September Is Historically the Worst Month for Markets. Should You Worry?

Welcome to September, historically the worst month of the year for stock markets.

Since 1928, the S&P 500 has lost money in September more often than in any other month - but it’s not as straightforward as that.

And there's a less-discussed flip side: what comes after September has historically been some of the strongest market performance of the year.

So should you be worried? No, not if you know what you're looking at.

Here's what history shows, and what to make of it.

Why September Has a Bad Reputation

There's no single explanation, and that's actually part of the story.

The September Effect, as it's called, doesn't trace back to one recurring event or policy cycle. It's a statistical pattern with several plausible contributing factors, none of them definitive on their own.

One theory is behavioural. Summer tends to be a quieter period for trading, with many institutional investors and fund managers on holiday. When they return in September, they often execute selling decisions they'd been putting off, bringing increased selling pressure to markets all at once.

Another factor is mutual fund activity. Many funds used to end their fiscal year in September, which could mean fund managers sold underperforming positions before the books close. But this explanation is less relevant today, since relatively few US mutual funds now have a September year-end.

A third explanation is that September has simply been unlucky: two of the most devastating events in modern financial history happened in September.

On 11 September 2001, the twin towers attacks shut down the New York Stock Exchange entirely. When it reopened on 17 September, the Dow Jones Industrial Average fell 684 points, a 7% drop, to close at 8,921. It was the biggest single-day point drop the Dow had ever recorded at the time.

Seven years later, on 15 September 2008, Lehman Brothers filed for bankruptcy. The Dow fell 504 points, or 4.4%, that day alone, and the collapse went on to trigger the worst financial crisis since the Great Depression.

Both crises happened to land in September, but neither one is why the month has a reputation on its own. The average return you see quoted every year comes from a much longer, far less dramatic stretch of ordinary Septembers.

How Bad Is It Really?

The average S&P 500 return in September from 1928 to 2021 is roughly minus 1%. But the September Effect has weakened considerably since the 1990s.

Over the past 25 years, the average September return for the S&P 500 is around negative 0.4%, not the full 1% decline seen over the longer historical period.

And over the past 30 years, the S&P 500 has actually finished September with a positive return 16 times, and a negative return 14 times - almost exactly a coin flip.

One reason may be that investors have caught on. As the pattern became well known, more investors began selling in August rather than September, effectively front-running the seasonal weakness. That pre-positioning has smoothed out what used to be a more concentrated period of selling.

The reason the month has a bad reputation is that the down months have tended to involve sharper drops than the up months produce gains, not that September is reliably negative.

Why 2026 Might Not Follow the Script

2026 has one thing going for it that many past Septembers didn't. It's a US midterm election year, and midterm Septembers have historically been very kind to stocks.

According to research cited by market strategist Ryan Detrick, the four best Septembers on record all happened in midterm years, and five of the seven best. 2010 gained 8.8%, 1954 gained 8.3%, 1998 gained 6.2%, and both 1950 and 1958 gained more than 4.5%.

The market is also starting September from a position of strength. The S&P 500 was up 12.6% for the year through the end of August, after reaching record highs during the month.

None of this guarantees a good month. It just means the case for a rough one is weaker this year than the headline statistic suggests.

Is it An Opportunity?

There's one more angle worth mentioning, not as financial advice, but as context.

If September tends to be the weakest month of the year, it also tends to create the most attractive entry points.

Historically, investors who added to their positions during September's dips have often been well-positioned for the fourth quarter, which is typically one of the strongest periods of the year.

That's not a guarantee, and timing the market is never straightforward.

But if you've been sitting on cash and waiting for a moment to invest, a historically soft month is arguably a more interesting time to act than the peak of a rally.

What This Means for You

None of this is a reason to change what you're doing. The pattern is real, but it's not a forecast. Markets respond to news, earnings, rates, and sentiment, all of which are uncertain right now regardless of the calendar.

But if September does bring some market weakness this year, that's not a crisis. For anyone investing regularly through a dollar-cost averaging approach, it might even mean buying at better prices.

Either way, a difficult month is something a well-constructed portfolio is designed to absorb.

So keep it simple. If your portfolio dips this month, you already know why it might be happening. Keep your eye on the long-term, and remember that successful long-term investing is less about avoiding every dip and more about staying consistent through them.