Seasonal Trends and the September Swoon

Mark Twain may be right about which months are dangerous!

The September Swoon

The “September Swoon” is a seasonal trend in the stock market and one that has been well documented by researchers and the press. September has historically been the worst month on the calendar for stocks.

• The S&P 500 has averaged a 1% loss in September, dating back to 1928, according to Dow Jones market data.
• The same is true for the Dow Jones Industrial Average, dating back to 1896.
• Those are the worst monthly performances for both indexes in the calendar year.
• NASDAQ has also seen an average decline of 0.5% during September since 1971 (when NASDAQ was first established).

Seasonal Trends

Seasonality is based on the analysis of years of historical data. A seasonal trend is discovered if a pattern emerges in this analysis, in terms of average performance in a certain month. It’s important to remember, however, that due to financial, psychological, and political factors, stock market behavior can run completely contrary to the seasonal trend in any given year.

Because the stock market in September has shown markedly different behavior, on average, for almost a century, these results are not coincidental. There could be a genuine seasonal trend.

Possible Reasons for the September Swoon

So, why does the stock market generally drop in September? What causes the September Swoon?  Economists and financial analysts have studied this topic, but no one has reached a definitive conclusion. Here are some of the hypotheses:

Summer Vacation:  This hypothesis holds that traders and investors are more likely to sell their stocks after returning from their August vacations or long, Labor Day weekends. Trading volume tends to decline during the summer, and then investors – especially professional investors – get back to trading from their computers.

Third Quarter:  Many mutual funds have fiscal years ending in the fall, provoking them to sell their losing stocks for “window-dressing” purposes. This term describes the process of a portfolio manager making cosmetic changes at the end of the quarter because they list their holdings at the beginning of a new quarter – their list of holdings looks better without the poor-performing stocks.

Tax Losses: Investors begin to sell declining stocks to harvest their tax losses, getting ahead of other investors who sell at year-end. This hypothesis also draws support from the observed “January effect,” where investors buy back the stocks that they sold for tax purposes.

Tuition Time:  With this hypothesis, investors might sell large amounts of their stock holdings to pay their children’s tuition bills at colleges, universities, and prep schools. And for most, the school year begins roughly in September.

Seasonal Affective Disorder (SAD):  A university study suggested that the sharp drop in the amount of daylight in New York City in September might trigger Seasonal Affective Disorder (SAD), a type of depression related to changes in seasons. As a result, according to this hypothesis, some investors become more risk-averse, so they sell losing stocks, unwilling to wait for things to get better.

Cultural trends (summer vacations, back-to-school in the fall), regulations and taxes (third quarter, tax losses), and even psychological effects of weather (seasonal affective disorder) have been offered as explanations for this strange September market trend.

Unfortunately, none of these explanations has been proven, frustrating researchers who seek reasons for patterns in the stock market. Maybe each of these factors contributes to the trend. No one for sure knows the reason for the September Swoon.

Knowing the Reason

If someone discovered a convincing explanation for the September Swoon, would this help investors? Probably not. Savvy investors might jump the gun, selling stocks in August, and then others might try to beat them by selling in July. Of course, the seasonal September pattern would then disappear, replaced by some other trend.

Portfolio Changes

But even if there is no proven reason for this September Swoon, shouldn’t an investor make changes, anyway? Because of this seasonal trend toward declining stock values in September, traders and investors might be inclined to alter their portfolios. However, the September Swoon is based on an average, the average performance of the stock market since before the Great Depression, divided by month.

While September is, on average, the worst month for stocks, this doesn’t mean that each September is bad. In fact, since 1950, the month of September has delivered almost as many up-years as down-years.

No one knows the definite reason for this so-called Swoon, and trying to guess which year will be bad could be an unprofitable game.

Seasonality vs. Market Timing

There is a difference between market timing and seasonality. Seasonal trends reflect how the market will behave in particular months as part of a long-term trend. Market timing is based on short-term price patterns. Timing the market perfectly is, of course, impossible. As discussed above, seasonal trends are grounded in the analysis of years of data, but not every year is identical.

Dangerous Months

“October. This is one of the peculiarly dangerous months to speculate in stocks. The other are July, January, September, April, November, May, March, June, December, August, and February.”
~Mark Twain

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