Why September Is Considered the Worst Month for Investing

Why Do People Say September Is the Worst Month for Investing?

September has historically been the weakest month for stock market performance, a pattern that's often referred to as the “September Effect.” This market anomaly is commonly observed in the U.S. and global markets and is thought to stem from factors like seasonal investor behavior and mutual fund portfolio adjustments before year-end. Although the effect has lessened, it continues to attract analyst and investor attention, especially those who want to understand market trends.

Exploring the September Effect in Global Markets

From 1928 through 2021, the S&P 500 index has averaged a 1% decline during the month of September. This is an average exhibited over many years, and September is certainly not the worst month of stock-market trading every year.

The September Effect is a market anomaly and not related to any particular market event or news. In recent years, the effect has dissipated. Over the past 25 years, for the S&P 500, the average monthly return for September is approximately -0.4%, while the median monthly return is now positive.

In addition, frequent large declines have not occurred in September as often as they did before 1990. One explanation is that investors have reacted by “pre-positioning”—that is, selling stock in August.

Reasons Behind September's Market Dip

The September effect is not limited to U.S. stocks but is also associated with some worldwide markets. Some analysts consider that the negative effect on markets is attributable to seasonal behavioral bias as investors change their portfolios at the end of summer to cash in.

Another reason could be that most mutual funds cash in their holdings to harvest tax losses. Another particular theory points to the fact the summer months usually have lightly traded volumes, as a good number of investors usually take vacation time and refrain from actively trading their portfolios during this downtime.

Once the fall season begins and these vacationing investors return to work, they exit positions they had been planning on selling. When this occurs, the market experiences increased selling pressure and, thus, an overall decline.

Additionally, many mutual funds end their fiscal year-end in September. Mutual fund managers, on average, typically sell losing positions before year-end, and this trend is another possible explanation for the market's poor performance during September.

The Bottom Line

The “September Effect” refers to the historical trend of weaker stock market performance during September, with major indices like the DJIA and S&P 500 often showing declines. Analysts attribute this anomaly to factors like investor behavior, mutual fund year-end sales, and reduced summer trading activity. But since the 1990s, this pattern has become less consistent, with fewer significant downturns occurring in recent decades.