September has long had a reputation as a difficult month for stock markets.
Historically, September has often produced weaker average returns than other months, particularly in the US stock market. This seasonal pattern is sometimes referred to as the “September Effect.”
But does that mean investors should be worried every time September arrives?
A Historical Pattern, Not a Prediction
Historical market statistics can reveal interesting patterns, but they cannot tell us with certainty what will happen next.
There have been Septembers when markets declined sharply — but there have also been many Septembers when markets rose.
A weak historical average does not mean that September must be negative in any particular year.
Why Might September Be More Volatile?
There is no single accepted explanation for the September Effect.
Several factors may contribute, including:
- investors returning after the summer period and repositioning portfolios;
- institutional portfolio rebalancing;
- changes in trading activity and market liquidity;
- economic and corporate developments later in the year; and
- investor psychology surrounding a well-known seasonal pattern.
These factors can create volatility, but they do not make September's market direction predictable.
What Should Investors Take Away From This?
Seasonality is interesting, but it should be viewed in context.
For long-term investors, one month's historical performance is usually less important than factors such as:
- investment objectives;
- time horizon;
- portfolio diversification;
- valuations;
- economic and market conditions; and
- the ability to remain invested through periods of volatility.
A weaker market can also create opportunities for investors who have a long investment horizon and available capital.
The important distinction is between understanding a historical tendency and using it as a market-timing signal.
The Bigger Picture
Markets rarely move according to the calendar alone.
September's historical record is an interesting market statistic, but investment decisions should not be based simply on whether a particular month has historically been strong or weak.
Sometimes the most useful role of market history is not to tell us what will happen next — but to remind us that periods of volatility are a normal part of investing.