The 15 worst-performing S&P 500 stocks during a dismal September

September is historically a challenging month for equities, and this year has reinforced that reputation beneath the surface. While the S&P 500 itself has eked out a modest gain, roughly three-quarters of its constituent companies have posted declines — a divergence that indicates the index's headline performance is being carried by a relatively small group of large names rather than broad participation.

This gap between the index and its average member matters for how investors interpret market conditions. A rising benchmark can mask weakness across sectors, meaning many portfolios may not reflect the headline number. When gains are concentrated in a handful of heavyweights, the index's advance says less about overall market health than it might appear.

What to watch: whether market breadth improves in the weeks ahead, and whether the laggards stabilize or continue to drag. Sustained narrow leadership can persist, but historically, broader participation has been a more durable foundation for market gains.