September has earned its reputation as the market’s least favorite month, and 2026 is shaping up to be an especially spicy edition. Barclays is telling equity investors to trim risk heading into fall, citing a cocktail of seasonal weakness, midterm election jitters, interest-rate volatility, and a wave of blockbuster AI IPOs that could test how much capital markets can absorb at once.
September’s track record speaks for itself
Since 1928, the S&P 500 has posted an average September return of roughly -1.1%. That makes it the only month on the calendar with a negative long-run average. The numbers get worse in midterm election years. During those cycles, the S&P 500 has historically declined about -1.5% in September. 2026 happens to be a US midterm election year, and the August-through-October stretch in such years has produced mixed results at best.
Barclays isn’t the only firm sounding the alarm. Citadel Securities has also reportedly advised reducing exposure or adding portfolio protection ahead of September.
AI IPOs could absorb a lot of oxygen
OpenAI is anticipated to go public in September 2026, with valuations estimated between $850 billion and $1 trillion. Anthropic is reportedly preparing its own IPO around October 2026. Barclays has specifically flagged risks around AI positioning due to these seasonal factors and the lofty valuations already baked into the sector.
Rate volatility adds another layer
Growth stocks, which dominate AI-heavy portfolios, are especially sensitive to rate movements because their valuations depend heavily on discounting future cash flows. Market commentary from multiple strategists has emphasized hedging and diversification beyond mega-cap AI leaders.
Quality over momentum
Strategists are recommending that investors distinguish between AI companies with proven monetization strategies and those still burning cash in pursuit of scale. Trimming positions in high-momentum names, adding hedges through options or volatility strategies, and tilting toward quality factors — meaning companies with strong balance sheets, durable competitive advantages, and visible earnings trajectories — reflect the defensive playbook being recommended for this period.