Stocks tend to outperform in January — in a well-known market mystery that still defies clear explanation.

Why it matters: The so-called January effect is a seasonal stock market behavior that runs counter to the "efficient market hypothesis" which says stock prices are fundamentally random and unpredictable.

Backstory: Academics first quantified January outperformance in the 1940s, but the key paper on the topic dates from 1976.

• It found that, from 1904 to 1974, the average stock market return during January was nearly 3.5%, about eight times higher than the monthly return during the remainder of the year, which averaged 0.4%.
• Subsequent studies updated the findings. A 2003 paper that looked at the years 1947 to 2000 found sharply higher returns for January, as did a 2008 paper looking at data from 1927 to 2004.

Go deeper: Two main theories have been proposed about why this should be: tax-loss selling and window dressing.

• With tax-loss selling, the idea is that U.S. investors sell a lot of losing stocks in December to generate losses they can use to reduce their taxable income, and therefore lower their tax bills.
• That generates abnormal downward pressure on the market in December, subsiding in January, giving stocks a bit of a bounce.
• The wind0w-dressing hypothesis suggests professional money managers purge poor-performing stocks from portfolios in December to avoid reporting embarrassing holdings in end-of-year documents, then often repurchase them in January, providing a fillip to the market.

Yes, but: There are some weak spots in those arguments.

• For instance, data shows a January effect for stocks before there was an income tax — it was introduced in 1913.
• There are documented January effects for stock markets in many countries with very different tax systems.

💭 My thought bubble: While the machinations of money managers probably do play a big role in the phenomenon, I like to think the January effect is evidence of the human spirit at work, too.

• Some behavioral economists — who apply psychological insights to their analysis of the economy — agree.
• Citing findings that the turn of the calendar year typically injects a momentary bit of optimism into the market, a 2011 paper in the Journal of Behavioral Finance found that "the January Effect is at least partly driven by investor optimism at the turn of the year."

The bottom line: It may not last, but here's hoping you're feeling that bit of optimism as 2024 gets going.