However, the end of the October effect, if it ever was a market force, is already at hand.

Special Considerations

What is true is that October has traditionally been the most volatile month for stocks. According to research from LPL Financial, there are more 1% or larger swings in October in the S&P 500 than any other month in history dating back to 1950. Some of that can be attributed to the fact that October precedes elections in early November in the United States every other year. Oddly enough, September, not October, has more historical down markets.

More importantly, the catalysts that set off both the 1929 crash and the 1907 panic happened in September or earlier, and the reaction was simply delayed.

In 1907, the panic nearly occurred in March. Throughout the year, the public’s confidence continued to diminish in trust companies, which were considered risky because of their lack of regulation. Eventually, public skepticism came to a head in October and sparked a run on the trusts.

The 1929 Crash arguably began in February, when the Federal Reserve banned margin-trading loans and cranked up interest rates.

The Disappearance of the October Effect

The numbers don’t support the October effect. If we look at all October monthly returns going back more than a century, there simply is no data on average to support the claim that October is a losing month. Indeed, some historical events have fallen in the month of October, but they have mostly stuck around in the collective memory because Black Monday sounds ominous. Markets have also crashed in months other than October.

Many investors today have a better memory of the dotcom crash and the 2008–2009 financial crisis, yet none of those days were given the black moniker to bear for their particular month. Lehman Brothers’ collapse happened on a Monday in September and marked a large increase in the global stakes of the financial crisis, but it didn’t get reported as a new Black Monday. For whatever reason, the news media no longer leads with black days—and Wall Street doesn’t seem eager to revive the practice, either.

Moreover, an increasingly global pool of investors doesn’t have the same historical perspective when it comes to the calendar. The end of the October effect was inevitable, as it was mostly a gut feeling mixed with a few random chances to create a myth. In a way, this is unfortunate, as it would be wonderful for investors if financial disasters, panics, and crashes chose to occur only in one month of the year.

Article Sources
Investopedia requires writers to use primary sources to support their work. These include white papers, government data, original reporting, and interviews with industry experts. We also reference original research from other reputable publishers where appropriate. You can learn more about the standards we follow in producing accurate, unbiased content in our editorial policy.
  1. Federal Reserve History. “Stock Market Crash of 1987.” Accessed Sept. 29, 2021.

  2. Library of Congress. “The Black Monday Stock Market Crash.” Accessed Sept. 29, 2021.

  3. LPL Research. “Is October Really Scary?” Accessed Sept. 29, 2021.

Take the Next Step to Invest
The offers that appear in this table are from partnerships from which Investopedia receives compensation. This compensation may impact how and where listings appear. Investopedia does not include all offers available in the marketplace.