Ask anyone who has spent a few years invested in the markets, and you'll probably hear the same warning this time of year: September is trouble. Traders brace for it, financial media write about it every year, and the phrase "September Effect" gets tossed around like it's a settled fact. The interesting part is that the data mostly backs it up. What it doesn't back up is the idea that you should do anything differently because of it.

The Numbers Behind It

According to Bloomberg and monthly data through 7/31/25, since 1928, the S&P 500 has posted an average September return of roughly negative 1.2%. On average, every other month except February has ended positively. July, by contrast, has been the strongest performer of the year. Zoom in on more recent decades, and the pattern holds. Since 1990, September still comes in as the weakest month on the calendar, and the gap widens the closer you look at the last twenty-five years.

Some of that average gets pulled down by outliers. The 1931 crash, in the depths of the Great Depression, wiped out nearly 30 percent of the market's value in a single month. More recently, in September 2008, the collapse of Lehman Brothers led to a nearly 9 percent drop. September 2022 delivered another rough one, down over 9 percent as the Fed pushed rates higher to fight inflation. Months like these skew a century-long average more than people tend to realize.

Strip out the drama and September still loses more often than it wins, but not by as much as the reputation suggests. Stocks have finished the month lower roughly 55 percent of the time going back nearly a hundred years. That's a real tilt, but it's closer to a coin flip than a curse.

Why It Happens (Maybe)

Nobody has a clean answer for why September behaves this way, and that alone should tell you something. A few theories get repeated most often. Institutional investors frequently rebalance portfolios at quarter-end, and September closes out the third quarter for most funds. If equity allocations have drifted above target after a strong summer, fund managers sell to bring things back in line. Tax considerations play a role too. Many mutual funds close their fiscal year in October, which pushes some managers to realize losses in September to offset gains elsewhere. Add in a return to full trading volume after a quiet summer, with everyone back from vacation and reassessing positions at once, and you get a month where selling pressure tends to build.

Then there's the simplest explanation of all: everyone knows about the September Effect, so everyone half expects it, and that expectation shapes behavior. When enough investors trim positions early to avoid a dip they've read about for years, the dip becomes more likely to show up. It's a reasonable theory, and also nearly impossible to prove.

Worth noting, one long-running study of U.K. market data going back to the 1600s found no meaningful September pattern at all. That doesn't disprove what's happened in U.S. markets over the last century, but it's a good reminder that seasonal patterns can owe more to coincidence than cause.

What This Actually Means for Your Portfolio

Here's the honest answer: probably nothing. A monthly seasonal pattern, even a persistent one, isn't a reason to change how you're invested. The S&P 500 has still delivered strong long-term returns across the exact same decades that include all these weak Septembers. Investors who try to dodge the market's worst month often end up missing some of its best days instead, and that trade-off has historically cost far more than September ever has.

If anything, September is a good prompt to do what you should be doing anyway: check in on whether your portfolio still fits your goals and how much risk you're genuinely comfortable carrying. That's a conversation worth having with your XML Wealth Advisor regardless of what month the calendar says.

Sources: RBC Wealth Management; Yardeni Research via The Motley Fool; 24/7 Wall St.

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