Why Do Stock Markets Tend to Fall in September?

Every September, a familiar phrase resurfaces in market commentary: the "September Effect." It might sound like folklore, but the data tells a surprisingly consistent story. The S&P 500 has averaged a return of roughly -1.2% in September since 1928 — the only calendar month with a negative long-run average.

3 Reasons September Is Historically Weak

The first reason is fund fiscal year-end positioning. Many U.S. mutual funds close their books at the end of October. To tidy up portfolios — locking in losses for tax purposes or rebalancing — fund managers tend to sell heavily in September. The second reason is the post-summer trading surge. August is quiet: institutional investors are on vacation and markets drift. When September arrives, trading volume jumps sharply, and that sudden uptick in activity naturally brings more volatility. The third reason is a psychological shift toward year-end. As investors start reviewing annual performance, the urge to lock in gains or cut losses picks up pace.

The VIX Fear Index Also Tends to Spike in September

The VIX index measures expected volatility in the S&P 500 over the next 30 days, derived from options prices. It's widely known as the "fear gauge." Historically, VIX tends to reach its highest annual levels in September and October. In calm markets, VIX hovers around 12–15. When anxiety rises, it can jump past 25. A rising VIX in September signals that investors are pricing in a bumpier road ahead.

2026 Has an Extra Wild Card: U.S. Midterm Elections

This November brings U.S. midterm elections, where voters choose new members of Congress. If control of the House or Senate flips, it can shift the direction of tax policy, government spending, and sector regulation. Markets don't like that kind of uncertainty. Starting in September, investors typically begin repositioning portfolios based on their expectations of election outcomes — which sectors might benefit, which might be pressured. Historically, midterm election years have seen above-average volatility in September and October.

Korea's Market Is Feeling It Too

Today, KOSPI trading volume dropped to roughly half of where it stood two months ago. Thin trading volume is dangerous: even modest selling pressure can push prices down significantly. Adding to that, remarks from the U.S. Federal Reserve's annual Jackson Hole symposium rattled expectations for interest rates. When the Jackson Hole season and the September Effect collide, volatility can amplify quickly.