January Effect and October Effect, Are They Real?

Market timing does not work (1). However, there are still plenty of investors who believe in market timing, and a subset of these believe that January Effect and October Effect are real. In a true MythBusters‘ fashion, I did some historical data analysis and have a few statistical factoids to share with you. But first…

What is the January Effect?

Some people believe that stock prices generally increase during the month of January, because people are buying back stocks after end of the year sell-off by investors harvesting tax losses.

What is the October Effect?

There’s a theory that stocks tend to decline in October, because it’s cursed. Coincidentally, the dates of some large historical market crashes occurred during this month — for example, Panic of 1907, Wall Street Crash of 1929, and Black Monday, October 19, 1987.

Now let’s look at the data from the past 30 years

You can skip this table if it gives you migraine.

January and October Effect

The table above shows us monthly performance of the S&P 500 for the past 30 years:

  • Declines are noted in red and worst month of the year in yellow
  • Gains are noted in black and best month of the year in green

Here are some quick factoids based on the data above

  • October 1987 was the worst month, and January 1987 was the best
  • 2002 was the worst year, and 1995 was the best
  • On average, September was the worst month, and November was the best
  • July has the most declines at 17 down months, and December has the most ups at 24
  • August has the most worst months, and October has the most best

Even if you’re sure you see an unmistakable trend, don’t bet the farm on it, because other people sees it too. The efficient market hypothesis dictates that you can’t take advantage of any such trend. For instance, after reading this post you (and everyone else) think August is the best time to invest, then next year people will anticipate the sharp gain in August and invest in July instead, and so on and so forth…

Note: (1) There are a few exceptions of timing that work — e.g., tax-loss harvesting and timing mutual fund buys to avoid capital gains distributions.

About the Author

By , on Oct 29, 2007
Pinyo is the owner of Moolanomy Personal Finance. He is a licensed Realtor specializing in residential homes in the Northern Virginia area. Over the past 20 years, Pinyo has enjoyed a diverse career as an investor, entrepreneur, business executive, educator, and financial literacy author.

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Leave Your Comment (4 Comments)

  1. Jonathan says:

    The depth of the stats are great, they really highlight your points well. The difficulty is that as you rightly point out, if one person spots a trend then it’s not long before othres do.

  2. Pinyo says:

    @Raymond – good for you!

    @Patrick – damn, you’ve got me good. If you have the money, I think one shot deal is better. Take a look at “Does Dollar Cost Averaging Work?

  3. Patrick says:

    I believe in keeping my money under a mattress until the stocks climb for 3-6 consecutive months. Then I invest it all in whichever sector is the hottest!

    I’m joking… I dollar cost average through my 401(k), and this year I invested my entire Roth IRA at once (in January). I could probably DCA the Roth as well, but I just did it at once. I don’t know how I will do it next year.

  4. Raymond says:

    There is no trend that I can discern. I truly believe in the random walk, lemming theory.

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